[Collins] Interpretation of 30L-1 manual, sec. 4.7

Robert Jefferis jefferis at antelecom.net
Sun Feb 21 23:57:40 EST 2010


Yes, the instrument used is quite important:
1. OK, sure, if you have one of the "T" coax adapters to use with the ubiquitous HP AC probe used on the 410B and 410C meters, the reading is likely to be close to 159V. I guess this could justify the presence of the reference in the manual, even if they didn't mention a peak reading RF probe connection. Although, I believe that one of these probes will read a bit lower than 159V.
2. I agree, a good peak reading wattmeter will should read 506 Watts.
3. However, an average reading (actually, RMS - a misnomer) wattmeter like a Bird 43 or thermal (heating value) wattmeter should read half the power, i.e., 253 Watts. I just verified this with a pSpice simulation. Lazy man's way of calculating the result. The math is too tedious given the cyclic change of phase relation between baseband exciting sources and, you have to sort out an appropriate integration period.

Tnx for flogging the brain cells.  Bob KF6BC


On Feb 21, 2010, at 5:15 PM, Dr. Gerald N. Johnson wrote:

> On Sun, 2010-02-21 at 16:21 -0800, Robert Jefferis wrote:
>> Greetings all.
>> 
>> This afternoon I was reading 30L-1 manuals to clear memory 
>> cobwebs before adjusting a unit. I happen to have the 5th 
>> and 8th editions on hand. When I got to the "TUNE METER
>> ADJUSTMENT", section 4.7.g., I noted what I feel is a confusion 
>> factor. Once you have increased exciter drive to the point of 
>> distortion onset, you should see a minimum of 450 V p-p on the 
>> scope (if you use this method). The 450 V p-p minimum output 
>> voltage requirement is clearly associated with the peaks of 
>> the 2-tone RF waveform envelope. This would correspond to a
>> minimum output requirement of 506 Watts into 50 Ohms.
> 
> The envelope is not constant at that peak value and that affects the
> other instruments.
>> 
>> Since this check is done with 2-tone excitation, why would the 
>> RMS voltage, 160 VRMS (actually 159 if you run the numbers corresponding 
>> to 450 V p-p ) be stated in the manual ?
> 
> Because with a peak reading, calibrated in RMS, voltmeter such as the
> HP410B typically specified for such a measurement, will read the peak,
> not the heating power(RMS) or the average. And that matches the peak to
> peak value on the scope.
> 
>> Since my scope can only handle 
>> 400 V p-p without an external calibrated attenuator, I don't use the 
>> measurement method called for. Instead, I place directional couplers 
>> in the input and output lines of the amplifier to create an X-Y display 
>> on the scope. I can get much more reliable detection of distortion onset. 
>> I also place a wattmeter in the amp output line. My interpretation of 
>> all this is that the minimum power output with 2-tone excitation at 
>> distortion onset, as indicated by a wattmeter should be 506/2=206 Watts.
> 
> Depends. A peak reading meter should show 506 watts. An average reading
> meter will show less, but I think more than half. Probably about 70.7%
> of the peak since the envelope is a sine squared waveform.
>> 
>> Have I missed something?
>> 
>> Yes, I know that the Tuning Indicator can be problematic, but IMO, 
>> it needs to be set up correctly, regardless of how you tune the amp 
>> when in service.
>> 
>> Bob, KF6BC
>> 
>> 
> 
> 
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