A demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. The term is usually used in connection with radio receivers, but there are many kinds of demodulators used in many other systems. Another common one is in a modem , which is a contraction of the terms modulator/demodulator.
For AM, the most popular demodulator used are the Envelop Detector and Product Detector .
Figure 3.1 Receiver Block Diagram RF Section IF Section Demodulator AF Stage
Demodulation of DSBFC AM
Simplest demodulator for DSBFC is envelop detector .
The recovery of the baseband signal undergoes the process of rectifying the incoming signal, remove half of the envelop, then use low pass filter to remove the high frequency component of the signal.
Major advantage of AM = ease of the demod process.
No need for synchronous demodulator.
Figure 3.2 Envelope detection of a conventional AM signal
Demodulation of SSBSC AM
For SSBSC, product detector is used to recover the signal.
The product detector multiplies the incoming signal by the signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal.
After filtering, the original audio signal will result.
This method will decode both AM and SSB, although if the phase cannot be determined a more complex setup is required.
Figure 3.3 Product Detector for AM and SSB
D 1 rectifies the signal producing only positive result.
The rectified signal will quickly charge C 1.
RC time constant of R 1 and C 1 is made long enough so that C 1 does not have to discharge before the next pulse is received.
Voltage across C 1 follows the amplitude variation of carrier signal, not the carrier signal itself.
C 1 is the coupling capacitor block DC from the input source.
R 1 and R 2 establish base bias and R 3 establish collector bias.
Transistor is biased-for-class B operation that allows positive pulses on the output.
C 2 filter out carrier frequency.
C 3 removes DC component.
Process in a Receiver
Signal received at antenna is very low, need to amplify (LNA) and tuned to desired freq. to avoid interference.
Detector finds the info signal from the rf signal.
Further amplification needed to give it enough power to drive a loudspeaker.
Fig. 3.6 Simple block diagram of a receiver
Parameters used to evaluate the ability of a receiver to successfully demodulate radio signal :-
Bandwidth Improvement Factor
Ability of a receiver to accept a given band of frequency and reject all others.
Obtained using tuned circuits.
Selectivity Q, is given by:
The bandwidth curve from the tuned circuit is:
Higher Q the narrower the BW and the better the selectivity.
i.e. using the bandwidth of the receiver at the – 3dB points not necessarily show rejection characteristic
Most common used two points; another at -60dB ratio of the two called shape factor:
High-Q tuned cct are used to keep the BW narrow to ensure that only desired signal is passed. Assumed that 10 H coil with resistance of 20 is connected in parallel with 101.4pF variable capacitor. The circuit resonates at what freq.?
What is the inductive reactance?
What is the selectivity of the cct?
The bandwidth of the tuned cct?
Find the upper and lower cutoff frequencies?
Answer Eg. 3.1
5. One half on each side of center freq. of 5MHz is 318.47/2 = 0.159 MHz.
The minimum RF signal that can be detected at the input of a receiver and still produce a usable demodulated info signal.
Also called receiver threshold.
Depends on the noise power present at the input of the receiver, the receiver’s noise figure, sensitivity of the AM detector and the bandwidth improvement factor of the receiver.
The best way to improve sensitivity is by reducing the noise level reduce temperature, reduce bandwidth of the receiver, or improving receiving noise figure.
One way of reducing the noise level is by reducing the bandwidth of the signal
There is limitation for reducing the bandwidth to make sure information is not lost
As RF bandwidth at the input of the receiver is higher than the IF bandwidth at the output of the receiver, reducing the RF bandwidth to IF bandwidth ratio effectively reducing the noise figure of the receiver, thus reducing the noise
Bandwidth improvement expressed mathematically as
Noise figure improvement expressed as
NF improvement = 10 log BI
Bandwidth Improvement Factor
The minimum input level necessary to discern a signal and the input that will overdrive the receiver and produce distortion.
Minimum receive level is a function of front-end noise, noise figure and the desired signal quality.
Input that produce distortion is a function of the net gain of the receiver.
1 dB compression point is used for the upper limit for usefulness.
FIGURE 3.7 Linear gain, 1-dB compression point, and third-order intercept distortion for a typical amplifier
A measure of the ability of the receiver to produce, at the output of the receiver, an exact replica of the original source information.
