Re: LF: TXing 2200m WSPR

[Andy Talbot <andy.g4jnt@gmail.com>]

Well ...    Using exactly the values in the filter circuit diagram, 50R transforms through the filter to 48.5 - j2.86 (Ret Loss = 30dB,  VSWR = 1.07)   [Using GM3SEK's original Netcalc prog.]

So that's pretty conclusive the ideal filter values will not be upsetting things at the fundamental frequency.    

According to Google, the  T106-2 has a stated Al value of  13.5nH /turn^2  so 72 turns does indeed give 70uH.   So IF your core is correct, the filter should be OK.

It's a bit difficult from now on, at a distance, to try to work out what is happening.

Anyone else, any suggestions ?

BTW ...

Peak to peak of a { symetrical }square wave needs to be multiplied by 4/pi to get the peak-to-peak of the fundamental component.   So the amploitude you see will be lower by about 1.3 times for teh same fundamental power component. 

Andy  G4JNT

On 24 December 2017 at 21:22, N1BUG <paul@n1bug.com> wrote:

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