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The following resources come from the 2009/10 BEng (Hons) in Digital Communications & Electronics (course number 2ELE0064) from the University of Hertfordshire. All the mini projects are designed as …

The following resources come from the 2009/10 BEng (Hons) in Digital Communications & Electronics (course number 2ELE0064) from the University of Hertfordshire. All the mini projects are designed as level two modules of the undergraduate programmes.

The objectives of this module are to investigate the following functions in a radio receiver for a communication system
• RF amplification
• Frequency conversion
• IF amplification and filtering
• Demodulation
This project involves working from a specification for the performance of a radio receiver and making laboratory measurements on various building blocks. These will be connected to form a complete receiver whose overall performance will be assessed using both test equipment and 'off air' signals.

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  • 1. Mini Project- Radio Receiver Design & Evaluation Author: University of Hertfordshire Date created: Date revised: 2009 Abstract The following resources come from the 2009/10 BEng (Hons) in Digital Communications & Electronics (course number 2ELE0064) from the University of Hertfordshire. All the mini projects are designed as level two modules of the undergraduate programmes. The objectives of this module are to investigate the following functions in a radio receiver for a communication system • RF amplification • Frequency conversion • IF amplification and filtering • Demodulation This project involves working from a specification for the performance of a radio receiver and making laboratory measurements on various building blocks. These will be connected to form a complete receiver whose overall performance will be assessed using both test equipment and 'off air' signals. Contents Mini Project- Radio Receiver Design & Evaluation.........................................................................................................1 Section 1. Project Overview...........................................................................................................................................2 Section 2. Project Day 1 Brief........................................................................................................................................4 Section 3. Project Day 1 Guidance................................................................................................................................5 Section 4. Project Day 2 Brief......................................................................................................................................11 Section 5. Project Preparation.....................................................................................................................................12 Project Introduction .....................................................................................................................................................13 1. Radio Receivers......................................................................................................................................................14 2. Architecture of a radio receiver.................................................................................................................................15 3. Building blocks of a radio receiver.............................................................................................................................17 4. Principal performance parameters of a radio receiver...............................................................................................21 5. Electronic design of selected parts of a radio receiver..............................................................................................23 6. Radio Wave Propagation..........................................................................................................................................27 Appendix A...................................................................................................................................................................31 Credits..........................................................................................................................................................................34 In addition to the resources found below there are supporting documents which should be used in combination with this resource. Please see: Mini Projects - Introductory presentation. Mini Projects - E-Log. Mini Projects - Staff & Student Guide. Mini Projects - Standard Grading Criteria. Mini Projects - Reflection. © University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License.
  • 2. Section 1. Project Overview Learning Outcomes assessed (as taken from the DMD) All Learning Outcomes specified in the Definitive Module Documentation are assessed as part of this miniproject, the specific Learning Outcomes are: Knowledge and Understanding Successful students will typically • Identify and enhance knowledge gained from other studies in areas relevant to the project topic selected Skills and Attributes Successful students will typically • Synthesise a solution to a defined electronic engineering problem relevant to communications systems • Take, and analyse appropriately, test results from that solution. • Carry on a simple critical evaluation of the results taken. Project Title: Radio receiver design and evaluation Project Objectives: (technical, specific to this project) To investigate the following functions in a radio receiver for a communication system • RF amplification • Frequency conversion • IF amplification and filtering • Demodulation Project Summary: (50 words max) This project involves working from a specification for the performance of a radio receiver and making laboratory measurements on various building blocks. These will be connected to form a complete receiver whose overall performance will be assessed using both test equipment and 'off air' signals. Introductory Lecture (2hrs) Content: • General project area: radio receivers. • Architecture and building blocks of a superhet radio receiver • Principal performance parameters, frequency range, sensitivity, selectivity, etc. • Electronic design of selected parts of a radio receiver, e.g. tuned amplifier. • Outline of MF/HF/VHF radio wave propagation Preparation Session (3hrs): Laboratory session, familiarisation with RF test equipment and measurements • RF spectrum analysers • RF signal generators • RF measurements with oscilloscopes Day 1 Expected Outcomes for the day: To have built and tested a tuned RF amplifier stage. To have measured characteristics of a mixer, IF amplifier, detector and Received Signal Strength Indicator (RSSI) Assessment criteria: Schematic diagrams of RF amplifier stage. Results of laboratory measurements such as spectrum analyser plots, gain measurements, selectivity curves, bandwidth measurements. Analysis and discussions of results, contrasting between AM and FM receivers. Key Tasks: • Design, build and measure characteristics of a tuned RF amplifier stage, e.g. gain and tuning range • Measure characteristics of a pre-built RF amplifier stage. • Measure characteristics of a mixer • Measure characteristics of IF amplifier stages, e.g. gain and bandwidth • Investigate properties of AM and FM detectors and Received Signal Strength Indicator (RSSI) Page 2 of 34
