[Test-Equipment] FS: Heath IM4180 FM deviation meter

[James Duffer]

Thanks for the link.  I stand corrected that the carrier does not change frequency as I guess if it decreases to zero then frequency is mute Hi!  Back to the point, the question was a good question and brought up a lot of good points.  Glad he asked what deviation was.  I suspect many learned something except those who ridiculed the question and questioner!
73, Jim wd4air

> Date: Tue, 22 Apr 2008 11:53:49 -0400
> From: lhwill at verizon.net
> Subject: Re: [Test-Equipment] FS: Heath IM4180 FM deviation meter
> To: test-equipment at mailman.qth.net; test-equipment at mailman.qth.net
> 
> Hi all,
> 
> The correct analysis of FM requires the use of Bessel functions, not 
> a simple item unless you dig math.  We used the 'n" order of the 
> function to observe the carrier going to zero to accurately set FM 
> deviation.  For standard 75 khz FM broadcasting, At a 15 khz audio 
> single tone, the modulation index is 5, that is there are 5 pairs of 
> significant sidebands each spaced 15 khz apart.
> 
> There are many combinations possible but the object is to use the 
> correct precision audio tone and slowly increase the amplitude until 
> the correct carrier null occurs.  This point equates to 75 khz 
> deviation.  As the audio is increased the systematic peaking and 
> cancellation of the carrier can easily be observed on a good spectrum 
> analyser.   Here is an excellent link that says and shows this better 
> than I ever could.
> 
> http://www.cliftonlaboratories.com/bessel_nulls.htm
> 
> Larry
> W3LW
> 
> 
> 
> At 09:58 PM 4/21/2008, WA5CAB at cs.com wrote:
> >Actually, that is not the case.  The actual carrier does change frequency.
> >F3 (FM voice) is perhaps harder to wrap your mind around but F1 (FSK) is easy
> >to understand.  The carrier alternates between two discrete frequencies
> >commonly called Mark and Space.  With an up and running FSK station 
> >it is easy to
> >demonstrate this.  In this case, the deviation (or peak deaviation, 
> >which is the
> >same) is equal to the shift.  The bandwith is greater than the deviation.
> >
> >
> >
> >In a message dated 4/21/2008 8:15:57 PM Central Daylight Time,
> >dufferjames at hotmail.com writes:
> > > While the provided answer to your question will suffice, actually 
> > Frequency
> > > Modulation (FM), the carrier does not change in frequency, the amplitude
> > > varies, this can be observed with a spectrum analyzer and proven
> > > mathmematically.  The modulated wave has various side frequencies 
> > that vary in both
> > > frequency and amplitude depending on the modulating source.  AM (Amplitude
> > > Modulation) the carrier is constant but the modulated composite 
> > varies in amplitude as
> > > a result of the combination of the carrier and side frequencies.
> > >
> >
> >Robert Downs - Houston
> ><http://www.wa5cab.com> (Web Store)
> >MVPA 9480
> ><wa5cab at cs.com> (Primary email)
> ><wa5cab at comcast.net> (Backup email)
> >
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