Lab 8 - Mixer
Figure 1 - Mixer Concept
The purpose of the mixer in our radio circuit is to take the RF frequency and multiply by an oscillator signal (LO) to produce an intermediate frequency (IF).
This week, my partner and I I used only one circuit since we needed two of the precision function generators. Overall, this lab was successful. We were able to produce a very clean IF output with a respectable gain.
As we observed the output across the load of the circuit without the RFC choke, we saw the difference frequency vary as the oscillator frequency changed. To observe the difference frequency, we used the FFT mode on our oscilloscope.
Once we added the RFC choke to our mixer, we noticed a large gain increase by at least 100 V/V.
Next, we added the bandpass filter from the last lab. What was interesting here was that only the IF frequency was amplified while most other frequencies were filtered out.
Figure 2 - Mixer and Bandpass Filter
The problem we encountered during this lab was that our IF, or difference, frequency was not at 200kHz in our final circuit. We noticed a spike on the FFT plot near 45kHz instead. It was suggested that the cause my be due to poor quality components not rated for these frequencies.