Tuesday, May 26, 2009

The LO for the PSKmail-TRX

First steps first... investigate if I can reach 10148kHz using standard crystals.
The idea, as shown in an earlier post, is, to use a 6144kHz LO and and I.F. of 4000kHz. This short of 4kHz. A lot, one may think.

However, when building my 30m-MEPT, I experienced that oscillators based on 74HC86 generate a frequency much higher than the crystal's nominal. Using a 5068.8kHz XTAL, I thought I was way to low, but then, I had hours of experimentation, before I got the frequency down into the qrss-band.

Thus, today, the idea was, if I can repeat this with 6144kHz crystals to close the 4kHz gap.
Well, the gap is closed, by far exceeding my expectations. The 6144kHz crystal produced a 6154kHz signal. Yes, 10kHz above the nominal frequency. Further experiments revealed, an inductor of 15┬ÁH and a capacitor with 47pF both in series with the crystal, brought the frequency down to 6146kHz. Close, but still more room for improvement appeared. The photograph shows the LO with the experimental capacitor (the long legged blue thing).

Two more gates of the 74HC86 will be used to generate an in-phase and an out-off-phase LO signal for use in a 74HC4066-mixer.

The remaining fourth XOR-gate will become the 4000kHz BFO.

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