1. Principles & Applications
Electronics
Principles & Applications
Sixth Edition
Charles A. Schuler
Chapter 12
Communications
© Glencoe/McGraw-Hill

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

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

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

5. On a spectrum analyzer, AM looks like this:
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:

6. Oscilloscope fC = carrier frequency Spectrum Analyzer
amplitude
time
fC = carrier frequency
amplitude
frequency
Spectrum Analyzer
LSB = fC - fAUDIO
USB = fC + fAUDIO
AM produces sum and difference frequencies called sidebands.

7. An amplitude modulatorAF
+VCC 2p LC 1
fC = L C RF
(fC)
An amplitude modulator

8. 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

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

10. A very basic AM receiver
Antenna
A very basic
AM receiver
Transmitter
Diode
Headphones

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

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

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

14. Some typical frequencies:
fSTATION = kHz
Some typical frequencies:
fIF = 455 kHz
Mixer
IF amplifier
Detector
Oscillator
fLO = kHz
Note:
The two inputs to the mixer have a difference of 455 kHz.

15. Superheterodyne receivers can also respond to the image frequency.
fSTATION = kHz
fIMAGE = kHz
( = 455)
A tuned circuit before the mixer is required.
fIF = 455 kHz
Mixer
IF amplifier
Detector
Oscillator
fLO = kHz

16. 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

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

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

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

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

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

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

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

24. Bluetooth Range: 10 to 100 meters Power: 1 to 100 mW
Sensitivity: 0.1 nW (-70 dBm)
Frequency: 2.4 GHz
Data rate: 1 Mbit/s

25. 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.

26. 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

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