  • 3. Day 2 Expected Outcomes for the day: To have designed and built a superhet receiver using a combination of pre-built modules and modules designed by the student. To have measured the overall performance of the receiver in 2 different frequency bands for test signals and off-air broadcast signals. Critical evaluation of performance compared to specification given. Assessment criteria: Block diagram of superhet radio receiver designs. Measurement of overall response of radio receivers in two different bands using modulated test signals at the antenna input with varying modulating frequency. Evaluation of performance in relation to design specification. Assessment of radio receiver performance in three different bands using off-air signals. Assessment of any degradation in quality of received signal and an analysis of the reasons for this Key Tasks: • Design a superhet radio receiver using pre-built modules and modules designed by the student. • Measure sensitivity, bandwidth and response to modulated test signals for the overall receiver in two different bands - MF or HF AM broadcast, VHF/FM broadcast • Measure sensitivity, bandwidth and response to modulated test signals for the overall receiver in two different bands - MF or HF AM broadcast, VHF/FM broadcast • Assess performance of the overall receiver for off-air signals in two different bands - MF or HF AM broadcast, VHF/FM broadcast. • Assess and justify any degradation that occurs. Facilitator Guidance (key ideas to draw out from students): Day 1: Measurements of communication building blocks - use of signal generators and other test equipment Day 2: Operation of a superhet receiver, gain, selectivity, frequency conversion, track tuning of RF and L.O. Required Resources: Laboratory Facilities and Teaching Support. Laboratory Resources: Day 1: Superhet receiver modules (RF amplifier, mixer, IF amplifier/detectors), RF signal generator, oscilloscope (min 20 MHz) (1 complete set per group of students). Day 2: As Day 1 Teaching Resources: • Preparatory Session; tutorial exercises. • Day 1; briefing pack containing instructions for the day • Day 2; briefing pack containing instructions for the day Page 3 of 34
  • 4. Section 2. Project Day 1 Brief Expected Outcomes for the day: To have built and tested a superhet MF receiver with balanced mixer, using an RF signal generator as the Local Oscillator (L.O.) source. To have received at least 3 different MF broadcast signals. Assessment criteria: Schematic diagrams of the various radio receivers and the tuned RF amplifier stage. Results of laboratory measurements such as gain measurements, selectivity curves, bandwidth measurements. Analysis and discussions of results. Key Tasks: • Construct audio amplifier using audio amplifier I.C. • Evaluate characteristics of AM IF amplifier/detector board, i.e. measurement of centre frequency, gain, bandwidth and plot selectivity curve. • Design and build an MF 'crystal set' receiver with ferrite rod antenna. • Design and build MF superhet receiver with voltage regulator, balanced mixer, ferrite rod antenna, using an RF signal generator as the Local Oscillator (L.O.) source. Page 4 of 34
  • 5. Section 3. Project Day 1 Guidance Application Notes on components and circuit boards These notes are circuit ideas that you can use but you will need to work out how to use them and also calculate some component values. Please take care to avoid destroying semiconductor devices by applying excessive voltage, reversed (+) and (-) polarity or by connecting power to the wrong pins. The IF amplifier circuit boards have been custom-built and some of the electronic components used in this mini project have been ordered specially, so the number of spares available may be very limited. Your progress may be delayed or you may lose marks if you 'blow up' (destroy) a circuit board or an electronic device that has been ordered specially. Please remember to: • Switch off the power supply before making any changes to your circuit. • Set the power supply to the right supply voltage • Connect the (+) and (-) polarity the right way round. • Connect the supply voltage to the right pins. • Get it right first time • Check before switching on the power supply. 1. The audio amplifier Start by building an audio amplifier for your radio receiver. A suitable circuit is shown in Fig. 1 below. Yellow +9V * Use 2 x 4.7Ω in parallel if no 2.2Ω Keep C 5 resistors are available C5 close to IC 1 100 nF 14 2 C3 Red lead of coaxial + 470uF C1 C2 IC1 8 + - 100nF 47pF LM380N C4 Input from signal - 3 100nF 6 5 generator 7 LS1 R1* 8Ω 2.2Ω Green Black lead of coaxial 0V Fig. 1. Audio power amplifier using LM380 IC C3 is an electrolytic capacitor and it must be connected with the correct (+) and (-) polarity. Although the LM380 is an audio amplifier IC, it is capable of oscillating at radio frequencies if the circuit layout or supply decoupling are not good enough. The circuit in Fig. 1 also illustrates some useful principles of radio frequency circuit electronics. • IC1 has 3 ground pins. These should all be grounded via very short wires (approx. 1 cm). • C5 is a power supply decoupling capacitor. It must be located close to IC1. • R1 and C4 form a Zobel Network to prevent RF oscillation due to the inductance of the loudspeaker coil. You will also need to add an Audio Frequency (AF) Gain Control (i.e. a volume control) at the input to the LM380. A potentiometer with a resistance of 47 kΩ or 100 kΩ is suitable but you should use a logarithmic potentiometer and connect it so that clockwise rotation increases the gain. 2(a). The AM IF amplifier board. Page 5 of 34
  • 6. Fig 2(a). Schematic diagram of the AM IF amplifier/detector board Please do not attempt to adjust the cores in the coils IFT1 and IFT2. These boards have been set to various different centre frequencies and are marked with frequency codes A - F. Note which one you are using and keep the same IF board for Day 2. Although the supply pin is marked +10V, any supply voltage between about 8 V and 12 V can be used but this will affect the gain. The RF In pin requires a DC path to ground to bias the gate of the FET. If the signal is AC coupled into this pin, a resistor to ground is required. As the gate current of the FET is insignificant, this resistor can be relatively high, e.g. 100k - 1 MΩ. The FET Q1 could be used as a simple unbalanced mixer by feeding a local oscillator signal at 4 V p-p into the L.O. In pin. Alternatively, Q1 can be used as an amplifier by leaving the L.O. In pin disconnected. When using Q1 as an amplifier, grounding the L.O. In pin will increase the gain (why?) although this may give more gain than necessary. When measuring the gain of the IF amplifier, find out how much difference grounding this pin makes. The IF Out pin can be used to attach a "times 10" 'scope probe for gain measurements. Do not use a "times 1" probe and do not attach a wire to this pin as this is likely to cause instability (unwanted oscillation). When testing the IF amplifier, do not over-drive the input otherwise the gain measurement will not be valid. If the IF output level is more than about 1 V p-p, reduce the input signal level to avoid over-driving the amplifier. Find the centre frequency then measure the -6dB bandwidth and the -20dB bandwidth. The Audio Out pin is the demodulated AM output. It also contains a negative-going d.c. voltage that can be used to produce an AGC voltage, to reduce gain on strong signals. 2(b) Automatic Gain Control (AGC) You will find that strong signals cause clipping and severe distortion. It is possible to improve the handling of strong signals by means of Automatic Gain Control (AGC) as shown in Fig 2(b) below. Page 6 of 34
  • 7. Fig. 2(b) I.F. Amplifier with Automatic Gain Control (AGC) The carrier detected by D1 causes a negative voltage to be fed to the gate of Q1 via R7 and R6. This reduces the gain of Q1 on strong signals. R7 and C6 form a low-pass filter with a cut-off frequency of approx. 3 Hz to remove demodulated audio from the AGC line. 3. Balanced mixers The Philips/NXP SA602/612 is a Gilbert Cell active mixer. You can use an NE602, SA602, SA602A or SA612A as there ICs are all very similar. The absolute maximum supply voltage Vcc is +9 V but the recommended operating voltage is 5 - 6 volts. Instead of using two different d.c. power supplies, you are recommended to use a 5 volt regulator such as a 78L05 to regulate a 10 - 12 V supply down to 5 V. Fig. 3. How to couple signals in and out of the SA602/612 mixer via d.c. blocking capacitors. Work out how to use the SA602/612 in your receiver and draw a schematic diagram showing how to connect it. It is important to a.c. couple signals in and out of this mixer via d.c. blocking capacitors (e.g. 10 nF), as shown in Fig. 3. The recommended Local Oscillator (L.O.) drive level should be 200 - 300 mV p-p. Do not short-circuit either of the output pins (4 or 5) to ground as this will destroy the chip. 4. Design, construction and testing of a varicap-tuned HF RF amplifier This is a major part of the Day 1 mini project and at least half the time should be allowed for this task. Page 7 of 34
