[Elecraft] [P3] Power measurement with the P3

[Al Lorona]

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