Any amplitude, frequency or phase variations present in the demodulated waveform that are not included in the original signal are consider as distortion.
Loss occur when a signal enter the input of the receiver.
Parameters associated with the frequencies that fall within the passband of a filter.
Defined as the ratio of the power transferred to the load with a filter in the circuit to the power transferred to the load without a filter.
Tuned Radio Frequency Receiver
Tuned RF Receiver (TRF)
It is the earliest and simplest receiver design (Fig. 3.8).
TRF consist of RF amplifiers stages, detector and audio amplifier stages (Fig. 3.9)
The received signal is tuned by LC circuit to a passband centered at carrier frequency.
Selectivity pass only the desired signal, others are rejected.
The tuned signal is boost up by an amplifier for better info detection.
Signal info detection is made at the demodulator and further amplified for the speaker output.
FIGURE 3.9 Noncoherent tuned radio frequency receiver block diagram
TRF has high sensitivity – ability to drive the speaker to an acceptable level (to amplify).
BW is inconsistent and varies with center frequency when tuned over a wide range of input frequencies selectivity changes, (means the extent to which a rx can differentiate between the desired signal and other signal).
Instability due to the large number of RF amplifier all tuned to the same center frequency oscillation.
Gain is not uniform over a wide range of frequency.
Superhets was designed to overcome the problems in TRF.
Complex circuitry compared to TRF but excellent performance under many conditions (Fig. 3.10).
to mix 2 frequencies together in a nonlinear device or
to translate one frequency to another using nonlinear device.
Rx tunes to desired signal and converts the signal to intermediate frequency via a signal mixing circuit.
Then IF signal is optimized to fully recovered the modulated info signal.
FIGURE 3.10 AM superheterodyne receiver block diagram
Stages in Superhets
Which takes the signal from the antenna and amplifies it to a level large enough to be used in the following stages.
Mixer and Local Oscillator:
Converts the RF signal to IF signal.
Further amplifies the signal and has bandwidth and passband shaping appropriate for the received signal.
Recovers (demodulates) the info signal from the carrier.
The received signal is amplified for loudspeaker or interconnections to comm systems.
RF Stage and Characteristics
RF Stage and Characteristics
The RF section is a tunable circuit connected to the antenna.
It is where the wanted signal is selected and the unwanted signal is rejected.
Some basic receiver does not have amplifier but for rx that has one is much more superior in performance.
The main advantage having RF amplifiers are:
Greater gain – better sensitivity
Improved image frequency
Two main characteristic of RF stage are:
Sensitivity – ability to amplify weak signals
Selectivity – ability to reject unwanted signals
Path and Frequency Changing
Path & Frequency Changing
Converter / Mixer (Fig. 3.11)
RF is down converted to IF, but shape of the envelope remains the same info is conserved, bandwidth is unchanged.
Output of the mixer : infinite no. of harmonic and cross product including f RF , f LO , f RF + f LO , f RF – f LO.
LO is designed so that its frequency is always above or below the desired RF carrier by an amount equal to IF center frequency.
f LO is usually higher than f RF because up conversion leads to a smaller tuning range (smaller ratio of the maximum to minimum tuning frequency) much easier to design an oscillator that is tunable over a smaller frequency ratio.
If mixer and LO are in a single stage, it is called converter.
Common IF : 455 kHz.
Adequate selectivity because it is difficult to design sharp band bass filter if the center frequency is very high.
Center frequency is fixed and factory-tuned effectively suppressed because of its high selectivity.
Figure 3.11: Mixer input - output
f i ,f o f o + f i f o – f i or f i - f o f o f i
Intermediate Frequency & IF Amplifiers
IF & IF Amplifiers
Sum or difference in the output of a mixer that enters the IF stage.
A down-converted frequency that carries the information.
One or more stage(s).
Provide most gain and selectivity.
IF is much lower than RF easier to design and good sensitivity is easier to obtain with tuned circuit.
Image Frequency & Rejection
It is formed after the mixer circuitry.
It is an image of the input frequency that enters the mixer.
Represented in two form: high side injection and low side injection.
The image is an equal distance from the LO frequency on the other side of it from the signal.
An image must be rejected prior to mixing, because it’s indistinguishable and impossible to filter out.