03 April 2014

RF signal source. Prototype build and test 2.

I have now rerouted the main variable capacitor wire away from the bandspread capacitor wire. This issue is now fixed. Next I replaced the 10pf blocking capacitor with a 6.8pf capacitor. No real difference seen in performance.

The inductor was also rewound using much tighter turns in an effort to try to improve stability. When I reinserted it back in the circuit the drift was far worse than the more loosely wound inductor. After doing some reading it was noted that often it takes time for coils to settle down due to stress in the wire caused by tight winding. I then boiled the inductor in boiling water for a few minutes. This appeared to have stabilized the coil.

I then changed out the JFET MPF102 for a J310. This resulted in a very distorted waveform (see pic). My assumption was that this was caused by some parasitic oscillation. I noted that the output voltage was in fact similar to the MPF102. Upon changing back to the MPF102 the waveform was once again clean. No further investigation carried out.

Next I made some measurements of Time, vs Temperature, Vdd and Frequency. I left the circuit powered on for a few minutes before beginning the tests so as to not measure warm up drift. The shack temperature varied between 23.5C and 23.4C with a stable Vd-d = 12.12V. The frequency varied between 7007430Hz and 7007404Hz according to my frequency counter. A variation of 26Hz. More tests over a wider temperature range needed once the buffer and amplifier stages are built.

Next steps: Build the buffer stage.
Waveform showing distortion when a J310 was deployed. Is this parasitic oscillation occurring?

Thermal stability tests. A wider range in temperature is needed to measure frequency stability correctly.

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