Note that while this local oscillator redesign produces a better sinusoidal output it didn’t produce better audio output from the S-Pixie. This is because the mixer loads the local oscillator circuit, producing a signal very similar to the original design. I found that coupling the local oscillator and mixer with a 100 pF capacitor instead of the 10 nF capacitor solved this problem and somewhat decreased noise at the audio output.
In my last post, I attempted to redesign the S-Pixie local oscillator to obtain a more sinusoidal output signal. Most of my effort was modifying the oscillator transistor DC operating point so it would operate within its active region. This wasn’t very successful, I believe because the feedback signal was too strong. For this redesign, I decided to directly attenuate the feedback signal by adding a resistor and capacitor in its path.
This design proved successful but at the cost of three additional components which definitely won’t fit on the crowded S-Pixie PCB.
This design has an output signal six time the signal level of the alternative design in my last post and significantly reduced harmonics.
I need to check how the reduced local oscillator signal affects the S-Pixie operation. If a greater signal level is needed, the attenuating capacitor and resistor can be changed. A 2k ohm resistor gives a signal of about 3.4 volts.
But with an increase in harmonics.
This feedback attenuation can also be used with the original S-Pixie local oscillator biasing, thus saving one resistor. This increases the output signal somewhat from the design above.
And without much penalty in harmonics.
As above, the signal can be increased more by decreasing the attenuating resistor, but with increased harmonics.
I also tested various attenuating capacitors, but found that harmonics increased with larger capacitors. I didn’t look more into this because I was trying to avoid harmonics. As we saw in my post on the S-Pixie low pass filter, it isn’t particularly good at suppressing the harmonics generated by the radio.
Getting the Colpitts Crystal Oscillator to Oscillate
In my last post I referenced a great video describing how to design a Colpitts oscillator. At several points the presenter lists the conditions required for the oscillator to oscillate. I got the impression that these calculations were somewhat critical but as I worked through these experiments I got oscillation with almost everything I tried. I began to wonder if I could make any changes that would cause the oscillator to cease oscillating.
Obviously, oscillation will stop if the feedback signal is insufficient. This happened with an attenuating resistor of 5.6k ohms or above. What surprised me somewhat is that the circuit wouldn’t oscillate with much higher capacitors in the capacitor divider. For example, I thought that using 1 nF capacitors in the divider made sense as these matched the calculated impedance of the crystal at 7.023 MHz (similar to the process in design video). However, I couldn’t get the circuit to oscillate with them or even as low as 200 pF capacitors. I’ll leave this for future investigations though. It’s time to move on to the S-Pixie mixer.