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

[Phil Alexander]

On 1 Feb 2006 at 19:33, Alan Alsobrook wrote:

> You should use sample of the audio being applied to the input terminals 
> of the transmitter. A spare processor output should work fine as long as 
> the wave form is the same as being feed to the transmitter. Also be sure 
> that you run it through an isolation 600:600 transformer, as you will be 
> grounding one leg of audio to feed the scope.

That depends on the linearity of the modulator. "Back in the day" when 
such things were less dependable, we would tap a blocking cap on the
top side of the modulation reactor and feed a big voltage divider with
it. Thus, you had a pure analog of the B+ going to the PA driving the
H plates of the CRT and we used a pick up coil in the tank compartment
or a tap off the mod. monitor sample for driving the V plates. See, 
those were the days when most scopes had jumpers on the back so you
could drive the deflection plates of a CRT directly. Just remove the
jumper from the V amp and sweep output and put enough voltage to get
a screen filling pattern.

> X axis is typically the Transmitter input audio and the Y axis is the 
> NON demodulated RF sample from the transmitter. Any appearance of a flat 
> line to the small end of the trapezoid would indicate -100% modulation.

Actually, when we used this method extensively, we called it something
else, as suggested by a protruding horizontal peak on a solid triangular
display. <g>

> If you set the unmodulated carrier at +/- 1.6 Div then anything 
> exceeding 4 div would indicate more than 125% modulation. Alternately if 
> you move the base line to the bottom of the display then you can set the 
> unmodulated carrier to 3.2 div so that anything above 8 div is over 125%.

When monitoring glowing tubes, one was fortunate to get much more than
100% positive mod., although there was no limit to the + allowed. In
fact, the rules called for a minimum capability of 85%.

I once had an FCC RI (in the days when the Field Engineering and
Monitoring Bureau existed and had them) question my modulation level
because while the jock was talking the modulation meter never showed
more than 65%. On music it hovered around 90-95%. He asked one of
those "hang yourself" mild mannered questions they were famous for,
saying something about the modulation looking a little low. I had
a scope parked permanently by the side of the monitor rack so I
said just a minute as I reached over and turned it on. By that time
the jock had a record on the air, so I asked the RI to watch when 
it ended and the patter began again. On music the scope was showing
a perfect trap and sometimes a bright point on triangle peaks. When the
voice started, he saw the same pattern with a few more bright points
at the tip of the triangle on peaks although the mod. mon. never kicked 
over 65%. I told him the monitor calibrated within less than 2% on sine
wave, and that 'you fellows approved it'. He smiled, didn't say another 
word about it. I got through that inspection cleanly. This was in 
the days long before fines when they usually found some minor point 
for writing a 'ticket' just to keep you on your toes.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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