On 09/01/18 20:33, Alan de G1FXB wrote:
> Hi Paul , Nick and any others using that PA design,

Just as a reply to another message - yes a dummy load in perfect up to
silly voltage

I had not really thought that the reactance 'off frequency' might be an
issue but I can see it could be a possibility

> (I 've being subscribed to the RSGB LF feed for 4-5 years ? and of
> course seen reference to the blacksheep feed but interpreted that as
> being a legacy rather than where I recently I subscribed and found to be
> where the activity is.
> I make it brief  in-case I'm so wide off topic I'll fall of the edge of
> the planet, and NOT to sound like I know what I'm talking about....

Well, you may know more than me ...but I am trying to learn :-)

> What MAY explain that circuits instability WRT to antenna loads as
> opposed to a 50 Ohm load)
> 
> Tony's design
>  http://qrp-labs.com/ultimate3/u3mods/lfamp.html
> was made around the era of the QRP Labs U3 using the AD9850 module with
> a sine wave + associated DDS spectra through the BS170 PA
> IF you are using the current U3S design that uses the Si5351 synthesiser
> it is driving through the untuned PA a square wave with a very different
> spectra.
> I think Hans has described HIS PA as just being untuned  broadband in
> the sine wave fed U3
> and 'Class D in the biased adjusted U3S design, not class E as there are
> no frequency dependant components.

Ah!, yes I have the OCXO

> To further blur the situation on the referring page,
> http://qrp-labs.com/ultimate3/u3mods.html#lf
> Tony details coupling capacitor value changes, which are now used in the
> U3S design?? But also a change to what will be your driver stage ferrite.
> So his PA would probably be driven by this modified U3?

OK the U3s is stock, other than 3 x BS170s and the ability to use 5V or
13.8V for the PA stage.

> (Everybody has their own goals, favourite topology and most importantly
> junk box....
> If it were a clean sheet of paper and you wanted a class E PA using a
> common switcher FET device, rather than an RF device.)
> Work with the fact the Si5351 outputs square waves, use that to drive
> the final stage rather than through an (RF) driver stage.
> If you need a pulse shaper or just like the idea of a buffer stage, then
> do it at logic switching levels in the PA Gate rather than RF.

Ah, a small glimmer may be about - I do have the LPF in the U3s so I
'should' be driving it with a sine wave.

However the fact that an increase in drive level means less tendency to
oscillate and lowering the drain voltage improves things - looks like it
is drive related, at least in part - but of course FET gates are not a
static impedance thing.

I accept that the output ferrite may need to be adjusted - I need to
look at G4JNT's article from 02/2013 where, whilst he is talking about a
push-pull arrangement, there is some maths around impedance matching to
input & output of the FET(s)

> Hopefully not being interpreted arrogant interloper. :-[
> regards Alan
> 

Not all, I am happy to pick the brains of anyone, the more the merrier
as far as I am concerned.

All the best

Nick
M0HGU

-- 
Mrs Evadne Cake was a medium, verging on small.
(Reaper Man)
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