[AMRadio] Modulation power required

Gary Schafer garyschafer at comcast.net
Thu Feb 14 18:01:36 EST 2008

Thank you John!
You are absolutely right. That should have been; at least one RF cycle at
the peak of the audio envelope. That's what I thought I wrote but I guess
the gremlins messed it up.  :>)

What you are talking about, seeing it on a scope can be easily done as you
describe if you have a fast enough scope and you trigger on the envelope. By
expanding (using a faster sweep time) you can see each individual RF cycle
under the audio envelope waveform. You will see many RF cycles for each
audio cycle and see the different amplitudes of each RF cycle across the
audio waveform.

Gary  K4FMX

> -----Original Message-----
> From: John Coleman [mailto:jc at pctechref.com]
> Sent: Thursday, February 14, 2008 4:56 PM
> To: garyschafer at comcast.net; 'Discussion of AM Radio in the Amateur
> Service'
> 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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