Lab 9 - Oscillator
The purpose of the oscillator in our circuit was to provide an oscillator signal (LO) that would be sent to the mixer, in place of using a function generator.
During construction and testing of the oscillator, my group had mixed results. Our oscillator did not necessarily provide the correct signal to our mixer, but showed promise that it could be done with more time, or perhaps a better trimming capacitor.
Figure 1 - Oscillator Circuit
To start this lab, we constructed the oscillator circuit shown in Figure 1. This circuit utilizes a trimming capacitor that we could use to adjust the oscillator frequency going to the mixer circuit.
Adjusting the trimming capacitor, we observed the following low and hi frequencies:
f-lo = 81.5kHz
f-hi = 1.26MHz
The quality of the sine wave at the low frequency was very cut off, almost 'spiky' looking, as shown in Figure 2.
Figure 2 - Oscillator Output (bad)
Figure 3 - Ideal Oscillator Output
After a bit of tinkering with our oscillator circuit, we changed our C4 capacitor to 100uF. With this change, our output was much more ideal and is shown in Figure 3.
We obtained a much clearer signal at approximately 1kHz, but with a reduced gain. This was ideal for our mixer circuit.
Once we connected the oscillator and mixer to the rest of our radio circuit, we were disappointed. Our sound output was making an undesireable 'chirping' noise. After some testing the following week, we concluded it was something in the already built radio portion of our heterodyne radio. We replaced every single component, but had no luck in finding the culprit.