|[AMRadio] Use of an oscilloscope to monitor am audio|
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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