[Elecraft] Elecraft K2 Power Readout

Wed May 21 12:40:18 2003

Even if you only have a low frequency scope, you can read the Peak voltage across the dummy load by using a diode detector.  The
voltage from the diode peak detector can also be read with a DMM.  In this case the power into a 50 ohm dummy load is:

Power_out = Vpeak*Vpeak/100

Use a low power diode like the 1N5711 schottky or the 1N4148 silicon computer diode.  (Don't use a
power diode like the 1N4007 or another member of that family.)

The Vpeak reading can be corrected for the small diode drop by adding 0.3V for a schottky diode or 0.7V for a
computer diode.  Even without this correction, the power reading will be off less than 5% at 5 watts when using a schotty diode.

By running the diode into a 0.001uf cap with a 10K resistor in parallel with the cap, the peak detector will follow the
envelope of the RF so you can see the modulation or CW waveform, even with an audio scope.

73/ Bob - AG5Q

----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Wednesday, May 21, 2003 10:09 AM
Subject: Re: [Elecraft] Elecraft K2 Power Readout Discrepancy

> Another measurement alternative... use your oscilloscope!
>
> Assuming you're feeding a 50 ohm dummy load, measure the peak-to-peak
> voltage (Vp) of the (hopefully) output sine wave while transmitting "key
> down".
>
> Power = (Vp * Vp) / 400
>
> This depends on a calibrated vertical scale on your 'scope.  Also, take
> into account any multiplication factor of your 'scope probe:  X1, X10,
> etc. when calibrating the vertical scale and reading the voltage.
>
> This method falls apart if the output wave is complex (not a sine wave),
> or the dummy load is not 50 ohms.  The underlying formula is derived from:
>
>         Power in watts = Vrms * Vrms / resistance across which the voltage
> is measured
>
> and
>         Vrms = 0.707 * (V peak-to-peak / 2)
>
> Steve
> aa8af
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