02 June 2013

W1FB DC Receiver experiments 6

Last week I made good progress towards a reasonably stable oscillator circuit. First I was able to procure an amount of small value Ceramic Caps from a discount store in Pretoria. I also bought a number of small value trimmer Caps. I changed out the existing circuit for the new components and tweeked the values in line with what I had. Upon firing up the receiver I was very disappointed in the result. The oscillator drifted wildly both up and then down in frequency. The caps that I acquired were clearly not COG/NPO and almost definitely not even ClassII. They certainly look like ceramics and will probably be ok for non frequency sensitive applications.

I was able to confirm that the CAPS were the contributors by blowing on each CAP with a WD40 thin straw. This was the perfect tool for this test. I can direct hot air directly at even the smallest CAP. When I did this I was able to generate a frequency shift of at least 2Khz.

I went back to a previous store and was able to buy discontinued COG hole through caps. I also bought some COG SMT caps to experiment with. I have read that these SMT caps do not have acceptable Q. To test this I will first have to build a signal generator. I replaced all CAPS except the feedback CAPS in the Colpitts circuit. The result was much improved oscillator. Next I will replace the feedback CAPS because I can see that they are still contributing.

I will also try to locate a small air variable in order to improve the ease of tuning. Bearing in mind that the goal of this RX is to primarily receive a single frequency on 7020Khz.

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