Tuesday, July 12, 2016

First steps towards a transmitter

I would like to add a CW transmitter to my little DC receiver.  I'm thinking of using an IRF510 running as a class C amplifier.

I got a 74AC240 to use as a driver for an IRF510.  However, the output of the DDS is fairly low.  I was going to use a comparator to convert to a 5v square wave, but was having trouble getting it to work.  I think the issue is that the LM399 I pulled out of the junk box isn't fast enough :/

I then tried making a Schmitt trigger out of an op amp I had laying around, but it wasn't fast enough either. One I tried was a bit faster, and if I got the source down to 1 Mhz it sort of worked.

Then I tried the DDS buffer to directly drive the '240 - success!  The buffer distorts a bit, so the signal out of the '240 isn't that great, either.  But I can clean that up with my low pass filter.  I'm measuring about 400 mW into a 50 ohm load when I use 5 volts on the '240.  I'm using a 12V source for the buffer and the Arduino's 5V output for the '240.

I seem to be running out of breadboard space.  Hopefully the IRF510 stage won't take up too much room.  Once I have something together that works, will see about moving the PA stage out to something more permanent.

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