[BC] Modulation Monitor - a necessity?...

[Robert Meuser]

Burt I. Weiner wrote:
> I also believe in having properly operating modulation monitors at all 
> stations I have ever been responsible for.  I no longer allow myself to 
> be in the position of being the responsible person.  Now they can either 
> take my advice or leave it.  Most take it seriously.

That blinky light thing is good for CYA, assuming the monitor is properly 
calibrated. I have seen stations where the mod monitor, FCC certification and 
all, was past being ready for a museum. A scope would be much better.

The other problem is that a mod monitor does not show WHAT you are modulating. I 
put a brand new station on - all new properly installed top of the line gear, 
etc. The mod monitors (1 Belar and 1 TFT)both showed we were banging out 125% 
plus. Problem was the station sounded horrible on air, much worse than the 40 
year old site it was replacing. The scope showed a different story. Ultimately 
we found a lot of RF feedback induced from STL coax thanks to what we saw on the 
scope.

In trapezoidal mode, you see a lot of things the TX is doing that a mod monitor 
will never show.

> 
> Using a scope is good - if you know what you're doing.  You can't simple 
> turn it on every once in a while to see if you're legal other than for 
> negative peaks for AM and depending on some scopes I've seen pressed 
> into that service, that's even questionable.  

No you rack mount a new scope suited to the job and connected to a proper RF 
sampling system. I usually have all monitoring equipment, mod monitor, scope, 
distortion analyzer, monitor amp, etc on a separately activated circuit. These 
devices are always off if no one is in the building and always on when they are.

>However, I still stand by 
> my earlier comment about (not) being able to resolve within a dB or so 
> of positive peaks or FM deviation.  I've been there and fought that 
> battle.  As you know, at the top end a single dB is about 10 percent.  
> Not so many years ago we didn't have a cap at +125 percent and since 
> there was no limit on positive peaks the only concern were the negative 
> peaks.

The thing here is the governing authority WILL be using a scope for these 
things. I would think you'd like to see things as they see it.

> 
> When the FCC relaxed many of the rules that move was misread by many 
> that certain, old aspects of compliance were no longer an issue.  
> Nothing could be farther from the truth.  Almost all technical operating 
> tolerances are still in place and we are as responsible to operate 
> within tolerance as we ever were.  How we achieve that is now left up to 
> the licensee.
> 
True, see above.


> As far as no one being concerned about positive peaks on AM, that 
> reminds me of the conversation a few weeks ago about breaking the 
> rules.  I like to play by the rules - in all aspects of life.  I'm 
> certainly not saying that by not having a modulation monitor you are not 
> in compliance it's just that those who choose not to have one have 
> chosen to go a different route for monitoring than I would.

A very large number of stations that have new up to date mod monitors will show 
the positive meter slamming 130% pretty hard. I think 145% possibly the industry 
standard.

I think the point is that a station with no mod monitor (especially AM) can do a 
more than adequate job of staying compliant with a relatively inexpensive scope.

R