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Communication
Electronics
Principles & Applications
Chapter 7
Communications Receivers
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
Tuned Radio Frequency (TRF)
Receiver
RF Amplifiers
Detector
Tuning capacitors mechanically
ganged
Audio
amplifiers
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.
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
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
SUPERHETERODYNE
RECEIVER
Audio
amplifier
RF amp
Mixer
IF amplifiers
Detector
LO
AGC
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.
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.
RF AMPLIFIERS
• Many receivers use an RF amplifier
ahead of the mixer to improve receiver
sensitivity.
• The best RF amplifier is a special metalsemiconductor field effect transistor
(MESFET) made with gallium arsenide
(GaAs).
• MESFETs contribute very low noise.
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.
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.
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.
+Vcc
Audio
amplifier
R1
speaker
Power
amplifier
IF amplifiers
Detector
Q2
C1
High-pass
filter 6kHz
Noise amplifiers
C2
Squelch gate
Q1
D2
R1
+
D1
C3
Rectifier -voltage doubler
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.
BASIC PLL FREQUENCY
SYNTHESIZER
Reference
oscillator
Phase
Detector
1.5
12 MHz
MHz
LPF
VCO
100 kHz
Frequency
divider
 20
10
15
Divider
control
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.