Mixers in Communication Receivers Dr. Charles Baylis Faculty Candidate [email protected] April 11,...
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Mixers in Communication Receivers Dr. Charles Baylis Faculty Candidate [email protected] April 11, 2008
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Transcript of Mixers in Communication Receivers Dr. Charles Baylis Faculty Candidate [email protected] April 11,...
Mixers in Communication Receivers
Dr. Charles BaylisFaculty Candidate
April 11, 2008
School of Engineering & Computer Science
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.
School of Engineering & Computer Science
Receiver System
RF Filter/Amplifier
IF Filter/Amplifier
Local Oscillator fLO
fRF
To Demodulator
Antenna
Mixer
School of Engineering & Computer Science
Multiplication of Cosines
• Recall the following trigonometric identity:
• Take a multiplier and input two sinusoids of different frequencies:
)cos(2
1)cos(
2
1coscos BABABA
tAtv RFRF cos)(
tAtv LOLO cos)(
tBtAtv LORFIF coscos)(
tABtAB LORFLORF )cos(2
1)cos(
2
1
School of Engineering & Computer Science
A More Realistic Mixer
• Sum the RF and LO signals, then input them into a diode.
• Diode Current-Voltage Characteristic:I(V)
V
2)( cVbVaVI Combiner
RF Input
LO Input
School of Engineering & Computer Science
A More Realistic Mixer
• Mixer Input:
• Mixer Output:
tQtPV LORF coscos
22
2
1
2
1)( cBcAaVI
tPQ RFLO )cos( tbP RFcostbQ LOcostcP RF2cos
2
1 2
tcAB RFLO )cos(
tcQ LO2cos2
1 2
DC
Difference
RF
LO
2nd Harmonic of RF
Sum
2nd Harmonic of LO
School of Engineering & Computer Science
“Squarelaw” Mixer Output Spectrum
f
Spectrum (Power or voltage)
fIF fRF fLO 2fRF fRF+fLO 2fLO
School of Engineering & Computer Science
A Microwave Diode Mixer
Harmonic Tuning Stub
RF Choke Inductor
Diode
λ/4
λ/4
λ/4 DC return path
λ/4 near fRF, fLO
50 Ω
RF Input
LO Input IF Output
