Tuesday, 7 February 2012

17m RockMite troubles

I haven't really got very far recently with radio stuff. I have been trying to tame my RockMite, built Manhattan style with 18.086MHz crystals. I can't get the RX/TX shift to work properly.

RockMites have in effect two 'channels' of operation. The frequency is shifted during keying by changing the voltage supplied to a varactor which 'pulls' the crystal. The voltage is either held high and pulled low by the keying, or held low and pulled high by the keying, effectively swapping the RX and TX frequencies, giving two 'channels' of operation.

My trouble is that the amount of difference between RX and TX is not the same for both 'channels'. At the moment I consistently have 500Hz difference in shift between each 'channel', for example, on one 'channel' I get around 800Hz shift between RX and TX - (this is good), but on the other 'channel' I get only 300Hz shift between RX and TX (this is bad!).

I've been experimenting with moulded inductors and different combinations of resistors. I think my 9V supply (actually less, from PP3) is making things more difficult. I scaled the resistors supplying the varactor for 9V instead of 12V, but this hasn't improved things much.

Maybe it's my crystals at fault. I hope to sort this problem out soon, it's driving me a little crazy!

On a plus note, I finished wiring up my SW-20+ yesterday evening and it has passed it's smoke test today. I tend to wait a while before applying power to circuits after building them, I like to think it gives you the chance to spot any mistakes you've made. I was pretty tired last night when I finished wiring the SW-20+, so I checked my work again this morning. All was good.

My SW-20+ is showing around 1 Watt output from 12 Volts, rising to considerably more with increased supply voltage. The quoted 1.5 Watt output can be achieved with 13.5V supply.

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