COPY OF EMAIL FROM IAN G3SEK TO ME FROM THE OTHER DAY:
Hello Luther
>To summarize what I understand (please dissect, clarify, correct and
reply):
>
> The crest has nothing to do with the amount of power or the cycle,
>itself. It has to do with identifying a cycle to measure.
You got it!
>So, first locate
>the crest - in fact, locate the highest crest that is transmitted in order
>to measure for legality.
>
> Then, within that crest pick the cycle with the highs voltage peak
>and measure its power.
>
> As a practical matter, that crest will consist of thousands of
>cycles (e.g., if the crest is 1/10th of a second in duration and one is
>transmitting on 3.795 MHz then there would be something like 379,500 FR
>cycles within that crest). And, most all of the cycles within that crest
>would have the same voltage peak. Right so far?
>
It's probably much shorter than 1/10th of a second. The narrowest,
highest peak could be at the upper frequency limit of the modulation.
Say that is 3kHz, so one audio cycle will be about 333 microseconds in
duration. If it's a sine-wave modulation, the crest value will hang
around within 1% of the absolute maximum for about 9% of the time (the
top of a sine-wave is pretty flat) so that is 30 microseconds. At the
crest of the modulation, that gives you about 111 cycles of 3.795MHz RF
that are all within 1% of the same peak (and RMS) value.
So, it isn't quite as many RF cycles as you thought, but still plenty to
hang a measurement on!
> So, to obtain power of that cycle, just apply a power formula to
>.707 peak voltage - - I guess is that is because we can't hear the highs
and
>lows of a cycle - we can only hear the average of it - it occurs too fast.
>Is that right?
>
Well, it's more that the power in an AC/RF waveform is DEFINED as being
averaged over a complete cycle, or several identical cycles. (You can
*calculate* "instantaneous" power at any moment within a single cycle,
but it doesn't have any physical meaning.)
> It appears that based on the above a cw signal's PEP would be
>computed in the very same way. So, if I tune my transmitter for max power
>with a cw tone and view it on the scope and measure it to be 100W - and
>then - I switch to SSB and adjust the mic gain so that some of my voice
>peaks reach, but do not exceed, the cw level, then those particular peaks
>will have a PEP power of 100 watts, also. Right?
>
That's right. However, the 'scope is mainly your way of identifying the
biggest cycle of *modulation*, and you don't even need a calibrated
'scope to do that. You can measure the CW power by many other ways, most
of which will be more accurate than measuring V_peak using a 'scope.
> If that is the case, then my average power out during the duration
>of one syllable will be something much less - depending on my voice, etc.
>That, of course, is one explanation as to why a cw tone can be heard
>better - it is always at the same PEP. Even so and most of the time SSB
PEP
>and CW will be equal for legal measuring purposes - unless I have (for
>example) a transmitter that is capable of transmitting more power in the
SSB
>mode than in the CW mode. Right?
>
That's correct. And most power amplifiers will generate slightly more
power on SSB because it tends to pull the HV rail down less than CW.
> I have now noted that the ARRL, as per the glossary in the 1995
>handbook, defines PEP using the FCC definition. Is that because that is
>what PEP is and only is?
>
Yes, that's EXACTLY it!
> What would be the terminology for power at the crest computed using
>the voltage peak value (not just 70.7% of it)?
Peak instantaneous power (see above).
> Is that what some
>manufacturers use to define PEP for marketing reasons? Is that how we
>computed PEP in the old days - when legal power was based on input power
>instead of output power?
>
Nobody ever has, as far as I know. Amp manufacturers may make
exaggerated claims about the achievable PEP, but multiplying PEP by 2 is
not one of the methods they use.
> I think I am beginning to see more of this now - - back when
>measuring input power, if a transmitter could withstand a cw signal of
1000W
>then it could handle roughly twice as much SSB power because only a few of
>the crests would be at the 2000W level. If one could maximize SSB under
>those rules - he could legally put out a maximum SSB signal that would be
>close in average power to the CW limit. That is not true at all today -
one
>could only approach that under today's rules by maximizing compression with
>corresponding increases in drive (limited by intelligibility). Anyway,
that
>is where the idea that PEP is 2xCW power - which isn't and never was
>correct.
>
Something like that. In the USA, I believe it was also historically
tangled up with power limits that were 1kW DC input on CW or AM, or 2kW
INPUT on SSB as indicated on the plate current meter (with some weird
rules about time constants). Later, all modes were rationalized to 1.5kW
PEP output.
> Hopefully, my last question: I set my xcvr drive so that my voice
>peaks hit the same level on the scope as full output CW (100W). Why then
>does my PEP reading wattmeter never read above about 50 or 60 watts? I
know
>that I am transmitting lots of crests that are at the 100W level.
Most PEP reading wattmeters are actually very poor at grabbing the peak
value of the modulation waveform. The electronic part is like fast
attack / slow decay AGC response, but the meter needle is usually too
slow to reach the true peak reading before it starts to decay again.
> I think
>that it must be giving me an average of my PEP output.
That's about it - the electronics may grab the PEP, but the meter then
averages it down.
> Based on all of the
>above, that meter is of no use to me in determining whether or not I am
>legal.
Probably not...
>Help me out here.
>
Well, you can help the meter along by cranking the compression way above
normal, in order to get a steadier reading - so long as you remember to
back it off again! The PEP will probably still not be exactly the same
in both cases, but you've done all you reasonably can.
Remember that the FCC's peak reading power meters are probably no better
than yours!
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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