Saturday, April 14, 2012

Progress on the ARNSW Home Brew 40m AM challenge

In spare time I've been working on my entry for the ARNSW home brew group challenge project which is to build a low power AM transmitter for 7.125MHz.

There have been a couple of false starts here. First I ambitiously tried to make a pulse width modulated transmitter but couldn't get it working. Next I tried high level AM modulation using an audio transformer but I think it saturates and I could get only very small modulation with it.

Now I'm trying low level modulation of the IRF510 power stage. This looks distorted but seems to be working pretty well.

The AF board uses a pre-amp with AGC picked up from a recent trash and treasure at Dural (thanks John VK2ASU). For the power stage is a module from Jaycar - I know, a bit slack on my behalf, but an audio power amp is the least interesting bit for me.

The RF board starts with parallel 7.122 crystals pulled as close to 7.125 as I can get, a buffer stage, a driver, an IRF510 final and a low pass filter.

Along the way, the bias on the IRF510 got out of control making a big bang and releasing its smoke. I now use a current meter at all times so I can spot thermal runaway before catastrophe strikes.

Happily they are cheap.

Lots of work still to do and I'm very much looking forward to having a contact with this tangled mess.


Anonymous said...

Peter - keep us posted. I should be able to hear it down here - especially if at a low noise location.

Just wondering about the purpose of the inductor in series with the crystals. For me they're a last resort and only needed if you want wide pulling range or a downward excursion. I'd be tempted to remove it and add some capacitance in parallel with the trimmer if you can't get it low enough. 73, Peter VK3YE

Peter Marks said...

Thanks Peter,

Yes, I'm really after upward frequency pulling so I'll remove the inductor as you suggest.

I ran the transmitter for a while today playing a tape and it sounds reasonably good on a short wave receiver.

Not much power out at this point.

All great fun.

Peter, VK2TPM