Re: [AMRadio] Use of an oscilloscope to monitor am audio

[Donald Chester]

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 
tube.

Don K4KYV