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

[Larry Will]

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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