  • 8. The objective is to design, build and test an RF amplifier for an HF receiver that covers the "49 metre", "41 metre" and "31 metre" international broadcast bands. You will need to find out the frequencies of these bands and design your amplifier accordingly. You can use RFSim99 software to calculate resonant frequencies of L-C tuned circuits. You can use a 10 µH fixed inductor in the resonant circuit. You also have some Zetex ZC836B varicap diodes available for tuning. You could also consider using a PNP BJT so that one side of the inductor L1 is grounded, as shown in Fig 4 below. This will simplify the varicap tuning arrangement. Fig 4. A tuned RF amplifier using a PNP BJT Note that C2 is returned to ground rather than Vcc as this improves stability. Nevertheless, it may be necessary to omit C2 to reduce the gain. Varicap diode tuning Varicap tuning can be added to replace C1. The following type of varicap diode is available and the typical characteristics are shown below. Type C @1 V (pF) C @ 2 V (pF) C @12V (pF) C @ 20V (pF) ZC836B 150 100 ~60 <20 pF Various varicap diode configurations are shown below. The 'back-to-back' configuration is preferred because it offers better strong signal handling. It is better not to use tuning voltages below 2 V but if this is necessary to achieve the required tuning range, the 'back-to-back' configuration should be used. It is also better not to use tuning voltages above 12 V, to avoid the need for a separate tuning voltage supply. Page 8 of 34
  • 9. Select a suitable type of varicap diode and a suitable circuit configuration, then update your schematic diagram. The tuning voltage potentiometer should be a linear type, not logarithmic. Output buffering The tuned amplifier load RL needs to be relatively high (e.g. tens of kilohms) to avoid significant damping of L1/C1. It should also be noted that even with a 'times 10' 'scope probe, adds a capacitive load of about 13 pF, which will prevent the tuned amplifier from tuning up to its highest design frequency. In order to drive a lower impedance load, such as the 1.5 kΩ input impedance of the SA602 mixer, and to reduce the effects of load capacitance, a FET buffer amplifier could be used as shown below: Page 9 of 34
  • 10. 5. Suggested layout on Solderless Connection Board (SCB) Although Solderless Connection Boards (SCB) are not generally suitable for constructing radio frequency electronic circuits, it is possible to use this type of construction for this mini project provided a suitable layout is used, interconnecting wires are kept as short as possible and attention is paid to decoupling power supply rails. The photo below shows the recommended SCB layout. Page 10 of 34
  • 11. Section 4. Project Day 2 Brief Expected Outcomes for the day: To have designed, built and tested a superhet HF receiver with balanced mixer, varicap tuned RF amplifier stage and AGC, using an RF signal generator as the Local Oscillator (L.O.) source. To have evaluated the performance of the various stages. To have received an HF radio broadcast signal that originates outside the UK. To have received a VHF/FM signal and to have evaluated the shortcomings of the receiver for this type of signal. Assessment criteria: Schematic diagrams of the various radio receivers and the tuned RF amplifier stage. Results of laboratory measurements such as gain measurements, selectivity curves, bandwidth measurements. Analysis and discussions of results. Key Tasks: • Using the receiver from Day 1 (MF superhet with voltage regulator, balanced mixer, ferrite rod antenna and RF signal generator as the Local Oscillator (L.O.) source), evaluate the effect of impedance mismatch between the ferrite rod antenna and the mixer. • Add a FET RF buffer amplifier between the MF ferrite rod antenna and the mixer of the receiver and evaluate the benefits of using this buffer amplifier. • Design and build an HF superhet receiver with voltage regulator, balanced mixer and FET RF buffer amplifier, using an RF signal generator as the Local Oscillator (L.O.) source. Using the MF/HF antenna distribution system in laboratory, receive an HF radio broadcast signal that originates outside the UK and attempt to identify the source. • Design a varicap-tuned RF amplifier stage for an HF receiver, to meet a specification given in the Application Notes for this mini project. Construct the amplifier and measure its characteristics, e.g. gain, bandwidth and tuning range. • Connect the RF amplifier stage to the mixer of the HF superhet receiver via the FET RF buffer amplifier. Using a 50 cm length of wire as an antenna, receive an HF radio broadcast signal that originates outside the UK and attempt to identify the source. • Disconnect the HF RF amplifier and use the VHF/UHF antenna distribution system in laboratory to receive a VHF/FM radio broadcast signal in the 88 - 108 MHz band. Evaluate the shortcomings of the receiver for this type of signal. • Add Automatic Gain Control (AGC) to reduce the gain when strong signals are received. Page 11 of 34
  • 12. Section 5. Project Preparation Preparation Session (3hrs): Laboratory session, familiarisation with RF test equipment and measurements. Due to the limited number of spectrum analysers, some students will need to do part 2 before part 1. 1. Spectrum analyser/RF signal generator (A) Set the RF signal generator to give the following output: Frequency: 6 MHz, amplitude 22.3 mV RMS, modulation, AM 50% at 10 kHz (using internal modulation source) Set the spectrum analyser to display the above signal in such a way that the sidebands are visible and save a trace to your own USB flash drive or you personal files area on Studynet and include it in your E-log. (B) Set the RF signal generator to give the following output: Frequency: 100 MHz, amplitude 223 mV RMS, modulation, FM 75 kHz deviation at 10 kHz (using internal modulation source) Set the spectrum analyser to display the above signal in such a way that the sidebands are visible and save a trace to your own USB flash drive or you personal files area on Studynet and include it in your E-log. 2. Measurement of bandpass response of tuned circuit. Using a ferrite rod antenna with tuning capacitor, couple an unmodulated signal from the RF signal generator into the 2-turn coupling winding on the ferrite rod. Connect an oscilloscope across the black and pink wires with ground to black, using a "Times 10" probe. Set the variable capacitor to maximum capacitance and find the resonant frequency. Make measurements at a suitable number of frequencies and plot a frequency response curve down to the -20 dB points. Find the -3dB bandwidth and hence the 'Q' factor. Repeat with the variable capacitor set to minimum capacitance. 3. A 'crystal set' radio receiver. Connect a simple AM detector consisting only of a diode to the pink and black wires (black is ground) so that the demodulated output is fed to a crystal earphone. Connect approx. 50 cm of wire to the pink wire as an antenna. Tune the variable capacitor and find an AM broadcast signal that can be heard in the earphone. 4. Resonant circuit calculations Use the calculator and other tools in RFSim99 to find the following: • What capacitance resonates a 10 µH inductor at (i) 5 MHz (ii) 10 MHz • What inductance resonates with a 50 pF capacitor at 100 MHz? • If the above inductor is air-cored with a diameter of 6 mm, how many turns are required and how long should it be? 5. Tuned amplifier simulation (if time permits) Design a BJT tuned amplifier with a 10 µH inductor in the collector load, a centre frequency of 6 MHz and a load resistance RL of 20 kΩ. Simulate the circuit using TINA and estimate the 'Q' factor. Page 12 of 34
  • 13. Project Introduction 1. Cohort: DCE2 [B.Eng] 2. Project Title: Radio receiver design and evaluation 3. Project Objectives: (technical, specific to this project) Investigate the following functions in a radio receiver for a communication system • RF amplification • Frequency conversion • IF amplification and filtering • Demodulation 4. Project Summary: This project involves working from a specification for the performance of a radio receiver and making laboratory measurements on various building blocks. These will be connected to form a complete receiver whose overall performance will be assessed using both test equipment and 'off air' signals. 5. Introductory Lecture (2hrs) Content: 1 General Project area: radio receivers. 2 Architecture and building blocks of a superhet radio receiver 3 Building blocks of a superhet radio receiver 4 Principal performance parameters, frequency range, sensitivity, selectivity, etc. 5 Electronic design of selected parts of a radio receiver, e.g. tuned amplifier. 6 Outline of MF/HF/VHF radio wave propagation Appendix A Physical constants and RF fundamentals 6. Preparation Session (3hrs): Laboratory session, familiarisation with RF test equipment and measurements Page 13 of 34
