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From: Alan de G1FXB <G1FXB@yahoo.co.uk>
Message-ID: <0aceecaa-9b43-d255-316e-5fa994e01f11@yahoo.co.uk>
Date: Tue, 9 Jan 2018 20:33:15 +0000
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Subject: Re: LF: Progress? with the PA
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Hi Paul , Nick and any others using that PA design,

(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 briefĀ  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....

What MAY explain that circuits instability WRT to antenna loads as 
opposed to a 50 Ohm load)

Tony's design
 Ā 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 untunedĀ  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.

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?

(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.

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


On 09/01/2018 17:36, N1BUG wrote:
> Nick,
>
> I was very interested to read your email. I am sorry that you are also 
> having problems with this PA but at least it isn't just me. A few 
> comments threaded in below...
>
>> I've been, sort of, following this as I have a similar problem with the
>> amp at <http://www.qrp-labs.com/ultimate3/u3mods/lfamp.html> I have not
>> used the 2n4 cap to ground on the gate (yet) but I have added a series
>> resistor of 15R to the gate prior to the DC blocking cap (or should I
>> have put it after the DC blocking?)
>
> I put my 15R directly at the gate.
>
>> With out the 15R it was very fussy and would go into, what I presume,
>> was oscillation - the voltage trace on my scope match was 'fuzzy' -
>> looked like AM? with almost no provocations ... however lowering the
>> supply voltage cured this.
>
> That is similar to my experience. There are two different scope 
> patterns I see when it starts acting up. Sometimes it will be one, 
> sometimes the other.
>
> One is as you describe, the voltage trace goes fuzzy, more so at the 
> peaks than at the zero crossing. This will range from mild fuzz at the 
> peaks to extreme where it is fuzz from the peak to the zero line.
>
> The other is different. I usually have my scope set to fit about two 
> full cycles (720 degrees) on the display. Sometimes the first and 
> second cycle will be of drastically different amplitude even though 
> both are crisp traces without any fuzz.
>
>> I have just re-worked the matching unit at the aerial and now have an
>> almost perfect 50R j0 match. so I know this is not a reactive issue
>
> Have you tried it into a 50R dummy load? Mine behaves perfectly into a 
> dummy load but not into the antenna, even though the antenna is tuned 
> for 50R j0. I am thinking the antenna being reactive *off* the 
> operating frequency sets off the oscillation.
>
>> It still oscillated if I run it at 13.8V and drive of around 250mW (U3s
>> 3 BS170s at 5V), drop the voltage to about 11V and all is well.
>
> Yes, lowering the voltage makes mine more stable also.
>
>> Increase the drive to about 800-900mW (U3s 3 BC170s at 13.8V nominal)
>> and I can wind the supply up to a little over 12V before it goes wild.
>> The bias is set to give about 75mA at idle.
>
> Mine is far more stable when I run the bias down into the class C 
> range (no idling current). For any given voltage what I usually do is 
> start with the bias at zero (wiper of the pot at ground end), apply 
> drive and turn the bias up until it goes into oscillation... then back 
> off a bit below the point where it stabilizes again.
>
>> If I reduce the bias & therefore idle current then the oscillation is
>> earlier & easier to provoke,
>
> Unless I am misunderstanding, mine is the opposite. It is more stable 
> when the bias is set for class C (well below the point of having any 
> idling current).
>
>> if I go much over about 150mA the FET takes
>> off in thermal runaway with the current going skyward with no change in
>> bias setting - so a little under 100mA it is.
>
> Mine does that too.
>
> > However if I give the amp a capacitive load it is much
> > happier ... and will take over 15V and no oscillation - that
> > ought to tell me something but I'm not sure what
>
> My amp is no longer like yours as I have 4 x 10 turns on the 
> transformer now, but yesterday I noticed it is happier operating 
> slightly below the antenna resonant frequency than dead on or above. 
> So I believe I am seeing the same thing.
>
> I think the output transformer is at least part of the problem. 
> Changing it in some way seems to have more affect on the instability 
> issue than any other change I have made in circuit. I've also been 
> told by those with the ability to model them that the low pass filters 
> for this amp are terrible designs. The specifics went over my head.
>
> Paul N1BUG
>