[BC] Modualtion Monitors (do we need them?)

[Cowboy]

On Tuesday 18 July 2006 01:54 pm, Bailey, Scott wrote:
> Jerry,
>    I have a Fluke, Pocket Scope" to see modulation. 

 I was given one of the first Scope-meter's to evaluate for broadcast
 uses by Fluke years ago.
 Unless they've greatly improved since then ( I suspect they have )
 it was nearly useless for mod levels.
 When I gave Fluke a negative evaluation, they justified the instrument
 as having been intended for 3 phase power work, not RF.
 My suspicion was that they were looking for a broadcaster endorsement
 at the time, which I could not give.
 It simply could not resolve pinch-off with program material, though it
 was fine with steady state tone, and certainly OK for audio work.

On Wednesday 19 July 2006 02:25 am, Burt I. Weiner wrote:
> Any station that does not have a properly working modulation monitor 
> for their transmitter/s is fooling themselves and whoever is 
> responsible should have their head examined.

 In most cases, I would agree with this sentiment.

> If you think the  
> bargraphs on the front of the transmitter are going to tell you 
> what's happening, you're wrong.

 Good as a rough indication.
 If the 100% blinky light is constantly illuminated, it does serve as
 an indicator that something else is immediately called for to determine
 compliance.

> A scope is fine to a point but it  
> won't tell you if you're modulating 115, 120 or 125 percent positive 
> peaks under dynamic program conditions.  You cannot resolve 
> modulation close enough with a scope under program conditions.
> 
> Burt

 Well, that depends on whether you are trying to read what it *is* on any
 given peak, in which case I would agree, or whether you are
 simply trying to see if *any* peak hits limit, in which case I would
 disagree.
 I would say the same about negative peaks.
 As with almost all equipment, it depends on the skill of the operator.
 There are far fewer who can properly use a scope to it's full capabilities
 than many would like to believe.
 In the right hands, it's a wonderful tool.
 In the wrong hands, it's a disaster.

> At 01:11 PM 7/18/2006, you wrote:
> >From: Alan Alsobrook <radiotech at bellsouth.net>

> >You don't need a "modulation monitor" but you do need to have a method
> >of determining your modulation.

 For which the licensee is responsible, by whatever method works.

> >Modern processors do a good job of keeping the modulation consistent and
> >limiting at their set point, but you need something to determine that
> >you have them set correctly. If caught in violation, without a means to
> >determine modulation not only could you be written up for the over
> >modulation, you can add unable to determine modulation. 73.1350(c)

 It's been my experience that as long as you can reliably demonstrate that
 the method you choose will accurately resolve "legal" or not, any method
 one might invent, though possibly viewed sideways, will be accepted.
 ( provided also that one doesn't need to call in an engineer to use it, and
 that it gives a constant indication )

On Wednesday 19 July 2006 09:43 pm, Burt I. Weiner wrote:

>    Many  times I've used a scope to set up a station's modulation when no
>    monitor  was  available  and  there  was  a  question about modulation
>    levels.   Any  decent  oscilloscope will certainly tell you if you are
>    cutting carrier. 

 As have I, and that part so far, is pretty easy.

>    The problem comes when you try and set your positive 
>    peaks  to the legal maximum of  125%.  If you have a scope with a peak
>    reading  digital  display indication then you can probably come pretty
>    close  depending  on  how  well  you  calibrate  for  a zero modulated
>    carrier.

 Even without a digital display. If one knows what one is doing, and the
 limitations of the instrument, one can get within 1/4% or so on some
 half-decent analog CRT scopes.
 I think I can get closer than most, but if you want to run THAT
 close, you probably better get someone else to sign off on it.
 I'm much too fond of guard bands, and safety margins to leave anything
 that close to the edge.

>    The problem is, how well
>    did you calibrate and how well can you resolve the trace.  How good is
>    the  focus  and  astigmatism  of  the spot.  You may think this is nit
>    picking  but  my  comments  personally come from desiring accuracy.

 On all of that, they'll be no argument from me.
 Once again, the skill of the operator is key.

>    I 
>    highly   recommend  to  my  clients  that  they  maintain  a  properly
>    calibrated modulation monitor.  It's good, responsible engineering.
>    The  issue  may very well come home to roost if there is ever an issue
>    with the FCC over your modulation levels.
>    Burt

 With the skill of the FCC personnel these days, it could be cheap insurance.

On Saturday 22 July 2006 09:01 pm, Robert Meuser wrote:
> Burt:
> 
> First I will plead guilty of having a mod monitor in every North American 
> station where I have been - often two or three.

 Same here, if I had a say in it at all.

> That being said, I never except as a matter of last resort used it to determine 
> compliance or as a detector for measurements.

 I have, but not running the hairy edge at legal limit.
 Allow some headroom, so that I know that the levels within the margin
 of error are still within limits in the worst case.

 I'm just a bit uncomfortable with how the hair on that edge might
 be interpreted.

> I will admit to using them on FM  
> for pilot injection. The flashing lights do enhance transmitter tours.

 Blinky lights do seem to fascinate management types too often !

 From the GM.....
 "We appear to be running slightly behind schedule to insure completion
 of this project by the deadline set by upper management. Can you absolutely
 guarantee on-time completion ?"

 Look, Charlie, blinky lights !!
 ;-)

> I have never seen any government entity anywhere use anything other than a scope 
> and/or spectrum analyzer for compliance measurements.

 TRUE, but other methods are acceptable as long as they are in substantial agreement
 with whatever the government agency is using.

> I do think that Belar has come up with some interesting tools that go way past 
> the average mod monitor.

 Ain't computers great ?
 :-)

-- 
Cowboy