[AMRadio] Use of an oscilloscope to monitor am audio

Donald Chester k4kyv at hotmail.com
Sun Mar 19 13:59:03 EST 2006

>>I don't find a "waveform" (amplitude vs. time) display difficult to 
>>interpret.   Over modulation in the negative direction shows up clearly as 
>>"pearls" on the baseline and positive peak levels can be easily read by 
>>slowing down the trace.   Flat-toping can be clearly seen at higher sweep 
>>speeds.   Admittedly, a trapezoid display can provide additional 
>>information but I've never found it worth the trouble.

I find the trapezoid useful for determining peak positive and negative 
modulation percentage, since it is a little easier to interpret that looking 
at the modulated waveform.  But it tells nothing about the waveform of the 
modulating signal.  A severely distorted flat-topped signal looks almost 
identical to a pure sinewave.

The trapezoid is most useful for observing the modulation linearity of the 
final.  I have one of my transmitters set up where I can instantly switch 
from one to the other.

One problem with the trapezoid is if you use it a lot, it will burn a 
vertical line on the face of the scope tube, since it displays a bright 
vertical line whenever there is no modulation.

On envelope pattern, I have a relay to kick in bias voltage to deflect the 
patter off the screen when not transmitting.  Leaving the horizontal base 
line on all the time will burn a horizontal line into the face of the  scope 


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