|[AMRadio] Re: MODULATION POWER REQUIRED|
garyschafer at comcast.net
Fri Feb 15 16:20:36 EST 2008
The average VOLTAGE of a sine wave is meaningless. You don't use it to
calculate average power. We are looking for the RMS value.
The RF signal is always composed of sine waves no matter if there is
modulation or no modulation. It also doesn't matter what the wave shape of
the modulating signal is either, tones, voice or whatever, the RF coming out
of the transmitter is still going to be sine waves. If it were not there
would be serious harmonics generated.
When we are talking about PEP and the "average power over one or more
cycles" we are talking about the RF cycles of the transmitter. Each part of
the audio envelope, even the narrowest peaks, are going to be composed of
(contain) many RF cycles in each of those peaks. Being that all of those RF
cycles are pure sine waves the rules of .707 times the peak voltage to find
RMS voltage will always apply. The amplitude of those RF cycles will vary
with the modulation envelope level. To measure PEP we pick a point that is
the highest part of the modulation envelope (peak) and there will be many
many pure RF sine waves contained there. We want to find the average power
in those sine waves at that particular time.
If you have a fast enough scope you can see exactly what is happening. Look
at a transmitter modulated with a tone and you will see the familiar
modulation envelope. Spread that way out by increasing the sweep speed and
turn up the brightness and you will see all the RF cycles under the
modulation envelope. Those are all pure sine waves and will always be.
A 3.8 MHz signal with 1000 cycle modulation on it will show a 1000 cycle
modulation envelope and contained in that envelope there will be 3800 RF
cycles. You can break that down further and say that half of that audio
envelope will be positive and half negative. So then the positive half will
have 1900 RF cycles. We could break that down further and find how many
cycles are near the peak if we wish. But the point is they are all RF cycles
and good sine waves on which power is measured.
If you are modulating the transmitter with speech the RF power still comes
out in perfect sine waves regardless of the modulating waveform.
If you were trying to measure audio power at the audio frequencies in this
manner then it would not be pure sine waves with speech.
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jim Candela
> Sent: Friday, February 15, 2008 9:54 AM
> To: amradio at mailman.qth.net
> Subject: [AMRadio] Re: MODULATION POWER REQUIRED
> It seems that we have defined PEP power pretty well:
> >"Well, PEP is defined as the AVERAGE power over at least one RF cycle at
> > the most powerful point of the envelope."
> Now, since we do not speak with sine waves, the average "power point of
> the envelope" is going to be less than 0.636 (sine wave average) of the
> peak value. With many voices the average might be 0.2 to 0.5 of the peak.
> Doesn't this mean we can increase the peak power until the PEP as we have
> defined it hits 1500 watts? It seems that many of us confuse peak power
> with Peak Envelope Power. The definitions are different. If I have this
> correct, then unprocessed voice peaks can be increased until the PEP legal
> limit is achieved, and the carrier level might be a lot more than 375
> watts. Maybe the KW1, or Johnson Desk KW at Hi-Tap are still legal?
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