W1FB DC Receiver experiments 7

I reworked the Oscillator components in an effort to try to:

1. Improve the stability
2. Improve the bandspread

Both objectives were achieved.

This was an interesting learning experience. I started out with 2x150pf CAPS in the circuit but the drift was still unacceptable. I narrowed the culprits down to these two caps using my 'straw blowing' method which is extremely effective at least in identifying the 'drift monsters'. I then spent about 5 hours trying different combinations. I had decided to try to eliminate the band edge trimmer cap in order to ensure that there was one less drift contributor. This made the fixed cap selection very challenging. The lesson learnt here was that the inductor should not have cement applied to it before the oscillator is trimmed. This allows a compressing and widening of turns possibility. To prove the point I ended up with a combination of 12 capacitors to achieve the result. These all appear to be COG/NPO Caps although they are not marked as such. I noted that it appeared that the bigger CAP values appeared to drift more than the smaller CAPS. This could be supported by the fact that the bigger caps would be handling more current and thus the heating effect may be greater. Especially as the bigger value caps are in fact not bigger in physical size.

The system is definitely now stable enough for use in my 7020Khz field system. I left the rig on in the sun for a couple of hours this morning with little noticeable drift. I have not measured the frequency with a frequency counter. The trimmer will allow tuning from 7006KHz - 7043Khz which is satisfactory. I would like to lower the band by 6KHz in order to allow the option to listen to DX when the band is open. Perhaps this will be a tweek that I can do in the future. I am not exactly sure how I would achieve this in practice since a 6Khz change constitutes a fraction of a picofarad change. Still even this narrow band is not easy to tune with a half turn trimmer. The good news is that this RX will be very compact on its small board. It will be fine to tune the rig with a small trimmer.

NPO/COG CERAMIC CAPACITORS ON HAND AS OF 4/6/2013 (pF)

10, 12, 18, 22, 27, 56, 68, 100, 150, 220, 270, 330, 820.