[SADXA] PEP vs N Tone Average Power - Amplifier Tuning

Sat Mar 19 18:56:10 EDT 2016

Dick, 

My FT2000D exhibits a 50% duty cycle with continuous dits, without
programming.  This would be equivalent to a two tone test. In some cases,
this may not be a sufficient reduction in the Average power for a full 1500W
PEP output. 

Unfortunatly,  most low dollar wattmeters do not properly exhibit the
average power for pulse or N tone tests. My DAIWA Wattmeter shows only 30%
average power with 50% RF power duty cycle. 

Pulse tuning down to a 10% a duty cycle [for example - the ATP-100], can
have this same wattmeter issue. 

However, if any of these wattmeters are pre-calibrated, [for a given PEP
output, the average power ratio (error) is determined], then the inaccurate
wattmeter can still be used for accurate amplifier tuning. 

I noted that even in these days, there are still many new RF amplifiers,
that still require tuning. And then.. there are the refurbished SB200,
SB220, Drake L7s.. etc.  But, I agree, no tuning with a solid state amp is
nice. 

73s, Bill W7EXG

-----Original Message-----
From: SADXA [mailto:sadxa-bounces at mailman.qth.net] On Behalf Of Richard W.
Solomon
Sent: Wednesday, March 16, 2016 3:28 PM
To: 'Southern AZ DX Association E-Mail Reflector'
Subject: Re: [SADXA] PEP vs N Tone Average Power - Amplifier Tuning

Back in the day when I had an Ameritron AL-1200, I would just send 
a string of dits and tune for maximum output.

Today with the Solid State Amp, it's punch and go.

Times have changed.

73, Dick, W1KSZ

-----Original Message-----
From: SADXA [mailto:sadxa-bounces at mailman.qth.net] On Behalf Of W7EXG (Bill)
Sent: Wednesday, March 16, 2016 1:45 PM
To: sadxa at mailman.qth.net
Subject: [SADXA] PEP vs N Tone Average Power - Amplifier Tuning

Hi SADXA Group.. 

If you are interested in Key Down Tuning of your Linear Amp [without frying
your output devices], you might find value in my comments below. No Pulse
Tuning required. 

Most of you are probably familiar with 2 tone SSB testing:

Key Points:  1. Two Tones of different non-harmonically related frequencies
are used as input, but with identical amplitudes.
		 2. Distortion/Linearity can be directly observed with scope
voltage (V) measurements across the load resistor (R).  
		 3. Average output Power (Pavg) is 1/2 the PEP output power
P(pep), while testing. 

NOTE: Pavg is defined by V(rms)^2/R for any waveforms, which defines the
Joule Heating of a load Resistor (R). 

At the Last SADXA meeting, I had mentioned the theoretical relationship
between P(pep), and an N tone Pavg. 

Basically:
	P(pep) = N Pavg

For example, using 4 different frequencies, but identical amplitude tones, a
1500W pep output will produce a 375W average output power. 

Most 1500W output PEP amps, can tolerate 375W key down average output power,
during the 1500W PEP Output tuning process. 

Ideally, access to a peak-reading RMS calibrated Wattmeter would be of
value, since it should measure the RMS value of the non-sinusoidal waveform
as well, and thus the correct average output power. 

NOTE: Most Watt Meters [with an AM or FM designation], are usually accurate
in measuring Pavg, for the sinusoidal waveform. However, for non-sinusoidal
voltages, these meters often determine average Value of the waveform to
determine the so-called Average Power. The DAIWA CN-101L series has the FM
designation (only in the Manual).

For the 2 tone case, the Average output Power on THIS DAIWA meter will be
40% of the PEP output power (not 50%). [2.5 Pavg] = P(pep) For the 3 tone
case, [3.75 Pavg] = P(pep) For the 4 tone case, [5 Pavg] = P(pep)

In general, a simple power test on any given wattmeter, can determine the
ratio of PEP output power vs Pavg for N input tones for that meter.  This
ratio can then be used with the meter to tune the Linear Amplifier.  

I hope someone in the SXDXA will find value in these comments. I found this
method useful, in testing my Drake L7 Linear Amp to maximum potential, after
I completed the refurbishing of this amplifier. 

Best 73s, and happy testing/tuning. 

Bill W7EXG

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