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ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsSixth EditionSixth Edition
Chapter 12Communications(student version)
©2003 Glencoe/McGraw-Hill
Charles A. Schuler
• Modulation and Demodulation
• Simple Receivers
• Superheterodyne Receivers
• Frequency Modulation
• Single Sideband
• Receiver Troubleshooting
INTRODUCTION
Dear Student:
This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.
Concept Preview• Modulation is the process of adding
information to an RF signal.
• The information signal controls the amplitude of the RF signal when amplitude modulation is used.
• The envelope of an AM signal has the same shape as the information signal (oscilloscope display).
• AM produces upper and lower sidebands.
• A spectrum analyzer displays an AM signal’s carrier and sidebands.
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 muchhigher than the modulating frequency,
an actual oscilloscope displayof AM looks like this:
On a spectrum analyzer,AM looks like this:
time
amp
litu
de
Oscilloscope
amp
litu
de
frequency
Spectrum Analyzer
fC = carrier frequency
LSB = fC - fAUDIO
USB = fC + fAUDIO
AM produces sum and difference frequencies called sidebands.
+VCC
L C
2 LC
1fC =
AF
RF
(fC)
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
Concept Review• Modulation is the process of adding information to
an RF signal.
• The information signal controls the amplitude of the RF signal when amplitude modulation is used.
• The envelope of an AM signal has the same shape as the information signal (oscilloscope display).
• AM produces upper and lower sidebands.
• A spectrum analyzer displays an AM signal’s carrier and sidebands.
Repeat Segment
Concept Preview• Information signal recovery is called detection. AM
receivers often use a diode detector.
• Tuned amplifiers provide selectivity so that only the desired station will be received.
• Superheterodyne receivers use an intermediate frequency (IF) before detection.
• A local oscillator is mixed with the desired station to convert it to the intermediate frequency.
• An image frequency will also mix with the oscillator and produce the intermediate frequency.
• Selectivity before the mixer eliminates the image.
An AM Detector
This capacitorapproaches a short
circuit at the carrier frequency.
AM in Audio outDiode
Transmitter Diode
Antenna
Headphones
A very basic AM receiver
A practical receiver needs tuned amplifiersto provide selectivity and sensitivity.
gain
frequency
IF amplifier
Oscillator
Mixer
Antenna
Detector
It’s too difficult to simultaneously tuneseveral circuits. The IF amplifier is
permanently tuned to one frequency.
IF passbandCarrier andsidebands
The desired station frequency is mixed to the IF frequency.
Audio
Frequency mixing is also called convertingor heterodyning. Receivers like this are
known as superheterodyne types.
IF amplifier
Oscillator
Mixer
Antenna
Detector
This is called the local oscillatorand it is tuned above the
station frequency by an amountequal to the IF frequency.
IF amplifier
Oscillator
Mixer Detector
fSTATION = 1020 kHz
fLO = 1475 kHz
fIF = 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
fSTATION = 1020 kHz
fLO = 1475 kHz
fIF = 455 kHz
Superheterodyne receivers can also respond to the image frequency.
fIMAGE = 1930 kHz
(1930 - 1475 = 455)
Receiver QuizRecovering 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 eachsignal to an __________ frequency. intermediate
A superhet can respond to one additionalfrequency called the __________. image
Concept Review• Information signal recovery is called detection. AM
receivers often use a diode detector.• Tuned amplifiers provide selectivity so that only the
desired station will be received.• Superheterodyne receivers use an intermediate
frequency (IF) before detection.• A local oscillator is mixed with the desired station to
convert it to the intermediate frequency.• An image frequency will also mix with the oscillator
and produce the intermediate frequency.• Selectivity before the mixer eliminates the image.
Repeat Segment
Concept Preview• With frequency modulation (FM), the
information signal controls the frequency of the carrier.
• FM produces more sidebands than AM and thus has greater bandwidth.
• Noise and static can be removed from an FM signal by clipping.
• The carrier in an AM signal can be eliminated by using a balanced modulator.
• Single sideband AM also eliminates one of the sidebands.
Audio Frequency (AF)
Frequency Modulation
RFOscillator
One way to accomplish this is to use a varicap diode in the oscillator tank circuit.
The audio signal changes thevaricap bias and the resonantfrequency of the tank circuit.
On a spectrum analyzer, FM shows more sidebands than AM.
fC
Uppersidebands
Lowersidebands
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 signalplus noise
Noiseremoved
An FM receiver can use an amplitude limiter to remove noise. An AM receiver cannot since the modulation would be defeated.
Modulationpreserved
Audio Frequency (AF)
Radio Frequency (RF)
DSBSC = RF x AF
DSBSC Modulation
Balancedmodulator
Audio Frequency (AF)
Radio Frequency (RF)
Spectrum analyzer
DSBSC Modulation
Balancedmodulator
LSB USBNo carrier
frequency
Balancedmodulator
Bandpassfilter
The lowersideband
is not in thepassband.
Since the sidebands are redundant, one can be filtered out to decrease bandwidth.
SSBSC
Only theupper
sideband istransmitted.
IF amplifierMixer
Oscillator
Detector
Oscillator
A superheterodyne SSB receiver requires a second oscillator to replace the missing carrier.
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
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
Concept Review• With frequency modulation (FM), the
information signal controls the frequency of the carrier.
• FM produces more sidebands than AM and thus has greater bandwidth.
• Noise and static can be removed from an FM signal by clipping.
• The carrier in an AM signal can be eliminated by using a balanced modulator.
• Single sideband AM also eliminates one of the sidebands.
Repeat Segment
• Modulation and Demodulation
• Simple Receivers
• Superheterodyne Receivers
• Frequency Modulation
• Single Sideband
• Receiver Troubleshooting
INTRODUCTION