[Elecraft] [P3] Power measurement with the P3

[Brett Howard]

Amplitude calibration is not yet implemented in the firmware.

As for SSB peaks do you have peak hold turned on when you are viewing
this?  You need to be looking at the signals with averaging disabled
(and preferably with peak hold on) in order to see the power peaks of
an SSB signal.

~Brett (N7MG)

On Fri, Aug 13, 2010 at 12:29 AM, Al Lorona <alorona at sbcglobal.net> wrote:
> I note my P3's amplitude readings are about 6 dB low-- that's with the IF output
> switched off. I'm not sure how to calibrate it.
>
> Even so, the P3 gives great insight into signals and their amplitude.
>
> Let's say you calibrated your S-meter to read S9 with a 50 uV input
> signal. Fifty microvolts is -73 dBm in 50 ohms.
>
> A CW signal which is S9 on your S-meter should peak at around -73 dBm (plus or
> minus 3 dB) on the P3. But an SSB signal which is S9 on your S-meter will not.
> It will be several dB less than -73 dBm on the screen of the P3. Why?
>
> A CW signal is one pure tone, and all of the power is concentrated in that one
> tone, whereas the human voice contains many different tones or frequencies
> within the 2 or 3 kHz bandwidth of the SSB signal. To measure the power in the
> SSB signal requires integrating or summing all of the power of all those
> different tones to come up with the total. Any one frequency falls far short of
> -73 dBm, but the total is far greater and should approach -73 dBm. This is easy
> to see on the P3.
>
> As an example of the above, if you had (10) -100 dBm in-phase tones in a 3 kHz
> bandwidth, the total channel power would actually be -90 dBm, 10 dB higher due
> to the summation. If the number of tones is greater than 10 as it can be with
> the human voice, the total can be even higher, that is, the difference between
> any single frequency and the total becomes greater. Fascinating.
>
> One final thing: my son is studying for his Technician license, and I plan
> to show him some real AM, SSB and CW signals on the P3 to help him see the
> difference. Do you suppose I can write the P3 off as an education expense? :^)
>
> Al  W6LX
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