Hi Andy,
Thank you very much for that. I am learning and knowledge sometimes
comes together from bits and pieces in discussions like this.
Anything is possible with this amp or the filter.
I tried without the filter but as it is now a somewhat spiky square
wave I have no idea how it compares power-wise. It did appear to be
lower. Except for a short spike at the leading edge of the positive
pulses the pk-pk amplitude was much less than with the filer.
This is the filer, for what it's worth:
http://n1bug.com/n1debug/ASB_LF_LPF-20171224.jpg
You may have helped me understand something I saw the other day. One
of the .01 capacitors on the output end of the filter became
disconnected and I saw more power out of the amp. Perhaps that was
due to an impedance change.
Paul N1BUG
On 12/24/2017 03:44 PM, Andy Talbot wrote:
The scopematch circuit looks reasonable. Arithmetic is correct
(as an aside, I always group constants together for calibrations
like that. Measuring peak to peak across a 50R load, power then
becomes Vpk-pk ^ 2 / 400
When my 30dB power attenuator is in circuit this becomes P = 2.5
Vp-p ^ 2 You could do the same grouping and simplifying for your
scopematch dividers and Vp-p readings)
Back to your amp ...
You get 25W into 50R going via the low pass filter
You don't show your low pass filter circuit.
It is possible that has the wrong values for 50R and is providing an
impedance transformation from your 50R true load to present the
amplifier with a lower RL, thus allowing more output
Terminate the amplifier directly with the 50 ohs load and see what
you get - so eliminating the filter
As Sherlock Holmes said "Once you have eliminated the impossible,
whatever remains, however improbable, must be so"
Andy G4JNT
On 24 December 2017 at 20:12, N1BUG <[email protected]
<mailto:[email protected]>> wrote:
Hi Andy,
This is very interesting, because 12 watts is what I think I
have consistently been able to generate into the antenna before
problems creep in.
I can easily accept that this amplifier may only be capable of
12 or 13 watts.
What I don't understand is why I think I am seeing a clean 25
watts into a pure resistive 50 ohm load. Can someone please
check my math?
I'm running the output of the amplifier through a low pass
filter, then a scopematch, then into a high quality 50 ohm load.
My scopematch circuit is here:
http://n1bug.com/n1debug/LF_ScopeMatch-20171224.jpg
<http://n1bug.com/n1debug/LF_ScopeMatch-20171224.jpg>
Note that it is configured such that on the current sense
output, 1V=1A and for voltage, 1V=50V.
Running into a pure 50 ohm resistive load I am seeing exactly 4
divisions peak to peak on the scope (2 divisions above center, 2
below).
500 mV/div * 4 divs = 2.0V peak-peak or 0.707V RMS.
0.707 * 50 (1V=50V on the scopematch) = 35.4V RMS.
35.4^2 / 50 ohms = 25 watts.
Where am I going wrong?
As a check on scopematch calibration I set my HP 3325B to 10V
peak-peak, ran it through the scopematch into the same 50 ohm
dummy load at 137.5 kHz. I set the scope to 50 mV/div and it
read exactly 4 divisions peak-peak = .2V * 50 = 10V which seems
to verify. Admittedly this verification is at a much lower power
level.
73,
Paul N1BUG
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