  • 14. 1. Radio Receivers Every type of radio communication system requires a transmitter and a receiver. In this section, the principles of some typical radio receivers will be studied. A radio receiver is required to provide the following: • Gain. The signal received by the antenna may be 1 µV or less, so high gain and low noise are required but a receiver must also be able to operate with received signals that are much stronger than the minimum detectable signal. For types of modulation that require a linear receiver (e.g. AM, QAM, SSB), automatic gain control (AGC) is required to avoid clipping or limiting. For types of modulation that do not require a linear receiver, such as FM, FSK and PSK, no AGC is required and the amplifiers in the receiver can be allowed to clip or limit. • Selectivity A receiver requires a bandpass filter to allow it to select and amplify the wanted signal and reject other unwanted signals on adjacent frequencies, which may be much stronger than the wanted signal. Hence a bandpass filter with a sharp roll-off is normally required. In practice, such a filter can generally only be designed to operate at a fixed frequency and it is not generally practicable to make such a filter tuneable. • Tuning Although some radio receivers receive a fixed frequency (e.g. GPS satellite navigation or 60 kHz MSF time code), most receivers need to be able to tune to a range of frequencies, usually controlled by a frequency synthesiser and microcontroller. • Demodulation A receiver requires a demodulator for the type of modulation that it uses, e.g. AM, FM, FSK, PSK, QAM, OFDM, etc. Page 14 of 34
  • 15. 2. Architecture of a radio receiver There are three main architectures for radio receivers: Tuned Radio Frequency (TRF) In a TRF receiver, all the gain is at the frequency of the received signal. Selectivity is achieved by bandpass filtering at the frequency of the received signal. The TRF architecture is seldom used because of practical difficulties in achieving sufficient selectivity and of tuning the receiver if it is required to cover a range of frequencies. Another practical difficulty with a TRF is that all the gain is at the same frequency, which can result in problems of instability, i.e. stray feedback from output to input that can cause unwanted oscillation in an RF amplifier. • Superheterodyne This is the most widely used type of receiver. In a superhet receiver, the received signal is fed to a mixer. The other input of the mixer is driven by a local oscillator (L.O.). The mixer generates sum and difference frequencies. One of these (normally the difference frequency) is passed through an Intermediate Frequency (IF) bandpass filter and amplifier. The IF amplifier and filter operate at a fixed frequency and together they provide most of the gain and selectivity of the receiver. The receiver is tuned by varying the frequency of the L.O. It may also be necessary to vary the centre frequency of the RF amplifier stage. • Direct conversion (Zero I.F.) A direct conversion receiver has a local oscillator and mixer like a superhet but the L.O. operates at the same frequency as the received signal. The difference frequency from the mixer is the baseband signal but unwanted signals on other frequencies produce difference frequencies that are above baseband. Hence, selectivity is achieved by low pass filtering at baseband. Although there are a number of practical difficulties in designing a direct conversion receiver, this architecture is used in some designs of mobile communication receiver. An MF AM Broadcast Band Radio Receiver An AM broadcast receiver covering the MF broadcast band 0.535 - 1.605 MHz will be used as the first example of a superhet receiver, e.g. the AM section of a car radio. RF amplifier and RF filter. Provides some gain before the mixer. The RF filter provides image rejection but does not make a significant contribution to selectivity. Local Oscillator (LO). Generates a sinusoidal signal that is fed to the mixer. Varying the frequency of the LO tunes the receiver. The RF filter may be 'track tuned' with the LO tuning. Mixer generates sum and difference frequencies between the LO and the incoming RF signal. IF filter. This is a bandpass filter, generally with a sharp roll-off but a fixed centre frequency. The IF filter provides most of the selectivity in a superhet receiver, i.e. only the wanted signal passes through the IF filter. Page 15 of 34
  • 16. IF amplifier. This operates at a fixed frequency and provides most of the gain in a superhet receiver. Demodulator takes the output of the IF amplifier and recovers the original baseband signal (AM, FM, SSB, etc). AGC is Automatic Gain Control. The gain of the IF amplifier (and possibly also the RF amplifier) is reduced if the received signal is strong. This is to prevent the IF amplifier from 'clipping' or limiting on strong signals. N.B. AGC is not normally required on an FM receiver as an FM IF is designed to 'clip' in order to remove amplitude variations from the signal. Example frequencies for an MF receiver: RF carrier frequency fc = 909 kHz, Intermediate frequency fi = 455kHz. Local Oscillator frequency fosc = 1364 kHz What happens if an unwanted signal at 1819 kHz reaches the mixer? It also produces a difference frequency of 455 kHz. In this case, 1819 kHz is the image frequency (spaced from wanted frequency by twice the IF) Image rejection is achieved by the RF filter which should have substantial rejection at the image frequency. An FM Broadcast Band Radio Receiver The block diagram of an FM broadcast band receiver covering 88 - 108 MHz is similar to the block diagram of the AM receiver above except that: • Different RF and IF frequencies would be used, e.g. a typical IF is 10.7 MHz not 455 kHz, why? - A 455 kHz IF is too low for use at around 100 MHz and would give very poor image rejection. • The RF sensitivity would be higher, why? - Natural and man-made atmospheric noise levels are lower on the VHF band than on the MF band. • A different IF bandwidth would be required, how wide? - The approximate bandwidth required can be found by applying Carson's Rule to FM stereo broadcasting where the baseband signal bandwidth is 53 kHz and the peak FM deviation is +/-75 kHz. • AGC is not required in an FM receiver, why? - An FM IF amplifier is designed to 'clip' or 'limit' in order to remove unwanted amplitude variations and impulsive interference from the received signal. A Dual Conversion Superhet Receiver In a dual conversion receiver, there are two mixers, two local oscillators and two different intermediate frequencies. Typically, the frequency of one L.O. is tuneable and the other L.O. is fixed. A dual conversion receiver generally uses a first IF that is sufficiently high to achieve good image rejection and a second IF that is substantially lower. This allows an IF filter with good selectivity to be implemented more easily. A dual conversion superhet is not required for the mini project. Page 16 of 34
