[hpsdr] PWM / PDM

[W2XJ]

There 8is a trick you can use in some PDM circuits to solve the negative 
modulation problem. Go analog below the 6 or 10% point. Harris did this 
with their broadcast transmitters and it worked well. You can derive the 
analog voltage from the PDM pulses. This make the switch saturated above 
the pinch off point and a linear series regulator below it.



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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