1. Mixers in Communication Receivers Dr. Charles Baylis Faculty Candidate baylis@eng.usf.edu April 11, 2008 baylis@eng.usf.edu

2. 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.

3. School of Engineering & Computer Science Receiver System RF Filter/ Amplifier IF Filter/ Amplifier Local Oscillator f LO f RF To Demodulator Antenna Mixer

4. School of Engineering & Computer Science Multiplication of Cosines Recall the following trigonometric identity: Take a multiplier and input two sinusoids of different frequencies:

5. 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 Combiner RF Input LO Input

6. School of Engineering & Computer Science A More Realistic Mixer Mixer Input: Mixer Output: DC Difference RF LO 2 nd Harmonic of RF Sum 2 nd Harmonic of LO

7. School of Engineering & Computer Science “Squarelaw” Mixer Output Spectrum f Spectrum (Power or voltage) f IF f RF f LO 2f RF f RF +f LO 2f LO

8. School of Engineering & Computer Science A Microwave Diode Mixer Harmonic Tuning Stub RF Choke Inductor Diode λ/4 λ/4 DC return path λ/4 near f RF, f LO 50 Ω RF Input LO InputIF Output