[BC] Re Oscopes Bessel Null and FM

[Xmitters@aol.com]

In a message dated 7/21/06 12:00:38 AM Central Daylight Time, 
broadcast-request at radiolists.net writes:

<< If the higher-order sidebands are a result of excessive carrier deviation 
--  
 one component of FM bandwidth.  As a rough approximation, Carson's Rule 
 states that ~ 98% of occupied bandwidth can be determined by use of the 
 following formula:
 
 BW  =  2(Dmax + Fmax) >>

Hifgher order sidebands will occur as the modulating frequency drops, but the 
deviation remains the same. An SA showing 100% FM modulation (75 kc 
deviation) will show fewed sets of sidebands if the modulating frequency is 15 
kilohertz as opposed to modulating with 400 hertz. So I don't think it's a sideband 
order issue per se. that causes the overmod flasher to blink.

It would be interesting to do an experiment with an FM system that is very 
wide, with little measurable group delay and then modulate with agressively 
processed programming and turn the system up until the overmod flasher starts 
winking at about one flash per second. 

Then kick in a Carson's filter that limits the occupied bandwith according to 
his well known formula. I would be willing to bet a round of beers that the 
overmod flasher would either stop winking or it would not wink as often as it 
would without the Carson filter. It seems that the lopping off of those higher 
order sideband pairs (that last 2% of occupied bandwidth) would do two things. 
One would be to very slightly reduce the amplitude of the recovered audio at 
the receiver. The other would be that distortion products would be created. 
The distortion products would probably be barely, if noticable at all. 

What are your thoughts?

Jeff Glass
Northern Illinois University
DeKalb, Illinois