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
73, Hugh, M0DSZ
Receiving the following content -----
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???
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
By the way I am
driving it with a sine wave. Maybe tomorrow I will
try a square wave to
see what that does.
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
> 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
> dirty. Seemed maybe it went into some lower frequency
> 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
> big choke. Maybe this is what you were saying, that a big choke
> not needed. I will try replacing it with just a few uH and see
> 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.