Trapezoidal modulation display (was "Re: [BC] Inovonics model 520")

[Robert Meuser]

You would have problems if you used the audio processor. Your trapezoid would 
have bowed sides due to TX phase shift in the audio path. Part the reason to use 
a trapezoid in the first place is to observe TX linearity which is observed 
along the sides of the trapezoid. In the days of plate mod TXs, the audio was 
sometimes taken from the modulator output (can be dangerous! ;-)  ) A demod does 
not have to consist of much more than a diode so I don't see a problem.

R

Kira Mead wrote:
> David - thanks for the information.
> No I am not using a trapezoidal display.
> 
> Does the audio for the scope have to actually demodulated from the rf 
> sample, or can i simply get it from a spare output of the audio processor?
> 
> 
> Thanks
> 
> 
> Warren
> 
> 
>> Perhaps <pirate at albanyis.com.au> is not using a trapezoidal display, 
>> which
>> makes things pretty clear.
>>
>> For those who aren't familiar with this technique, you apply a sample 
>> of the
>> modulated RF carrier to the vertical input of the 'scope, and the 
>> modulating
>> signal to the horizontal input (don't use a sweep).
>>
>> The resultant display resembles a trapezoid (hence the name) that 
>> becomes a
>> triangle pointing to one side when modulation reaches 100% negative 
>> modulation
>> (carrier pinch-off). (The side to which the trapezoid/triangle points is
>> determined by the relative polarity of the audio with reference to the 
>> audio
>> modulating the carrier).
>>
>> By setting the dead carrier in the exact center of the screen and 
>> calibrating
>> where the 100% neg. mod "point" occurs to, say, ten divisions from 
>> center,
>> you can then use the horizontal scale on the scope screen to determine 
>> pretty
>> accurately the positive modulation in the direction opposite from the 
>> negative
>> modulation "point."
>>
>> Like an X-Y display of stereo audio, this setup offers the user a 
>> plethora of
>> information about the signal. With the trapezoid, you can see modulation
>> level, density and amplitude and phase linearity.
>>
>> But, if nothing else, it's fun to watch. :-)
>>
>> Kind Regards,
>> David
>>
>>
>> David P. Reaves, III
>> TransLanTech Sound, LLC
>> Creators of the Award-Winning "Ariane Stereo Audio Leveler"
>>
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> 
> 
> 
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