Electronics Principles & Applications Fifth Edition Chapter 12 Radio Receivers ©1999 Glencoe/McGraw-Hill Charles A. Schuler

Modulation and Demodulation Simple Receivers Superheterodyne Receivers Frequency Modulation Single Sideband Receiver Troubleshooting INTRODUCTION

Oscillator A high-frequency oscillator can launch a radio wave. The process of adding information to the radio signal is called modulation. High frequencies are often called radio frequencies.

Audio Frequency (AF) Radio Frequency (RF) AM = RF x AF + RF Amplitude Modulation Modulator

Since the RF carrier frequency is much higher than the modulating frequency, an actual oscilloscope display of AM looks like this: On a spectrum analyzer, AM looks like this:

time amplitude Oscilloscope amplitude frequency Spectrum Analyzer f C = carrier frequency LSB = f C - f AUDIO USB = f C + f AUDIO AM produces sum and difference frequencies called sidebands.

+V CC L C 22 LC 1 f C = AF RF (f C ) An amplitude modulator

AM quiz The process of placing information on a carrier wave is _________. modulation With AM, the _________ of the carrier wave is controlled or varied. amplitude The oscilloscope displays a graph of ________ versus time. amplitude The spectrum analyzer displays a graph of _________ versus time. frequency A spectrum analyzer display of AM shows a carrier plus two ________. sidebands

An AM detector This capacitor approaches a short circuit at the carrier frequency. AM in Audio out Diode

Transmitter Diode Antenna Headphones A very basic AM receiver

A practical receiver needs tuned amplifiers to provide selectivity and sensitivity. gain frequency

IF amplifier Oscillator Mixer Antenna Detector It’s too difficult to simultaneously tune several circuits. The IF amplifier is permanently tuned to one frequency. IF passband Carrier and sidebands The desired station frequency is mixed to the IF frequency. Audio

Frequency mixing is also called converting or heterodyning. Receivers like this are known as superheterodyne types. IF amplifier Oscillator Mixer Antenna Detector This is called the local oscillator and it is tuned above the station frequency by an amount equal to the IF frequency.

IF amplifier Oscillator Mixer Detector f STATION = 1020 kHz f LO = 1475 kHz f IF = 455 kHz Some typical frequencies: Note: the two inputs to the mixer have a difference of 455 kHz.

A tuned circuit before the mixer is required. IF amplifier Oscillator Mixer Detector f STATION = 1020 kHz f LO = 1475 kHz f IF = 455 kHz Superheterodyne receivers can also respond to the image frequency. f IMAGE = 1930 kHz ( = 455)

Receiver quiz Recovering the information from a modulated signal is called __________. detection AM detection is often accomplished with a _________ rectifier. diode Radio receivers employ tuned amplifiers to provide sensitivity and ______. selectivity Superheterodyne receivers convert each signal to an _______ frequency. intermediate A superhet can respond to one additional frequency called the _______. image

Audio Frequency (AF) Frequency Modulation RF Oscillator One way to accomplish this is to use a varicap diode in the oscillator tank circuit. The audio signal changes the varicap bias and the resonant frequency of the tank circuit.

On a spectrum analyzer, FM shows more sidebands than AM. fCfC Upper sidebands Lower sidebands FM usually requires more bandwidth than AM.

Noise is always a problem in any communication system. FM has an advantage over AM since it offers better noise rejection. LIMITER FM signal plus noise Noise removed An FM receiver can use an amplitude limiter to remove noise. An AM receiver cannot since the modulation would be defeated. Modulation preserved

Audio Frequency (AF) Radio Frequency (RF) DSBSC = RF x AF DSBSC Modulation Balanced modulator

Audio Frequency (AF) Radio Frequency (RF) Spectrum analyzer DSBSC Modulation Balanced modulator LSB USB No carrier

frequency Balanced modulator Bandpass filter The lower sideband is not in the passband. Since the sidebands are redundant, one can be filtered out to decrease bandwidth. SSBSC Only the upper sideband is transmitted.

IF amplifier Mixer Oscillator Detector Oscillator A superheterodyne SSB receiver requires a second oscillator to replace the missing carrier.

Receiver troubleshooting Signal injection is standard practice. Both AF and RF signal generators may be required. Some receivers may require adjustments of their tuned circuits. This is called alignment.

FM and SSB quiz With FM, amplitude noise can be removed with a ___________. limiter FM needs more bandwidth than AM since there are more _________. sidebands A balanced modulator produces sidebands but no ___________. carrier In SSB, one of the sidebands can be eliminated by using a ____________. filter SSB demodulation requires an oscillator to replace the missing _________. carrier

REVIEW Modulation and Demodulation Simple Receivers Superheterodyne Receivers Frequency Modulation Single Sideband Receiver Troubleshooting