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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Date: Thu, 28 Jun 2007 18:39:48 +0100
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Subject: LF: 500kHz class D TX design
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Dear LF Group,

I drew up a circuit diagram of my new 500kHz transmitter design in case
anyone is interested.

It uses a "half-bridge" voltage-switching class D PA with complementary N
and P channel mosfets. The idea of this is that the push-pull design
minimises 2nd harmonic levels at around 1MHz, and the complementary mosfets
simplify the gate drive requirements somewhat. It uses cheap IRF520 and
IRF9520 devices, and produces a maximum output of about 11W with 13.8V DC
supply. It can easily run at different power levels by changing the number
of turns on the output transformer. I included DC current limiting  in the
keying circuit, which allows the PA to operate into open, short, and
reactive loads without apparent distress.

It has a VFO based on a 4MHz ceramic resonator and divide-by-8 counter. This
covered the whole band with several resonators I tried, and drifts by a few
hertz per hour, which is satisfactory for most purposes.

The circuit diagram is the basic CW transmitter for now. I have also added
various other bits and pieces such as variable output power, FSK for DFCW,
and a RF voltage/current/phase output metering system for easy antenna
tuning, but will have to draw  these up later. The image is slightly too big
to be sent via the reflector, but if anyone would like a copy, let me know
and I will e-mail it direct.

Cheers, Jim Moritz
73 de M0BMU