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Sunday, 19 January 2014

Minima BFO

Well,

Having brought home two piles of trouble this morning (they are solid Cocker Spaniels):


I suspect play time may be somewhat restricted for a while, but anyway, during a dog nap this afternoon I have built the BFO for the Minima. This uses a relay to switch between two trimmers which need to be adjusted for LSB and USB carriers. My build looks like this:


The output looks like this on the 'scope:


and this on the Spectrum Analyser:


I've also built the microphone amplifier and coupled it up to the post BFO mixer. This is a simple 2 diode arrangement with no adjustment for carrier suppression - this is dependent on good matched diodes. I matched a pair of 1N4148s and this is what I see with a 1500KHz audio injected into the mic input:


The signal in the centre is the BFO, so it's at least 40dB down on the audio - the peak to the right of the carrier or BFO is the USB and the peak to the left the LSB. The mixer has done what mixers do and we have the BFO plus the audio frequency and the BFO minus the audio frequency.

All good....

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