  • 17. 3. Building blocks of a radio receiver Small signal RF amplifier circuit configurations Small signal amplifiers are widely used in the front end and IF amplifier stages of radio receivers. There are a number of configurations that have various characteristics. Junction FET circuits are shown but similar configurations are possible using BJTs although the impedance matching and bias arrangements differ. The general arrangement is that the BJT or FET has some sort of tuned circuit as the collector or drain load. There may also be a tuned circuit at the input to match the impedance of the signal source to the input impedance of the base of a BJT or the gate of a FET. An RF amplifier with a tuned input and tuned output can achieve a relatively high gain from a single FET or BJT but careful design and layout are required to avoid unwanted oscillation, known as instability. Some specific design issues in tuned RF amplifiers will be considered in a later section. Bandpass filters Most RF and IF amplifiers used in radio receivers have a bandpass response. This may be achieved by means of L-C resonant circuits, crystal filters or ceramic filters. For example, most receivers use an IF filter that uses a ceramic filter, a crystal filter or a Surface Acoustic Wave (SAW) filter. Commonly used IFs are around 455 kHz for MF AM broadcast receivers, 10.7MHz for FM broadcast receivers, 39.5 MHz for TV receivers, 71 MHz or more for mobile phones. Mixers In radio systems, an RF 'mixer' is a non-linear circuit into which two signals are fed producing sum and difference frequencies (the wanted products) plus harmonics and other unwanted products at the output. The general principle is shown below; MIXER V1(t) OUTPUT V2(t) The wanted non-linearity in a mixer is a second order non-linearity. It can be analysed as a multiplier and it can be shown that multiplying two sinusoidal signals produces sum and difference frequencies. A mixer is an important building block in radio receivers. Mixers can also be used as modulators in radio transmitters to generate AM, DSBSC, PSK, QAM, etc. Mixer Configurations There are three main types of mixer configuration; unbalanced, single balanced and double balanced. All three types generate sum and difference frequencies but they differ in the other unwanted frequencies that are present at the output. An unbalanced mixer can be implemented as a single diode, BJT or FET operating in a non-linear way. Unwanted outputs from an unbalanced mixer include both of the input frequencies. In a single balanced mixer, only one of the input frequencies is present at the output. The other input frequency is prevented from reaching the output by the balanced characteristics of the mixer. In a double balanced mixer, neither of the input frequencies is present at the output. Both input frequencies are prevented from reaching the output by the double balanced characteristics of the mixer. In practice the balance of a single- or double-balanced mixer is not perfect so input signals that should not appear at the output do actually appear but at a low level. All mixers generate other unwanted outputs including harmonics of the input frequencies and higher order products such as third order, fourth order and above. Unbalanced, single- balanced and double-balanced mixers differ in their generation of harmonics and unwanted outputs. Mixer Technology Page 17 of 34
  • 18. Mixers can be based on various technologies including diodes, bipolar transistors (BJTs) , junction field effect transistors (JFETs) and dual-gate MOSFETS (metal oxide semiconductor FET’s) or CMOS devices, in fact, any device that can be operated in a non-linear mode. Mixers based on active devices may have a conversion gain whereas mixers based on passive devices (diodes) have a conversion loss. Example Mixer Circuits The diode ring mixer shown below is a simple example of a balanced mixer based on Schottky diodes. The Mini Circuits SBL-1 is a typical example of this type of mixer. It requires +7 dBm (5 mW) of local oscillator drive power. D1 TR1 TR2 V1(1MHz) D4 V2(10MHz) D3 D2 OUTPUT In this example, the inputs are single sine waves at 1MHz and 10 MHz respectively. The resulting output spectrum is shown below. The large terms at 9 MHz and 11 MHz are the wanted sum and difference frequencies which are second order mixer products. There are also a number of other unwanted mixer products such as 3rd order, 4th order and higher. There is also some leakage of the input frequencies V1 and V2 directly through to the output. The inputs and outputs of mixers must be carefully matched in order to optimise the performance. Conversion loss, noise figure and spurious suppression can be degraded if mixer ports are incorrectly matched. Mixers are also sensitive to overloads at either input which can lead to gain compression and hence distortion or de-sensing. A Schottky diode ring mixer generally offers high performance for professional communications applications but it cannot be implemented in an integrated circuit, due to the transformers. A Schottky diode ring mixer is seldom used in portable radio equipment because of higher cost, larger size and higher local oscillator power requirement, compared to integrated circuit mixers. A Gilbert Cell double balanced mixer using is an integrated circuit mixer configuration that can be found in the Philips SA602/612 integrated circuits or it may be integrated into RF ICs for mobile and portable radio equipment up to 2.5 GHz or more. A Gilbert Cell mixer may be implemented using BJT or MOS devices. It offers conversion gain although other parameters such as dynamic range are generally inferior to a high level diode ring mixer. The schematic diagram from the data sheet of the Philips SA612 is shown below. This shows the equivalent circuit of the mixer, which is implemented using bipolar transistors. Note that the amplifier stage between pins 6/7 and the buffer is intended for use as an on-chip oscillator and is not part of the Gilbert Cell mixer. If the on-chip oscillator is not used then pin 6 is used as the local oscillator input. N.B. the optimum L.O. signal level is approx. 120 mV p-p. It should not be driven with 5 V logic signals. The inputs at pins 1 and 2 are the signal inputs. These can be driven in a differential or single-ended mode. For single-ended input, the signal can be a.c. coupled via a capacitor into pin 1 with pin 2 RF grounded via a capacitor (e.g. 10 nF). The outputs at pins 4 and 5 are the mixer output. These can drive a differential or single-ended load. N.B. Pins 4 and 5 of the NE602 must not be short-circuited to ground as this will destroy the mixer. Page 18 of 34
  • 19. The Philips SA602/612 integrated circuit Gilbert Cell double balanced mixer. Demodulators There are various types of demodulators for different types of modulation such as AM. FM, QAM, PSK. For the purposes of the mini-project, only AM and FM demodulators will be considered. AM Demodulator The following circuit is a simple AM 'envelope' demodulator using discrete components. Other circuit configurations may be in integrated circuit AM demodulators. D1 should have a relatively low forward voltage drop. A Silicon junction diode is not generally suitable but a small- signal Germanium diode or Schottky diode are more suitable due to the lower forward voltage drop. FM IF/Demodulator ICs Discrete component FM demodulators include the Radio Detector and the Foster-Seeley Discriminator but these have largely been superseded by integrated circuits such as the RCA CA3089/3189 and its more recent derivatives. Such FM IF ICs normally include the following functions: o A high gain limiting IF amplifier (typical sensitivity 12 µV at 10.7 MHz) o An FM quadrature detector using an external inductor and tuned circuit. o A Received Signal Strength Indicator (RSSI) output. A block diagram of an FM IF amplifier IC with a quadrature detector is shown below: Page 19 of 34
  • 20. The FM quadrature demodulator uses the frequency/phase characteristics of and LCR resonant circuit together with a 90° phase shift network and a multiplier that is used as a phase-sensitive detector. Page 20 of 34
  • 21. 4. Principal performance parameters of a radio receiver Frequency range A radio receiver may be designed to tune to a fixed frequency, e.g. GPS or a range of frequencies such as: o MF AM broadcast band, 535 kHz - 1605 kHz o General coverage MF/HF communications receiver 100 kHz - 30 MHz o VHF FM broadcast band, 88 - 108 MHz o UHF TV broadcast band (analogue or digital) 470 - 860 MHz o Scanning receiver 0.5 MHz - 1300 MHz o GSM 900, GSM1800 or 3G mobile phone bands o Wireless LAN band 2400 - 2483.5 MHz In order to tune over a specified range of frequencies, the local oscillator (L.O.) must be tunable. Depending on the application, the tuning of the RF amplifier stage may be fixed or variable. If the RF tuning is variable, it generally needs to be 'track tuned' with the L.O. Sensitivity For most radio receiver applications, high sensitivity is required to receive weak signals although the receiver should also be able to receive strong signals without non-linear effects occurring. The sensitivity of a radio receiver can be specified in terms of the input signal level for a specified signal to noise ratio (SNR) at the demodulated output. An example of the sensitivity of an AM receiver could be 2 µV for a test signal with 50% modulation and 10 dB SNR at the demodulated output. In the case of an FM receiver, SNR at the demodulated output of an FM receiver is generally expressed in terms of SINAD, Signal to Noise and Distortion. SINAD measurements can be made using an RF signal generator and by feeding the audio output of the receiver under test into a PC sound card and using suitable software such as Spectrum Lab. Page 21 of 34
