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From: Alan de G1FXB <G1FXB@yahoo.co.uk>
Message-ID: <a173e1c0-6881-3f35-84da-1d367bd43f29@yahoo.co.uk>
Date: Thu, 11 Jan 2018 22:59:34 +0000
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Subject: Re: LF: Progress? with the PA
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Hi All,
If you are using an LPF in front of the PA then regardless of U3 / U3S 
'variability you are doing everything right to ensure a clean 
fundamental only signal to drive the PA.

Not recently but I think it was on the QRP Labs forum, there were 
discussions about this PA before, other builders not getting 
reproducible performance and choice of O/P ferrite material if it wasn't 
you guys??
i don't think anything productive got posted, I think it got side 
tracked to pro's & con's of real RF transistors.

(Isn't one of downsides to using switcher FET's in a linear mode a 
propensity for them to Osc ?

(Sod's Law = Amplifier always oscillate
and Oscillator circuits never do reliably?? :-) )

There are some HF & at least one Hi power 6 Mtr (Swedish / German?) 
design using multiple stages of IRF510's on the web to perhaps 
plagiarise what they do to tame parasitics.)
Else a quick and easy trial, but not good if you are drive limited.
  To put an input pad in the front end of the PA circuit.

I'm out of my depth, really......

Alan

On 10/01/2018 22:21, N1BUG wrote:
> 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
>