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Monday, 20 December 2010

How to generate drive to generate drive

Or perhaps I should call this post "recursion, see recursion".

I finished testing the phasing of the transformers on the linear amp board, you can see from this pic that the input gets split nicely into two signals that are out of phase with each other. These are effectively the inputs to the gates of the FETs. The output transformer is then doing a good gob of mixing these two signals back together again:

So, this suggests that the transformers are wound and installed correctly. So I've installed the FETs on a heatsync that is a little below the board, that bit's worked out OK I think:

I can bias the FETs as expected, I measured the idle current and then increased the FET bias voltage until each FET drew an additional 10mA, the board with the relay engaged and the FETs both biased on is now drawing about 50mA.

The problem I have now is that the board includes a RF sniffer to close the relay, my signal generator (homebrew of course) only generates about 150mV pk-pk output which isn't anywhere near enough drive to close the relay, if I use a cable to drive the relay (i.e. short out the relevant bits of the circuit), I'm not seeing much gain in the FETs as I dont think the drive is enough for them either.

So I need to build an RF amplifier for my signal generator before I can test my RF amplifier - recursion see recursion.

Hence another small project is born.

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