2007-02-05
Building the AM Modulator
I built up the modulator properly on a scrap of circuit board. It works quite well, a little capacitive feedback was added to assure stability and reduce the RF drive sensitivity so it better matches a crystal oscillator.
A crystal oscillator was also made and a simple AF speech amplifier, essentially completing the 80 metre AM exciter.
Some quick experiments were performed using the exciter to feed the old BC547-based half-watt amplifier through the step-attenuator (I also finally modified the amplifier to feed its DC through a feed-through capacitor while I was at it). It was simple enough to adjust the drive level to achieve about half a watt PEP with fairly low distortion.
So, I have a working AM transmitter. Now comes the hard part... I need to build an amplifier chain that delivers 20 Watt PEP. I figure MOSFETs are the best bet for the final device(s), at least price/performance wise. As the radio is for a single frequency it would be possible to achieve good efficiency by resonating the FET capacitances, but I don't particularly like the idea of a narrow-band amplifier, so I'll likely build it broad, so I can use it with other radios, and on 40 metres (and perhaps 20 metres) as well.
I had some thoughts about a matching AM receiver, if I am to dedicate this radio to an "80 metre chatbox" style unit. It would be simple enough to use an MK484 as an IF with AGC for the receiver, in fact I suspect it would make the receiver quite trivial and offer good performance. The MK484 could just about make 3.68 MHz itself as a TRF receiver, but a front-end mixer would allow running it around 400 kHz. The availability of 3.2768 MHz crystals and 455 kHz IF cans that can be tuned down to about 400 kHz suggests an obvious approach that I will investigate shortly.
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Parent article: "2007 80m Homebrew Challenge".