Dennis12Amplify@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/
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