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Re: Re: LF: PA problem

To: "RSGB LF group"<[email protected]>
Subject: Re: Re: LF: PA problem
From: "dhchurch"<[email protected]>
Date: Sat, 11 Nov 2017 17:02:28 +0000
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Hello Paul
Just a reminder, which you probably don't need, to be careful where the 'scope probe is connected, especially the earth/ground clip, and it's possible that even the 'scope earth/ground makes some difference to the measurements.
73, Hugh, M0DSZ
----- Receiving the following content -----
From: N1BUG
Receiver: rsgb_lf_group
Time: 2017-11-10, 22:37:32
Subject: Re: LF: PA problem

So as a final test today I put a 50 uH choke and then a bypass
capacitor on the power supply side of it.

With 100 uF I (oddly) found 2V pk-pk RF on the power supply. So it
is worse with the choke plus bypass capacitor than with just the
bypass capacitor alone???

With 330 uF it was better. With 1000 uF better still.

Finally with 2200uF there is only about 100 mV pk-pk RF there. That
is the best I have seen so far. The amp seems happy. It is producing
25W RF output which is the most I have seen. Efficiency comes out to
about 70% with bias well below the point of any idling current. The
drain waveform is hard to describe... looks a square wave with
rounded edges and some other irregularities... but at least there
are no big spikes.

Now there is no change when I insert / remove the DMM from the power
supply lead.

By the way I am driving it with a sine wave. Maybe tomorrow I will
try a square wave to see what that does.

Paul N1BUG

On 11/10/2017 04:53 PM, N1BUG wrote:
> 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

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