Hi Danny!
> Manfred, be sure an keep us posted on your MOSFET amplifier project. A
> bunch of us are following your work and would like to build up something
> similiar one of these days.
OK, so here it goes!
I built the complete scale model (for 15 Watts output), including the
modulator. I finished this just a few hours before I had to leave home
for a trip, so I didn't have time to fully measure performance, put it
on the air, etc.
A quick and dirty check, feeding the amplifier's output into a receiver
and putting its audio output into a computer sound card for spectral
analysis, shows an absolutely horrible IMD performance: The 3rd order
IMD is down by only 8dB! But that same test setup shows the exciter (a
TS450SAT running at 10 Watt PEP output on 160 meters) having the 3rd IMD
at only 12dB down, which is also horrible and PROBABLY NOT TRUE! So I
have to build a different IMD testing setup. Suggestions are welcome,
but if no better suggestions shows up, I will use a 74HC4060 chip with a
quartz to produce a nice clean stable square wave on the 160 meter band,
and use this local oscillator to drive a doubly balanced diode mixer.
That mixer will get a sample of the signal to be measured, and its
output will be fed into the sound card. Sort of a no-gain,
crystal-controlled, wide dynamic range direct conversion receiver.
Anyway, on the scope the signal looks good! At least, envelope-wise. If
I display the drive signal and the output signal together, they look so
similar that it's impossible to tell which is which.
I will not spend much time at home for the rest of this month, so no
further advances are to be expected soon. In february I will build this
better IMD testing setup, and then I hope to be able to systematically
measure the IMD effects of various delays in the envelope restoration,
and any phase modulation effects.
At this moment I'm away from home but connected to the web, so I have
been looking for parts for the big guy. I know suppliers for most
things, but some components are hard to find:
- Big ferrite cores. I usually buy my ferrites from Amidon or Bytemark,
but they don't have cores of the size required for this project. I need
a double-E core with round center leg capable of working at the 2kW
level at 100kHz (probably something 80 to 100mm wide), some slightly
smaller double E or RM cores for the chokes, and double-hole cores for
the RF stage that should ideally tolerate 300V rms per turn from 1.8 to
30MHz. Using cores from Amidon, I would have to do lots of stacking,
which is definitely not the best solution. I know that big cores exist,
but can't find a distributor willing to sell them in single quantity!
- Big porcelain chip capacitors with low ESR in the HF range.
- Silver mica or equivalent capacitors for the low pass filters. Values
of 82 to 3300pF are needed, with ratings of about 1000V and RF current
of 10A for some.
I would appreciate any hints as to where I can buy them!
Manfred.
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Visit my hobby website!
http://ludens.cl
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