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From: N1BUG <paul@n1bug.com>
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Date: Wed, 10 Jan 2018 17:21:52 -0500
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Subject: Re: LF: Progress? with the PA
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Hello Chris, Nick, and thanks Alan for your additions to this 
discussion.

> Should  this  amp be driven by sine waves?? I assumed you were driving
> it direct from CLK0 of the Si synthesizer in a U3S, with square waves?

As was pointed out to me earlier in this process, this was intended 
to be a *linear* amplifier, operating class AB and as such, should 
be driven by a sine wave. I've pushed mine into class C by 
adjustment of bias, but it is still not like a class D or E 
amplifier which may do better when driven with a square wave.

The designer noted that one might expect 25 watts out with 100 
milliwatts drive, and up to 50 watts if more drive is available. 
This is consistent with my result of seeing 50 watts with 250 
milliwatts drive.

Even if it were a sine wave, the Clk0 signal is, I believe, around 
+10 dBm or 10 milliwatts. I would expect that to give no more than 
two or three watts out of the amplifier.

Alan pointed out that there may be differences between the output 
spectra of the U3 and the U3S. I don't have a U3 so I can't check 
that. I presumed Toni intended his linear amp to be driven by a sine 
wave and that the high level output of the U3 was a sine wave after 
low pass filtering. I further presumed that his recommended 
modifications to the U3 were to get enough drive power to satisfy 
this amp and not for any other reason such as adjusting spectral 
purity. However this is all rather presumptuous of me and I really 
don't know the answers to some of this.

Mine seems to care little about what drives it (I've used multiple 
sources, some with 'cleaner' sine waves than others). It cares a 
great deal about output load and configuration of the output 
transformer, whatever that may say about it.

I agree that one should forget this amp and go with class D or E 
straight off. I chose this because my budget was extremely limited 
this year and I thought this looked like a very affordable way to 
get running on two bands. It has turned out to be more trouble than 
it was worth in my case.

73,
Paul N1BUG
(Still transmitting 2200m WSPR but this is probably my last night 
for some time with terrible weather moving in -- heavy rain, then 
ice, then into extreme cold before the ice can melt off the antenna)


> Wednesday, January 10, 2018, 4:22:39 PM, you wrote:
> 
>> 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