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From: N1BUG <paul@n1bug.com>
Message-ID: <b26ad024-2d39-7f2d-38eb-84a8ad89b383@n1bug.com>
Date: Sun, 24 Dec 2017 16:22:08 -0500
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Subject: Re: LF: TXing 2200m WSPR
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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 <paul@n1bug.com 
> <mailto:paul@n1bug.com>> 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