RF Systems Lab
Lab 8: Mixer
Lab 8 Goals:
Become familiar with how a mixer works
Analyze the mixer in LTspice, generating a spectrum using the FFT function
Build and test a mixer
Lab 8 Equipment:
AFG1062 Arbitrary Function Generator
Tektronix MDO 3032 Oscilloscope
Omega HHM90 Digital Multimeter
USB Device
1/2 Watt 8 Ohm Speaker
8.1 Mixing Theory
The mixer, as depicted in Figure 8.1, multiplies an RF signal with an oscillator signal (the local oscillator, or LO) to produce a number of signals at different frequencies. The desired output is known as the difference frequency, which in our radio is also termed the intermediate frequency, or IF. The mixing action occurs because of nonlinearity in the mixing element, be it a diode or a transistor. A transistor-based mixer, though slightly more complicated, has the advantage of providing signal gain, whereas the diode mixer does not, The pre-lab report for chapter 8 can be found below.
PreLab_8.pdf8.3 Building/Testing the Mixer Circuit
The circuit is constructed, and 20mV is used for the amplitude of Vrf. The spectrum output on the oscilloscope is used to view the FFT of the signal. Then, an RFC is added to the circuit to improve the gain of the mixer, which increases the gain by nearly double across the entire frequency spectrum. Figure 1 shows the frequency spectrum for the mixer.
Figure 1: Circuit Measurements at 100Ω and 1kΩ
Finally, the mixer is added to the bandpass filter from chapter 7, and the intermediate frequency of the radio is found to be 210kHz using the spectrum analyzer function of the oscilloscope.