Mini Project- Radio Receiver Design

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.

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
• Demodulation

Project Summary: (50 words max)

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

Day 2
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.

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

Section 2. Project Day 1 Brief
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.

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

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

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

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

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

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

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

Section 4. Project Day 2 Brief
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.
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

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

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
• Demodulation

4. Project Summary:


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Mini Project- Radio Receiver Design

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Mini Project- Radio Receiver Design

Similar to Mini Project- Radio Receiver Design (20)

More from University of Hertfordshire, School of Electronic Communications and Electrical Engineering

More from University of Hertfordshire, School of Electronic Communications and Electrical Engineering (20)

Recently uploaded

Recently uploaded (20)

Mini Project- Radio Receiver Design