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
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