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From: N1BUG <paul@n1bug.com>
Message-ID: <cba057fa-9d9b-5feb-21b8-049a4386c4f9@n1bug.com>
Date: Fri, 10 Nov 2017 16:53:53 -0500
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OK. I understand the problems with having RF reach the meter and 
power supply.

To be honest I don't fully understand what you are suggesting with a 
very small coil (few uH at best) and capacitor...

On my own lack of knowledge I put a 1.2 mH choke in the DC supply 
lead at the amplifier. This made very little reduction in RF I was 
seeing on the DC side of the choke!

So I put a big (1000 uF) capacitor to ground from the DC side of the 
choke. That removed all the RF there but make the amplifier output 
dirty. Seemed maybe it went into some lower frequency oscillation. I 
tried 330 uF, which seemed to make it dirty for a couple of seconds, 
then it cleaned up. With 100 uF it is always clean and there is very 
little RF component on the DC supply lead.

It seems almost as good to have the 100 uF capacitor but remove the 
big choke. Maybe this is what you were saying, that a big choke is 
not needed. I will try replacing it with just a few uH and see what 
happens.

It's supposed to be a class AB2 amplifier I guess, but when set for 
any amount of idling current, efficiency is extremely poor. About 
30%. If I set the bias lower, well below the point where it draws 
any idling current, efficiency becomes about 80%. As I understand 
it, the circuit is not correct for class E but that seems to be how 
it wants to run.

73,
Paul


On 11/10/2017 02:35 PM, DK7FC wrote:
> 
> 
> Am 10.11.2017 19:44, schrieb N1BUG:
>> I see a nice clean sine wave at the other side of the LPF.
> With a good LPF, the output often looks quite good on a scope, even with
> massively distorted input signals.
>> But I see a lot of RF (36 volts peak to peak) at the +13.8V power
>> connection to the PA!
> Yes and your power supply doesn't like it, it can be damaged from the
> high RF peak currents. Or the voltage regulator can do crazy things. And
> BTW the current measurement is likely to be totally wrong with such an
> AC component. With the coil you can decouple the AC component from the
> supply, at least most of it.
> Just take a piece of enameled cu wire, 1mm diameter, and wind a coul
> with 15 turns on something with 15mm diameter. Or use a T106-2 core for
> example. Of course the big C is needed as well. For the 1 uF cap i would
> use WIMA MKP-10 or so. Without calculating i would say that this
> improves things significantly.
> 
> 73 and have fun,
> Stefan
> 
>