[AMRadio] Use of an oscilloscope to monitor am audio

Jim Candela jcandela at prodigy.net
Sun Mar 19 15:27:15 EST 2006


   All that you said is true. The Trapezoid has
special significance to those running linear amplified
AM, or efficiency modulated finals. In these cases the
plate loading control may have a profound impact on
linearity, and this is usually not obvious with the
envelope pattern. For you plate modulators out there,
the trapezoid is less important. Still, take a Viking
I, put in on 10 meters with marginal grid drive, and
the linearity as seen on the trapezoid will be pretty
curvy, and will straighten out on the lower bands..


--- Donald Chester <k4kyv at hotmail.com> wrote:

> >>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
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