[hpsdr] PWM / PDM

[mike]

Hi Francis,

I am interested in what solutions you may have to the depth of 
modulation problem. I had been thinking (at least in my prototype) it 
was that the control loop error amplifier is not a "rail to rail" type 
and that it might be solved by offset biasing to keep the PWM in its 
most linear range.

I built my first solid state KW for commercial use around 1983 using 
MRF428s and the Motorola app notes. Pretty amazing work Helge Granberg did.

Very nice work you guys have done. I'll take another look at Steve's site.

I have a paper you might be interested in (not on this machine though) 
by Su and McFarland of HP describing a delta modulator that is supposed 
to get around this problem. It is intended for low voltage on-chip 
application, but the conceptual block diagram might be of use scaled up. 
You should be able to find it with a google search if you're interested.

Keep us posted on the buck modulator progress. Sounds pretty cool.

73,

Mike
WO1U



FRANCIS CARCIA wrote:
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 >
 > Hi Guys, I've been playing with PDM am since the early '80s and would
 > be glad to answer questions provide information links to anyone who
 > requests them. There is a basic problem with pdm and any switcher at
 > low duty cycles. The filter after the switch isn't perfect so at some
 > point a pulse goes in but no DC comes out. This is usually around 10
 > % modulation and lower when there is a step of distortion. My current
 > mode modulator in my 160 meter AM rig does a bit better close to 6 %
 > but I still see the step. I'm working on a modulator to support my
 > big 75 meter class e final motivated by a pair of 2.5 KVA isolation
 > transformers. The high voltage switching part is actually fairly easy
 > with isolated drivers or opto isolation and floating circuits. Check
 > out WA1QEX's web site on class e transmitters and modulators. Steve
 > has collected a pile of stuff over the past 10 years and sells boards
 > for different projects. I am presently looking at an interleaved buck
 > design using a TI chip. I'll be glad to share the design after it is
 > working. I've built the CCI- MRF 429 300 watt final for a friend and
 > it worked well. Efficiency went down a bit on 160. I think the
 > transformers were running out of steam. Quality stuff otherwise.
 >
 >
 >
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