  • 22. Selectivity The following diagrams illustrate how the selectivity of a radio receiver allows it to receive and amplify a wanted signal while rejecting unwanted signals on adjacent frequencies. Principal performance Page 22 of 34
  • 23. 5. Electronic design of selected parts of a radio receiver Resonant circuits and bandpass filters Quality Factor (Qo) and Selectivity Selectivity is the ability of the filter to select in-band frequencies and reject those out of band. An indication of filter selectivity is obtained in the form of its Q-factor (Qo). Higher Q circuits are more selective. Two Pole LCR Series Resonant BPF Resonant (tuned) circuits are widely used to obtain relatively narrow passbands for example in radio frequency applications where we may wish to tune to a specific transmission and reject adjacent channels. C L Vi Vo(f) f R Vo Vi Vo = θ Vo = − Tan−1 (ωL R ) − (1 ωCR ) 2 1 + (ωL R ) − (1 ωCR ) 1 ω oL 1 1 L fo fo = Qo = = = BW = 2 π LC R ω oCR R C Qo At some frequency we can note that XL = XC hence VL = VC, since VL and VC are of opposite signs the phasor sum will cancel, leaving all of the voltage across the resistor. This frequency is known as the resonant frequency (fo) at which the circuit is purely resistive giving zero phase shift. At frequencies below fo, the capacitor has more influence since XC > XL, the circuit is predominantly capacitive and so we have positive phase shift. At frequencies above fo, the inductor has more influence since XL > XC, the circuit is predominantly inductive and so we have negative phase shift. Page 23 of 34
  • 24. Two Pole LCR Parallel Resonant BPF (Tank Circuit) R Vo Vi Vo(f) L C f Vi Vo = θ Vo = −Tan −1 [ (ωCR ) − ( R ωL)] 1 + [ (ωCR ) − ( R ωL)] 2 fo is the same as for the series circuit Multiple pole filters In general, the roll off of a filter can be increased if more poles are present in the response. An ideal 'brick wall' filter has a perfectly flat passband and infinitely steep roll-off but this requires an infinite number of poles and cannot be implemented in practice. Numerous texts cover detailed filter design and many CAD packages offer simple facilities with which to design higher order filters using Butterworth, Chebychev or Bessel polynomials. Multi-pole bandpass filters are an important building block in radio receivers, for example for use in the IF or RF amplifier. Such filters are generally fixed frequency filters because it is not generally practicable to vary the centre frequency of such a filter. The example below illustrates what can be achieved easily using appropriate CAD; E.g. Butterworth BPF, fc = 10.7 MHz, BW = 25kHz s 21 Arg s21 Page 24 of 34
  • 25. It should be noted that any filter needs to be designed to operate with a certain source and load impedance, which normally needs to be resistive. This may be 50 Ω in some cases but other impedances such as 330 Ω or 2 kΩ may be used in crystal and ceramic bandpass filters for I.F. amplifiers. For an IF Filter, 4, 6 or 8 poles may be required in order to achieve a sufficiently sharp roll-off outside the passband. It may not be practicable to implement such a filter design using inductors and capacitors because the values of L or C or the required 'Q' factors may not be achievable in practice. For example in the above filter design, the values of the capacitors are all less than 1 pF and the stray capacitance of the inductors would need to be extremely low. Instead, such filter designs may require crystal or ceramic resonator elements. Such resonators use piezo-electric materials such as quartz or Barium Titanate. For frequencies up to about 70 MHz, the mode of operation is a bulk acoustic wave. For higher frequencies such as 100 - 2000 MHz, the mode of operation is a Surface Acoustic Wave (SAW). SAW filters are used for IF and RF bandpass filters in mobile communications equipment and other applications. Varactor (Varicap) Diodes Varicap diodes are voltage controlled capacitors that can be used in conjunction with a tuned circuit to vary the effective capacitance and hence the resonant frequency. Applications include voltage controlled oscillators and tuned RF amplifiers. A typical circuit configuration is shown below: U2 R1 +Ve TUNING VOLTAGE U1 L1 Note the diodes are reverse biased, this enables the tuning voltage to change the width of the depletion region hence its capacitance. R1 is a high value resistor, e.g. 100 kΩ to avoid damping the resonant circuit. Two 'back-to-back' varicap diodes are shown here, this gives reduced distortion on large signals when compared to a single device. Many types of varicap are available. Design of a simple tuned amplifier The following amplifier circuit uses an NPN BJT with a tuned collector load. In practice, there would normally be some sort of impedance matching at the input to match the impedance of the source to the input impedance of a BJT or FET amplifier stage. It should be noted that a simple constant-current bias arrangement via R1 is shown for simplicity but this results in a collector current that is proportional to hfe for the particular BJT. As there are large variations in hfe between BJTs in a particular batch, potential divider bias is preferable in practice. The maximum gain occurs at the resonant frequency of L1-C1. In practice, either L1 or C1 may be variable in order to vary the centre frequency of the tuned amplifier. The load resistance RL could be the input resistance of the next amplifier stage or it could be some other type of load such as a mixer, filter, detector or even an antenna (in a transmitter). RL needs to be sufficiently high to avoid excessive damping of the resonant circuit L1-C1 and reduction in 'Q' factor. C2 and C3 are DC blocking capacitors whose value is chosen to provide a low reactance at the frequency of operation. Typically, 1 - 10 nF would be suitable for many RF applications. C4 is a power supply decoupling capacitor. Its purpose is to ensure that the supply rail is a.c. grounded at radio frequencies. Power supply decoupling is important in all RF circuits. Page 25 of 34
  • 26. In practice, a tuned amplifier is often required to drive a relatively low impedance load. Matching can be achieved by means of an RF transformer. In the circuit shown below, the primary of TR1 is tuned by C1. If the turns ratio of TR1 is N1/N2, the effective load resistance seen at the primary of TR1 is RL(N1/N2)2. An alternative means of impedance matching is shown below. L1 has a 'tapped' winding. If the total number of turns is N1 and the number of turns from the Vcc end to the tap is N2 then the circuit is equivalent to the transformer matching arrangement shown above, except that a DC blocking capacitor C2 is required if one side of the load RL is grounded. Another means of impedance matching is shown below. This is sometimes called a 'capacitive tap' This has the advantage that L1 can be a standard inductor with a single winding and no 'tap' is required. The effective value of RL seen at the collector is T1 is RL(C2/C')2 where C' represents C1 and C2 in series, i.e.: 1/C' = 1/C1 + 1/C2 Page 26 of 34
