1. Principles & Applications Communications Receivers
Electronics
Principles & Applications
Chapter 7
Communications Receivers

2. KEY SPECIFICATIONS OF A RECEIVER
Selectivity - The ability of a receiver to tune and select the desired signal while minimizing interference from other signals
Sensitivity - The ability of a receiver to pick up and recover very weak signals

3. Tuned Radio Frequency (TRF) Receiver

4. TRF(cont’d)
A way to improve sensitivity is to add amplification, before and after demodulator.
To improve selectivity, the RF amplifiers uses tuned circuits. Whenever resonant LC tuned circuits tuned to the same frequency is cascaded, the overall selectivity is improved.
The greater the number of tuned stages cascaded, the narrower the bandwidth.

5. TRF(cont’d)-Example
Given a tuned radio frequency (TRF) tuning circuit consists of a 500μH loopstick, with a 50 to 500ρF variable capacitor for tuning. The equivalent tank parallel resistance is 200K Ohms.
Determine
i) the tuning range of the receiver
ii) the circuit bandwidth when tuned to 540KHz

6. TRF(cont’d)-Solution
i) Tuning range:
Fmin=1/(2π√LCmax);
Fmax=1/(2π√LCmin);
ii) Circuit bandwidth:
XL=2πfL
Q=R/XL
B=f/Q

7. SUPERHETERODYNE RECEIVER
Audio
amplifier
RF amp
Mixer
IF amplifiers
Detector
AGC LO

8. SUPERHETRODYNE PRINCIPLE
All incoming signals are translated in frequency by a mixer circuit down to a fixed intermediate frequency (IF) where it is easier and more convenient to obtain the desired selectivity and sensitivity (gain).
A mixer is a circuit that is similar to an amplitude modulator as it performs analog multiplication and produces sum and difference frequencies in the output.

9. INTERMEDIATE FREQUENCY (IF)
Mixers generate signals that are the sum and difference of the incoming signal frequency (fS) and the frequency of the local oscillator (fLO).
The difference frequency is more commonly chosen as the IF.
Some receivers use the sum frequency for the IF.

10. RF AMPLIFIERS
Many receivers use an RF amplifier ahead of the mixer to improve receiver sensitivity.
The best RF amplifier is a special metal-semiconductor field effect transistor (MESFET) made with gallium arsenide (GaAs).
MESFETs contribute very low noise.

11. MIXERS (and converters)
A mixer is any device capable of operating in a non-linear manner (diode, transistor).
Circuits to perform mixing or heterodyning or frequency conversion.
Up conversion-translating the input signal to a higher frequency.
Down conversion-translating the input signal to a lower frequency.
4 types of mixers usually found in communication:
Diode mixers
Doubly Balanced mixers
Transistor mixers
IC mixers
At low frequencies (<30 MHz) bipolar and field effect transistors are used.
At higher frequencies (>30 MHz), MOSFETs and diodes are used.
IC differential amplifier mixers offer gain.
Diode lattice mixers are popular at UHF and microwave frequencies.

12. IF AMPLIFIERS
Most of the gain in a receiver is obtained in the IF amplifiers.
Most IF amplifiers use IC differential amplifiers.
Most IF amplifiers have an automatic gain control (AGC) feature that permits gain to be decreased as signal strength increases.
Selectivity in an IF amplifier is usually obtained with a crystal, ceramic or surface acoustic wave (SAW) filter.

13. SQUELCH OR MUTE CIRCUIT
Most receivers contain a squelch circuit.
The mute circuit silences the receiver when no signal is received.
Squelch circuits amplify and rectify received noise and use the resulting DC voltage to block an amplifier circuit until a signal is received.
Tone activated mute circuits provide security and privacy.

15. FREQUENCY SYNTHESIZERS
A frequency synthesizer is a signal generating circuit that provides the carrier in a transmitter and one or more local oscillator frequencies in a receiver.
The synthesizer permits crystal frequency stability and accuracy.
The synthesizer generates signals of many frequencies in increments to permit channelized operation.

16. BASIC PLL FREQUENCY SYNTHESIZER
Reference
oscillator
1 MHz
 10
 15
1.5 MHz
 20
2 MHz
Phase
Detector
LPF
VCO
100 kHz
Frequency
divider
Divider
control

17. TRANSCEIVERS
A transceiver is the combination of a transmitter and a receiver in a single package.
All two way radios and cell phones are transceivers.
Circuits shared by the transmitter and receiver are the power source, antenna, frequency synthesizer, and filters.