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Saturday, 6 August 2011

BITXing yet some more


I've been having some difficulties with the clarity of the signal out of my BITX 20 from ages ago:

When I placed the unit back under test I found that the carrier suppression was as expected when the VFO was at the top of the band, but when it was elsewhere the carrier suppression was non existent. Now a quick look at the schematic will tell you that this simply isn't possible, however this video shows that it is:

Now, I've got the main board out of the box again and back under test on the bench:

Try as I may, I cant reproduce the problem when the unit is sat there. I have drawn the conclusion that I must have done something quite dumb in my wiring of the internals as the carrier suppression works just fine. The picture below shows the signal hooked from the top of the diode at Q11 on the BITX schematic. This is the LSB on the left, the supressed carrier in the middle and the USB on the RHS. The carrier to sideband bandwidth is 1.2 KHz because that is the frequency of the audio I'm injecting into the mic amplifier. It's worth noting that the spectrum analyser is a logarithmic instrument so this is showing the carrier at least 40dB below the sideband amplitude, given that 6dB is half the voltage, you wouldn't be able to see much of the carrier left on a scope at all!
Also the 14Mhz signal out of the exciter on this board looks clean enough on the scope and also on the spectrum analyser:
So tomorrow, I'm planning to re-do the internal wiring testing each stage as I go.

Damn cats haven't been helping much either:
Fun though, egh?

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