  • 27. 6. Radio Wave Propagation How radio waves travel. Although radio waves are sometimes colloquially referred to as "airwaves", they do not require air or any other medium in which to travel and can travel through a vacuum. Like light waves, radio waves travel in straight lines unless something reflects or refracts them. Like light waves, radio waves may be obstructed by obstacles, which can cast a radio "shadow". Although radio waves can pass through many non-conductive material without much loss, they do suffer some loss when passing through walls, floors and roofs. This loss depends upon the building materials used and increases at higher frequencies. Metal, water, ground and other electrically conductive materials cause large losses to radio waves passing through them and in some cases no usable signal may pass through. There are four main modes of radio wave propagation, • Free space wave • Tropospheric wave • Sky wave or ionospheric wave • Ground wave A free space wave travels through space with no reflection or refraction. On an unobstructed 'free-space' path, the path loss as a function of distance is determined by the inverse square law. An example of free space wave propagation is communication with satellites. In tropospheric wave propagation, the troposphere refracts radio waves slightly so that they can travel slightly further than the visible horizon. Examples of tropospheric wave propagation are terrestrial VHF and UHF radio and TV broadcasting and microwave point-to-point links used in telecommunication networks and links to mobile phone base stations. The distance that tropospheric waves can travel can be predicted by the 'Four Thirds Earth' method to calculate the distance of the radio horizon for transmitting ad receiving antenna of a given height. The troposphere does not introduce a significant additional loss to radio waves below about 1 GHz, i.e. the path loss in the troposphere is almost equal to the free-space path loss, which is determined by the inverse square law. At SHF and above, the troposphere introduces extra 'atmospheric loss' in addition to the inverse square law path loss. This 'atmospheric loss' varies with frequency and is caused by heavy rain, fog and absorption by gases in the atmosphere. In Sky wave or ionospheric wave propagation, signals are reflected by electrically charged layers in the earth's ionosphere and can travel thousands of kilometres, possibly by more than one reflection. This mode only supports frequencies in the MF and HF bands and occasionally low VHF. The Maximum Usable Frequency (MUF) is the highest frequency that can be propagated by a sky wave. Frequencies above the MUF pass through the ionosphere and into space. Ground wave propagation primarily supports frequencies in the LF and MF bands, such as long wave and medium wave broadcast bands. Ground wave propagation allows radio signals to travel well beyond a 'line of sight' path by propagating through a waveguide formed by the ground and the ionosphere. Only vertically polarised radio signals can propagate by ground wave. The maximum range is affected by ground conductivity. Page 27 of 34
  • 28. The Earth's atmosphere and its effects on radio waves For the purposes of radio communications, the atmosphere can be divided into two main regions, the troposphere and the ionosphere. The troposphere extends up to approximately 10 km above sea level and contains breathable air, clouds etc. The ionosphere contains various electrically charged (ionised) layers between 70 and 500 km above sea level. These are known as the D, E, F1 and F2 layers. These layers can reflect certain radio frequencies back to earth hundreds or thousands of kilometres away. The electrically charged layers are a result of solar activity and their properties vary greatly depending on time of day, season and sunspot activity. An outline of radio wavebands and propagation. Figure 6.2 shows an outline of the HF, VHF and UHF bands together with their principal propagation characteristics and selected uses. Abbreviations such as '49m' mean the 49 metre band (5.9 MHz). It should be noted that each band has ten times the bandwidth of the band below. For example the HF band has a bandwidth of 27 MHz while the VHF band has a bandwidth of 270 MHz and UHF has 2700 MHz bandwidth. Fig 6.2. Outline of the HF, VHF and UHF bands. Page 28 of 34
  • 29. The LF and MF bands. Radio transmissions in the Low Frequency band, 30 - 300 kHz, include the "Long Wave" broadcast band at 148.5 kHz - 283.5 kHz and UK time code transmissions on 60 kHz from MSF Rugby. The predominant mode of radio wave propagation in the LF bands and below is by "ground wave". This mode allows signals to travel far beyond the visible horizon but very large transmitting antennas are required and the available bandwidth is limited. 300 kHz - 3 MHz is designated Medium Frequency. It includes the "Medium wave" broadcast band 525 - 1605 kHz and the 160 metre amateur band, 1.81 to 2 MHz. Propagation during daylight hours is by ground wave. During hours of darkness, signals travel longer distances by "sky wave" propagation. Night-time "Sky wave" propagation is caused by reflection of signals from the E layer in the ionosphere which does not occur during daylight because of absorption in the D layer which is below the E layer. The HF band The High Frequency band extends from 3 to 30 MHz. Ionospheric propagation frequently permits long distance communication, sometimes world wide. HF propagation conditions are very variable and difficult to predict. They are affected by factors such as frequency, time of day and 11-year sunspot cycles. Many services use the HF band including commercial and military communications, 9 broadcast bands, 8 amateur bands and Citizens Band radio. A new service Digital Radio Mondiale (DRM) provides high quality digital international broadcasting on HF. Many parts of the HF band are extremely crowded because signals transmitted at one location may be received hundreds or thousands of kilometres away. In general the lower part of the HF band, for example 3 - 5 MHz, provides reliable communication around the UK during the day and further at night. Frequencies such as 7 - 10 MHz generally provide communication around Europe day and night but "skip zones" or "dead zones" exist between the point where the "ground wave" ends to the point where the first "sky waves" can be received. "Skip zones" make the higher part of the HF band unsuitable for local communications. HF frequencies from about 14 MHz upwards travel further during the day. Long distance HF communication is only possible up to the Maximum Usable Frequency (MUF) of the F2 layer. This regularly reaches around 14 MHz during the day at any time in the sunspot cycle, permitting long distance communication (e.g.:UK to USA, Asia, South Africa). The MUF frequently reaches 21 MHz during the day, except during sunspot minimum years. The MUF frequently exceeds 28 MHz during sunspot maximum years, permitting very long distance multi-hop F2 propagation (e.g.: South America, Australasia) during the day. HF communications can occasionally be disrupted by solar flares which cause a "fade out" lasting from a few hours to a day or two. Nevertheless, if such solar flares are very intense, these can also cause permanent failure of communication satellites. Although HF frequencies particularly around 5 MHz can provide reliable communications from one part of the UK to another, their use within the UK is largely confined to the army and to military aviation. Internationally, HF can be used for locations out of range of VHF communications, e.g. civil aircraft flying above the mid-Atlantic or an aircraft which has made an emergency landing. HF is also useful for locations shielded from satellite communications, e.g. in dense forest. The VHF band The frequency range 30 - 300 MHz is designated Very High Frequency. It contains a bandwidth of 270 MHz which is ten times as wide as the HF band. Under normal conditions, terrestrial VHF signals propagate by tropospheric propagation, with distances little further than "line of sight". VHF is seldom affected by sky wave propagation and normal propagation conditions do not vary between day and night. This contrasts with HF where a signal on a particular frequency may be received within a radius of hundreds or thousands of kilometres. Although the restricted range of VHF signals is a disadvantage if long distance communication is required, it is an advantage if only local communication is required because the same VHF frequency can be re-used by many different users simultaneously in different areas, provided they are not too close together. The lower end of VHF, 30 - 50 MHz and to some extent 50 - 70 MHz, can sometimes support long distance ionospheric propagation modes. During sunspot maximum years, the F2 MUF may occasionally reach 50 MHz. Another exceptional ionospheric mode is Sporadic "E" which mainly occurs from May - August at any time in the sunspot cycle. Sporadic 'E' often affects frequencies up to 70 MHz at on rare occasions up to 100 MHz or even 150 MHz. As these ionospheric effects are relatively uncommon at VHF, they are of little use for reliable long distance communication but when they do occur, strong signals from other countries can cause interference to short range VHF radio communication services, such as PMR/PBR. Page 29 of 34
