10.26.2021 -- The purpose of this lab is to become familiar with how a mixer works and to build on in lab. The frequency mixer basically multiplies signals into new ones with new frequencies.
8.1 Mixing Theory
The following definition of a mixer is from the lab manual: "the mixer 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."
Figure 8.1 Mixer
8.2 Mixer Simulation in LTSpice
1-3)
Figure 8.2 FFT plot for default mixer circuit
Figure 8.2 FFT plot for default mixer circuit with revised y-axis limit
4)
Figure 8.3 Answer to question 4 of section 8.2
5) See table 8.1
6) See Table 8.1. The VCE decreased significantly compared to that of the default Q-point. The IC current is about twice that of the IC of the default Q-point. The gains stay about the same compared to the default circuit.
7) See Table 8.1. Compared to the default mixer circuit, adding the RFC did not change the Q-point, but all the gains increase significantly.
8) See Table 8.1. By adding the bandpass filter to the mixer circuit, the gains calculated are much lower. I think the inductor and capacitor in parallel may cause the output gains of the BFP and mixer circuit to be extremely low.
Table 8.1 Data for LTSpice analysis of mixer circuit
8.3 Building/Testing the Mixer Circuit
The defualt circuit in Figure 8.2 is breadboarded and the output is shown below in Figure 8.4. The difference frequency is at about 1000 kHz. The two peaks in the graph are at about 1000 kHz and 1200 kHz.
Figure 8.4 Frequency peak at abouut 200 Hz
In Figure 8.5, the RFC was added to the circuit. The oscilloscope results are shown in Figure 8.6. the intermediate frequency (IF) is aat about 200 kHz.
Figure 8.6 Oscilloscope results
Conclusion
Overall, the mixer circuit worked successfully. I did need to adjust the circuit a little bit becuase the desired frequency output peaks weren't showing up on the oscilloscope at first. There may have been an issue with my oscilloscope that day because the frequency peaks were vissible when I switched equipment stations.
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