Ten Meter PDM project

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Ten Meter PDM project

Postby K9ACT » Sat Jun 11, 2011 8:47 am

I don't get much use out of my Class D transmitter because other than the great audio reports I usually get, there is not much incentive to letting my big tube rigs sit idle just to run Class D.

After frustrating over getting my tube rigs (big triodes) to behave on 10 meters, I gave up trying and launched a new stand alone 10 meter project.

After reviewing my options, it occurred to me that a solid state amp with series modulation would be the easiest to implement this as my PDM modulator is collecting dust.

My first problem was that my DDS VFO only goes to about 20 mHz. A friend (WA9CGZ) pointed me to:


and I copied the Push Push Doubler/Buffer from there, added an RD16 driver and got about 5W at 29 mHz.

Another friend (W9AD) gave me a couple of MRF150 spares that he no longer had anything to use them in.

I never could get them running as a pair so I copied some of the ideas from an Oct 1990 QST project for a 6 Meter transmitter. It used air wound coils and a bipolar transistor. After a bit of tinkering, I was able to get this to work well enough. I can get about 90 watts out with 25v and my first contact was RTTY in that mode.

Next step was to get it running with the PDM modulator which amounted to nothing more than adjusting the carrier level and tuning to work with the 50V max of the MRF150.

There are 4 compression caps in the output so tuning is rather cranky but the best combination so far seems to be about 14V carrier giving me about 25 W with 150% modulation. I can get up to 40W out just diddling with the caps but the price is always clipping on the positive modulation.

After a week of running the CQ robot with no joy, the band opened up last night (6/10) at about 2200Z and I worked 8 stations in 7 states.

Here is a pic of the RF deck followed by one of the modulator which still looks like it's not quite ready for prime time but works great.


I seem to have forgotten how to attach photos.
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Re: Ten Meter PDM project

Postby n4lta » Fri Jun 24, 2011 6:21 pm

That was my article in 1990 QST. I used that amp with a MRF140 on Six for several years.
I later moved up to a 4CX1500 amp but got rid of both when the cycle died and have not been active since.

The FET will run in Class C with a choke from the Gate to ground. Running it as a linear amp I got approximately 175 watts out of a single FET. It should be biased in Class C to modulate AM.
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