[BC] Scope for Mod Monitor...

[Paul Christensen]

> "100% modulation and observing the demodulated signal on an o-scpe.  They
then calibrated the vertical on the O-scope to that point, and started
watching the stations modulation.  The over modulation was obvious..."

That's still one of the best ways to measure analog FM modulation.  It's 
been a while, but I believe that method also captures the effect of SCA 
injection, making the total modulation reading on the scope graticule very 
accurate.

By setting the scope's sweep time to zero (e.g., in X-Y mode), it's possible 
to visually differentiate the effect of multipath against true instantaneous 
peaks.  It may not capture some transient peaks of the very shortest 
duration, but it's certainly accurate enough to satisfy the FCC when the 
demodulator output is calibrated against the Bessel function.

It's also possible to obtain highly accurate results of the competition even 
if you do not have access to a spectrum analyzer for the Bessel function. 
If you calibrate against your Tx monitor, and you are reasonably certain as 
to its accuracy, then other stations can be measured relative to your own. 
Even the older Belar FMM-1 series was a good reference, since it was pretty 
tough for the internal calibrate function to go awry, even after many years 
of use.

Probably the most crucial factor in these measurements is to ensure adequate 
signal strength into the tuner and perhaps even more importantly, that the 
antenna used in the measurement is aligned for zero multipath.

At one time, we used a Technics ST-9030 tuner with the inclusion of a 
customized 10.7 MHz I.F. output.  The ST-9030 was one of the best low-cost, 
high performance tuners ever manufactured and featured a highly-selective, 
eight-gang tuning cap along with variable I.F. bandwidth selection.

The I.F. output from the tuner was then routed to the I.F. input of a TFT 
mod monitor.  The neat aspect of the I.F. transfer between the tuner and mod 
monitor is that accuracy is independent of the composite baseband level. 
The result is a very low-cost and accurate tunable demod/mod monitor whose 
accuracy is pretty much limited only by the performance of the tuner.

Paul


====================================
Paul Christensen, CPBE, CBNT
LAW OFFICE OF PAUL B. CHRISTENSEN, P.A.
3749 Southern Hills,  Jacksonville, Florida 32225
Office: (904) 379-7802    Facsimile: (904) 212-0050
pchristensen at ieee.org