[Amps] RMS VS Peak VS Average power and the Bird 43 wattmeter

[David Kirkby]

Dennis12Amplify at aol.com wrote:

>For the EXPERTS.....,
> 
> I have a question which I believe will combign the two most  interesting and 
>informative threads we have had going here on the reflector  for quite some 
>time.
> 
> Why is it that a Bird 43 with a calibrated peak reading kit only  reads 
>THREE times the power for a peak reading of a 100% modulated AM  waveform, instead 
>of FOUR times the power as the textbooks all say it  should?
>
My first thought is to wonder whether the electrical output terminals of 
the element are capable of responding quickly?

The element will no doubt consist of a diode to rectify the RF, a 
resistor to develop the current into (otherwise the diode is not going 
to work), then *possibly* some capacitance to smooth this before passing 
it to the meter. If I were the designer of the element, when I thought 
the idea was to feed a physical meter, I would add some capacitance, to 
reduce any chance of RF getting into the circuit on the 'DC' side. The 
designer had no idea someone was going to come along with a peak reading 
kit years later.

I would guess the rise time of the physical meter (with its mechanical 
movement) is of the order of a 100ms or so. Now if the rise/fall times 
(from 10 to 90%) of the elements are even 100x faster (1 ms) an after 
market kit (or the fastest scope on earth) is not going to see the peaks 
properly throughout the audio (to 3 kHz) range. Only if the rise/fall 
times of the element is around 200us or less can you reasonably expect 
the element to be capable of accurately reproducing the peaks of an 
audio (to 3 kHz) signal.

Secondly, how does the calibrated kit work? Does it simply hang across 
the meter terminals, then you push a button and it displays what the 
peak value was? If so, the inductance of the meter will limit this.

Also, have you verified with a scope that 100% modulation is being 
achieved?

If you keep the modulation depth constant (as verified with a scope), 
does the kit perform more poorly as you increase the frequency of 
modulation? That would indicate the time constant of the elements is not 
fast enough - unless of course the kit is so badly designed that the kit 
limits the performance.

So I have no idea why the kit does not work as you expect, but can think 
of many reasons why it might not.

-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/