Hello Nick and Paul N1BUG
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?
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
--
Best regards,
Chris mailto:[email protected]
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