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
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: F min =1/(2π√LC max ); F max =1/(2π√LC min ); ii) Circuit bandwidth: X L =2πfL Q=R/X L B=f/Q
Detector Audio amplifier SUPERHETERODYNE RECEIVER LO MixerRF amp AGC IF amplifiers
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 (f S ) and the frequency of the local oscillator (f LO ). 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 metal- semiconductor 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.
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.
Reference oscillator Phase Detector LPFVCO Frequency divider Divider control BASIC PLL FREQUENCY SYNTHESIZER 100 kHz 1 MHz 10 15 1.5 MHz 20 2 MHz
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.