Mixers in Communication Receivers

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Mixers in Communication Receivers
Dr. Charles BaylisFaculty Candidatebaylis_at_eng.us
f.eduApril 11, 2008
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Overview

• Mixers
• Used for converting signals up or down in
  frequency.
• A mixer is a circuit containing a nonlinear
  device.
• Frequency conversion is accomplished by
  multiplying.
• The nonlinear device in the mixer performs the
  multiplication.

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Receiver System
Antenna
fRF
To Demodulator
Mixer
RF Filter/ Amplifier
IF Filter/ Amplifier
Local Oscillator
fLO
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Multiplication of Cosines

• Recall the following trigonometric identity
• Take a multiplier and input two sinusoids of
  different frequencies

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A More Realistic Mixer

• Sum the RF and LO signals, then input them into a
  diode.
• Diode Current-Voltage Characteristic

I(V)
RF Input
Combiner
LO Input
V
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A More Realistic Mixer

• Mixer Input
• Mixer Output

DC
Difference
RF
LO
2nd Harmonic of RF
Sum
2nd Harmonic of LO
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Squarelaw Mixer Output Spectrum
Spectrum (Power or voltage)
f
fIF
fRF fLO
2fRF fRFfLO 2fLO
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A Microwave Diode Mixer
?/4
Harmonic Tuning Stub
RF Input
50 O
?/4
?/4 near fRF, fLO
Diode
LO Input
IF Output
RF Choke Inductor
?/4 DC return path