Transcript superheterodyne receiver

Communication Systems
Prof. Chungming Kuo
Chapter 5
Superheterodyne Receivers (cont.)
Superheterodyne Receivers
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A communications receiver is an electronic
unit that responds to signals.
It separates the desired signal from all others.
It performs the demodulation and other signal
processing operations required to produce
the desired output.
Superheterodyne Receivers (cont.)
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While there are several types of receivers,
by far the most common is the
superheterodyne receiver.
It is based on the principle that all signals
are down-converted to a common
intermediate frequency (IF) for proper
filtering.
Receiver Terminology
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Selectivity - A measure of how well the
receiver can separate the desired signal from
all others present at the input.
Sensitivity - A measure of how well the
receiver can respond to very weak signals.
Classical Tuned Radio Frequency (TRF)
Receiver
ANTENNA
SPEAKER
RF
AMPLIFIER
DETECTOR
POST
DETECTION
AMPLIFIER
Limitations of TRF Receiver
Selectivity must be established in the RF
amplifier stage.
 The capability of designing highly selective
stages over a broad frequency range is a
difficult task.
 Moreover, if tuning is required, the task is
even more difficult.
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Superheterodyne Concept
 The
discussion at this point will be limited to
the single-conversion superheterodyne
receiver.
 The concept is to establish an intermediate
frequency (IF) amplifier stage in which very
high selectivity can be achieved.
Superheterodyne Concept (cont.)
 All
signals are then down-converted to this
frequency range for filtering.
Superheterodyne Block Diagram
(a)
ANTENNA
f LO + f c
MIXER
f LO - f c
fc
RF
AMP
fc
BRF > B
IF
AMP
X
f LO
LO
BASEBAND
f IF
DEMOD
AVC
BIF = B
AF
AMP
SPEAKER
Superheterodyne Spectral Chart
(b)
SPECTRAL
COMPONENTS
f IF
BIF = B
RF AMPLIFIER
AMPLITUDE
RESPONSE
f image
f LO
fc
BRF
f IF
f IF
f
IF and LO Frequency Relationships
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The LO frequency may be higher or lower than the
incoming signal frequency. When it is higher,
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When it is lower,
f IF = f LO - f c

f IF = f c - f LO
Image Frequency
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There is a potential problem with respect
to an image frequency. A frequency on the
opposite side of the LO frequency by the
amount of IF frequency can mix with the
LO frequency to cause a spurious output.
Image Frequency (cont.)
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For LO above input frequency,
f image = f LO + f IF = f c + 2 f IF
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For LO below input frequency,
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f image = f LO - f IF = f c - 2 f IF
Design Considerations
Image interference can be minimized by
having enough selectivity in the RF stage to
reject the image component.
 The higher the IF frequency, the easier it is to
reject the image component.
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Design Considerations (cont.)
However, selectivity at higher frequencies is
more difficult so there is an engineering
tradeoff.
 More sophisticated receivers employ dualconversion forms. Image rejection is
optimized in the first stage and selectivity is
optimized in the second stage.
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Double-Conversion Receiver
MIXER
1
RF
AMP
X
LO1
IF
AMP
1
MIXER
2
X
LO2
IF
AMP
2
DEMOD
AF
AMP
Example 1
• A single conversion receiver is tuned to 40 MHz
and IF frequency is 5 MHz.
• Determine LO and image frequencies if LO
frequency is higher than signal frequency.
f LO = f c + f IF = 40 + 5 = 45 MHz
f image = f LO + f IF = 45 + 5 = 50 MHz
Example 2
• Repeat analysis of Example 1 if LO
frequency is lower than signal
frequency.
fLO = f c - f IF = 40 - 5 = 35 MHz
f image = fLO - f IF = 35 - 5 = 30 MHz
Summary
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The superheterodyne receiver concept is the
basis for most modern receiver designs.
The selectivity of a superheterodyne receiver is
established in the IF stage.
Tuning of a superheterodyne receiver consists
of tuning the LO such that the desired mixer
output falls in the center of the IF band.