[AMPS] Peak Power Meters - some more thoughts

[Jon Ogden]

>Strictly, it's the PEP sum of two tones that is not twice the the power of
>each tone. The RMS power (i.e. power turned into heat) is another matter,
>because of conservation of energy. The two are very different. When you go
>to more tones, then it gets interesting. With 10 tones, the PEP that you get
>happens rarely, and an amp with lots of carriers can actually be pushed to
>what should be a higher than rated PEP because the peaks occur so rarely all
>together that the resultant IMD power averaged over the measuring period is
>low - although the IMD PEP is what you'd expect. This is found in cellular
>radio.

Correct, Peter!

Back in my days of building feed-forward linears at Motorola, we would 
run 20 tones into our amps and test for an IMD level of minimum -60 dBc.  
Typically we were around -70.  Modern amps today such as those sold by 
companies like Powerwave can easily accomplish -75 or -80 dBc.

However, these numbers were measured using AVERAGE IMD measurements.  If 
we took and measured the peak IMD the amplifiers would only give us about 
-40 dBc.  With roughly a 10:1 peak to average ratio and a composite 
output power of 150 Watts average, that made the peak 1500 Watts.  These 
beasts did NOT have enough silicon in them to stay super linear during 
those peaks (hey they already drew 80 Amps at 27 Volts DC! -  > 2KW DC 
input!) and were only about 2% efficient!  However, as one very seasoned 
engineer pointed out, the occurance of the peaks is so rare and 
short-lived that in terms of user noticeable signal degradation, you 
won't hear any.  So that's why we measured average IMD.

I had a customer a couple years ago who was doing a linear feed-forward 
PA and trying to accomplish -60 dBc at peak power.  They couldn't figure 
out why it wouldn't work.  I had to educate them!

Oh well...fun stuff!

73,

Jon
KE9NA


--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!


Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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