Rich Measures wrote:
>>When the FCC measures power, they use a regular peak reading
>>meter. .....
>
>? wanna guess how such meters are ultimately calibrated?.
>
Not against a 'scope, but ultimately against a terminating thermal
wattmeter which is the only system that is absolute and verifiably
broadband.
Scopes and all kinds of "through-line" wattmeters don't actually measure
power - they measure voltages and/or currents, maybe only proportional
to the voltages and currents in the line (and the ratios may vary with
frequency). Then either you or the makers of the instrument have to
calculate the power level - which requires certain assumptions.
For example, if you're using a 'scope, it requires the assumption that
the Y-amp and probe calibrations are both correct at that frequency, and
that you've read the peak voltage off the screen correctly (+/- the fuzz
on the trace). This isn't especially a criticism of the 'scope method;
all other methods have their own weaknesses and sources of error.
All this is for constant (CW) power. The peak-envelope-power function
is not an RF-related problem at all - it's a problem of accurately
finding and holding the peak of the modulation envelope, which in our
case is down at audio frequencies.
The definition of PEP does refers to "one cycle" but that means "one or
more identical RF cycles" as distinct from "the peak inside a single RF
cycle". In practice the peak of the modulation envelope contains many RF
cycles at that are substantially identical, because the modulating
frequency is thousands to millions of times lower than the radio
frequency.
With sine-wave modulation, you can go as much as +/- 5.7 degrees away
from the peak of the modulating envelope (along the X/time axis) before
the voltage drops enough to cause a 1% error in PEP. For 1kHz modulation
at 1.8MHz, this allows about 60 RF cycles that are all close enough to
the peak. A peak-reading instrument has this much time to grab its
rectified voltage sample, and if it is averaged over the whole period
the error in PEP is <1%.
I think that in practice the 'scope is no better at identifying the
modulation peak, because the user does the same averaging process by
eye.
The real problem is that there are so many meters out there with poor
peak-reading circuits. Some of them don't accurately grab the peak of
the modulation, and then some don't hold it for long enough for the
meter needle to respond accurately - or if they do, the total discharge
time is too long. A really good peak-reading converter would hold for
about one second after the peak, to give you time to read the meter, and
then discharge quite rapidly to allow the meter to respond again.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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