[AMPS] Peak Power Meters - some more thoughts

[Tom Rauch]

Hi Jon,

> We used HP power meters in our labs and with one CW tone (say at 5 dBm),
> the meter would read one power level.  If two tones were measured such
> that the total power out was stil the same (each tone would be 2 dBm -
> half the power of 5 dBm), the meter would read differently (I forget
> wether it read higher or lower - I think it was lower).  This is due to
> the way in which the envelope of the signal is detected.  Note: These were
> VERY expensive power meters.

Then you should have trashed them, regardless of price. 

Maybe what has confused you is the power sum of two tones is 
NOT twice the original tone. 

For example in a two tone test the sum of the two equal tones is 
six dB greater than a single tone, not 3 dB. That's why, if you look 
at the ARRL IMD settings, they set the individual tones six dB 
below the reference PEP to read IMD below PEP. PEP is not RSS, 
or RMS. 

I would hope your expensive HP meters said the same thing, or 
they were wrong. 

A  $ .20 diode, a 1 mA meter, and a resistor will read RMS power 
correctly across a given load impedance as long as you stay out of 
the non-linear range of the diode.

If you add a energy storage component, and insure the source 
impedance is many times less than the storage impedance and 
the load impedance is many times higher, the meter will read 
within a fraction of a percent of the true peak envelope power.   
 
> An SSB signal is in effect a much broader signal (than a CW signal) 
> almost equivalent of a multi-tone environment.  The envelope detection
> circuitry on the power meter just might not be able to do a good job of
> detecting peak power that is broader than a CW signal.

A typical meter responds to signals from 1.8 to 30 MHz with 
reasonable accuracy. The fact a separate tone appears .019 
percent (or much less) higher or lower in frequency has nothing at 
all to do with the problem.

The problem is pretty simple and straight forward. Why 
overcomplicate it? 

By adding a simple 2N3904 buffer to a relatively poor performing 
meter, the meter can be made to read within a few percent of true 
PEP instead of the pathetic original reading it produced.

There is no excuse for selling a PEP meter that reads peak power 
incorrectly, except cheapness on the part of the manufacturer.
   
> IMHO, the best way to read the actual peak power is to read it with a CW
> signal.  This is what I always did in my design days.

Then you would have no idea what the peak power is when you 
change modes.


73, Tom W8JI
w8ji@contesting.com

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