[AMRadio] Use of an oscilloscope to monitor am audio

W5OMR/Geoff w5omr at satx.rr.com
Sat Mar 18 13:50:39 EST 2006

Jim Miller WB5OXQ wrote:

> I have a Tektronix model 453a scope and I would like to know how it 
> can be used to monitor transmitted signals.  Do I need extra hardware 
> or an interface of some type to obtain the trapezoid pattern?  I know 
> some scopes designed for amateur radio have this feature built in so I 
> wonder if my scope can be used for this?

I see that there are already all kinds of answers, but honestly, what I 
do is just take another antenna outside - perhaps even something as 
small as a 10m loop (for a 75m antenna) and toss it up on the roof, 
under the antenna, and just use that to monitor the transmitted signal.  
You'll see the AM envelope and be able to tell exactly what kind of 
audio peaks you have, how close you are to pinching off the carrier - 
once the sweep-rate is set slow enough to see the audio, it's a simple 
matter of adjusting the volts/div to a level where your carrier consums 
2 centimeters (squares) tall.  One above, and one below the center 
line.  When audio peaks hit the top of the line of the centimeter/square 
ABOVE and BELOW the carrier quiescent point, and that co-incides with 
the negative part of the cycle just at the center-line (-not- to where 
the carrier is pinched off) then that's 100% modulated.  this is a 
positive-to-negative peak ratio of 2:1

Here's where some of the discussion has been, of late... it's -my- 
feeling (and a few of the guys here feel the same) that -over- 
modulation occurs when the negative going part of the audio that 
modulates the carrier goes 'beyond' the baseline.  At that point, you've 
exceeded the capability of your modulator, and are therefore 
overmodulating.  By allowing the typical male-patterned asymetrical 
voice peaks to rise to whatever peak they wish, as long as you're not 
overmodulating beyond 100% Negative, you're not going to cause 
distortion and splatter, -and- your peaks will be -much- higher.  In my 
case, I've got the capbility of having a positive-to-negative peak ratio 
of 4:1. 

do the math..

if a positive-to-negative peak ratio of 2:1 is equivalent of 100% 
modulation, then the positive peaks must be 4x the carrier level.  Must 
be, because when voltage doubles, current doubles.  Therefore, a 100w 
carrier, 100% modulated with a 2:1 positive-to-negative peak ratio, is 
producing instantaneous peaks of 400w Peak Envelope Power.

If I run my positive-to-negative peak ratio back up to 4:1, with my 100w 
carrier, my PEP level is 1,600w, *according to the scope*.

Who needs a PEP reading watt meter?  I've got a -much- better peice of 
test equipment!

73 = Best Regards,

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