[AMRadio] Modulation power required

Bob Macklin macklinbob at msn.com
Thu Feb 14 17:18:55 EST 2008

Why not just calibrate a Trapezoid display for MAXIMUM PEP? The just watch
the trapezoid display?

Watching a single cycle on a scope is a difficult thing to do.


Bob Macklin
Seattle, Wa,
"Real Radios Glow in the Dark"
----- Original Message -----
From: "John Coleman" <jc at pctechref.com>
To: <garyschafer at comcast.net>; "'Discussion of AM Radio in the Amateur
Service'" <amradio at mailman.qth.net>
Sent: Thursday, February 14, 2008 1:55 PM
Subject: RE: [AMRadio] Modulation power required

Perfectly and beautifully said Gary, except for one thing and some may not
have caught it.  I make this sort of mistake all the time so I'm glad I'm
not the only one, HIHI.  I hope you don't mind my adding in a little here
just for clearing up a little detail.

Original paragraph said:
 "Well, PEP is defined as the AVERAGE power over at least one audio envelope
cycle. So we need to use the average power of the carrier at least. Average
power is derived from RMS voltage so we use the RMS voltage of the carrier."

The above statement is defining the average power over one audio cycle would
be the 2 sidebands power plus the carrier power as you said before.

For PEP it should read:
"Well, PEP is defined as the AVERAGE power over at least one RF cycle at the
most powerful point of the envelope."

I have been asked this so many times so I feel compelled to expand on it.

While looking at a envelope display on the scope and picking a place on the
screen where the envelope is at the tallest peak then expand your scope
horizontally to a mile wide display so as to see one or two RF wave forms at
the place where the envelope peak was and measure the average power of the
one or two RF waves at that point.

Since we can't expand the scope to a mile wide display we must calculate the
values as Gary said and imagine what we might see.

John, WA5BXO

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