[BC] Re: FM Modulation, the O'scope and Bessel functions.

[Xmitters@aol.com]

In a message dated 7/19/06 8:47:07 PM Central Daylight Time, 
broadcast-request at radiolists.net writes:

<< 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. >>

Paul:

I often wondered back years ago when modulation measurements were a hot topic 
in Radio World and what constituted overmodulation, why the industry could 
not have been given the specs of the FCC's "wide band" so we cold build a 
modulation monitor around that. Correct me if i'm wrong, but high order sidebands 
that don't make it through the receiver's IF could add to the modulation 
percentage, right? Especially those little short duration spikes that light the 
overmod flasher.

Not having an SA for the initial recal is no big deal. Use a communications 
receiver tuned to the FM receiver's 10.7 Mhz IF. The first disappearance of the 
carrier (Jo) occurs when the modulation index is 2.4048 and with 75 khz 
deviation, that translates out to a modulatig frequency of 31.187; easily passed by 
the exciter wideband input. Tune the comms receiver to the unmodulated (10.7) 
carrier and apply modulation and turn up the audio until the carrier goes 
away on the comms receiver. Of course you could use whichever disappearance of 
the carrier you like best or you could even use the mod index for the 
disappearance of the first set of sidebands and tune the comms receiver to one of those. 
The carrier is easier.  Obviously the comms receiver has to have narrow 
enough IF to be able to pull out that carrier and not be influenced by the 
existence of it's next door neighbor sideband pair. The highest modulating frequency 
the better, because it relieves the bandwidth requirements on the comms 
receiver. My Drake SPR-4 worked quite well for the job :-) Of course it also helps to 
know what kind of distortion products the exciter is apt to put out, being 
modulated so hard with a rather high modulating frequency. You have to measure 
the test tone frequency accurately too, of course.

 Jerry Cerny of WLBK showed me this and I thought he was an absolute wizard! 
It was on the second or third day of my broadcasting career back in 1977. 
Very, very cool. Calibrating a mod monitor this way is such a blast and so much 
fun that I feel like I'm goofing off when doing it. I'm not sure how happy the 
FCC would be about 100% modulating the transmitter with 31 khz tone, so I used 
the exciter alone feeding a dummy load, then with a sample to feed the mod 
monitor and my wide band receiver.

"Modulatin DX'ing" (my words) as you have described is also Very 
entertaining. Once you get your stuff all calibrated, it's interesting to see what other 
stations are doing.

Jeff Glass
Northern Illinois University
DeKalb, Illinois