  • 30. The VHF/FM radio broadcast band is 88 - 108 MHz. Other VHF users include aeronautical 108 - 136 MHz, amateur radio at 50, 70 and 144 MHz and UK Digital Audio Broadcasting (DAB) 217.5 - 230 MHz. It is very rare for the higher part of VHF, above 100 MHz to be affected by ionospheric propagation but it can be affected by enhanced tropospheric propagation associated with settled areas of high pressure. This occasionally causes signals at VHF, UHF, SHF and EHF to travel considerably further than normal. Such conditions only exist for less than 1% of the year but may persist for hours or days at a time causing co-channel interference from distant transmitters on the same frequency that would not normally be received. The higher part of VHF has the following characteristics: * Propagation is relatively stable. * Natural noise levels are low compared to MF/HF * Mobile transmitting antennas are a manageable size * "Shadowing" effects by buildings and terrain are less severe at VHF than for UHF and SHF. Consequently, the available bandwidth is in great demand. The UHF band The 300 MHz to 3 GHz band is designated Ultra High Frequency, UHF. Although 2700 MHz of bandwidth is available, the lower part of the UHF band, below 1000 MHz is particularly useful. A large portion of this, 470 - 860 MHz is occupied by television broadcasting while most of the spectrum from 220 - 380 MHz is allocated to military aviation. Services using the UHF band include TETRA at around 380 MHz and 450 MHz, GSM cellular telephones around 900 MHz and 1800 MHz, 3G mobile phone networks around 2000 MHz and wireless LANs and Bluetooth at 2400 MHz. Antennas for these services can be relatively compact for portable use. Propagation is similar to the higher part of VHF, that is little more than "line of sight" apart from occasional enhanced tropospheric propagation associated with settled high pressure. No ionospheric propagation modes occur at UHF. The UHF band has the following characteristics and is in great demand. • Propagation is relatively stable. • Natural and man-made noise levels are lower than at VHF • The short wavelength permits the use of compact antennas on portable equipment • "Shadowing" effects by buildings and terrain and loss through walls are more severe at UHF than at VHF and these effects are more significant in the higher part of UHF. These are significant issues for mobile phone networks and wireless LANs. The SHF, EHF bands and above. The 3 - 30 GHz band is designated SHF, Super High Frequency. The SHF band is suitable for radar as its wavelength is sufficiently short to be reflected by relatively small objects and it allows a radar antenna to produce a sharply focused "beam". The SHF band is also used for links such as terrestrial microwave links for the public telephone network and links for mobile phone networks. It is also used for uplinks and down links for satellite communication. Highly directional dish antennas are normally required so these bands are unsuitable for two-way communication with mobile or portable users. The 11.7 to 12.5 GHz band is allocated to DBS, Direct Broadcasting by Satellite. Losses in coaxial cable are relatively high at SHF, even for lengths of a few metres and hollow waveguide is often used as an interconnection for microwave signals. The SHF and EHF bands have a very large amount of bandwidth available but their use is limited by such factors as high atmospheric absorption during heavy rain. Some frequencies also suffer high atmospheric absorption all the time due to water vapour absorption at around 24 GHz or oxygen absorption around 60 GHz. Transmitting and receiving equipment is relatively complex and expensive and is mainly used for fixed links rather than mobile or portable operation. 30 - 300 GHz is designated Extra High Frequency. Radio waves in the EHF band, with wavelengths of 10 mm to 1 mm are sometimes called 'millimetric waves'. Above EHF, wavelengths between 1 mm and 10 microns are on the borderline between microwave radio and infra-red light. The range from 10 microns to 0.4 microns covers infra-red and visible light. Beyond ultra-violet light are X rays and gamma rays, which are classed as ionising radiation, whereas radio waves are non-ionising. Page 30 of 34
  • 31. Appendix A A.1 Some physical constants and RF fundamentals (It is not necessary to remember the values of these physical constants as they would be given if required, e.g. in an examination) Free space velocity of light c = 2.998 x 108 m/s Relationship between frequency and free-space wavelength c = fλ where: c = free space velocity of light f = frequency in hertz λ = wavelength in metres Relationship between frequency f and angular frequency ω Angular frequency ω = 2πf radians/sec where f = frequency in hertz Power ratios expressed in decibels If the input power to a device such as an amplifier is Pin watts and the output power is Pout watts, the power gain in linear units is Pout/Pin and power gain in Decibels = 10 Log(Pout/Pin) If a power gain in dB is negative, this indicates a loss. A gain of -10 dB can also be expressed as a loss of +10 dB To convert a power ratio PdB from decibels to linear units, calculate Antilog(PdB/10) (note that the Antilog key may be marked 10x on some calculators) Absolute power units expressed in dBm or dBw dBm means dB relative to a reference power level of 1 mW To convert a power P to dBm, PdBm = 10 Log(P/PRef) Hence 0 dBm = 1 mW, -30 dBm = 1 µW +30 dBm = 1 W dBW means dB relative to a reference power level of 1 W Hence 0 dBW = 1 W -30 dBW = 1 mW +30 dBW = 1 kW Note that dBm is an absolute power unit so 0 dBm is the same power regardless of the resistance of the measuring equipment (e.g. 50 Ω, 75Ω or 600 Ω) Power can be expressed in dBm (or dBW) not only for electrical power but also for other forms of energy, for example optical power in optical fibre communications. Note that power gains or losses are power ratios, not absolute power units hence gains or losses may be expressed in dB but not in dBm. Page 31 of 34
  • 32. Voltage ratios expressed in decibels If the input voltage to a device such as an amplifier is Vin volts and the output voltage is Vout volts, the voltage gain Av in linear units is Vout/Vin and the voltage gain in decibels Av(dB) = 20 Log (Vout/Vin) N.B. this is derived from the definition of power ratio in dB and the two definitions are equivalent provided Vout and Vin are both measured across the same resistance. To convert a voltage ratio Av(dB) from decibels to linear units, calculate Antilog(Av(dB)/20) (note that the Antilog key may be marked 10x on some calculators) If a voltage in dB is negative, this indicates a loss. Hence a gain of -10 dB can also be expressed as a loss of +10 dB Absolute voltage units expressed in dB(µ V) dB(µV) means dB relative to a reference level of 1 µV Hence 0 dB(µV) = 1 µV, 60 dB(µV) = 1 mV and 120 dB(µV) = 1 V Note that as dB(µV) is an absolute voltage unit, 0 dB(µV) is the same voltage regardless of the resistance of the measuring equipment but in order to relate dB(µV) to dBm, it is necessary to specify the resistance of the measuring equipment. In the case of 50 Ω systems, 0 dBm = +107 dB(µV) = 223 mV RMS A2 Radio waves and the electromagnetic spectrum. A radio wave is an electromagnetic wave, that is, it has an electric and a magnetic component. Light waves are another type of electromagnetic wave but of a very much shorter wavelength than radio waves. Figure A2 shows radio waves in relation to other types of electromagnetic waves and the IEEE band designations of radio waves. Page 32 of 34
  • 33. Wavelength Frequency IEEE band Name λ f (Hz) Classification 0.1 µm 3 x 1015 Hz Visible light (0.4 - 0.8 µm) 1 µm 3 x 1014 Hz Infra-red light 10 µm 3 x 1013 Hz 100 µm 3 x 1012 Hz 1 mm 300 GHz EHF Extra High Frequency 1 cm 30 GHz SHF Super High Frequency 10 cm 3 GHz UHF Ultra High Frequency 1m 300 MHz VHF Very High Frequency 10 m 30 MHz HF High Frequency 100 m 3 MHz MF Medium Frequency 1 km 300 kHz LF Low Frequency 10 km 30 kHz VLF Very Low Frequency 100 km 3 kHz VF Voice Frequency 1000 km 300 Hz Figure A2 Radio waves in relation to the whole electromagnetic spectrum. Page 33 of 34
  • 34. Credits This resource was created by the University of Hertfordshire and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. The name of the University of Hertfordshire, UH and the UH logo are the name and registered marks of the University of Hertfordshire. To the fullest extent permitted by law the University of Hertfordshire reserves all its rights in its name and marks which may not be used except with its written permission. The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher. © University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License. Page 34 of 34