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

[Sheldon Daitch]

I am not sure I would agree with the relationship that too high a deviation
results in no modulation. 

What is probably happening is with too high a deviation level for the
receiver being used, the transmitted signal is too wide for the receiver
and that extra deviation level causes problems with the typical noise
squelch circuit.

100% modulation in FM is only a definition, assuming the equipment is
operating properly.   I think most of the amateur community now
looks at 5 kHz deviation as a defacto maximum for most modern two
way gear, while the TV industry (and the FCC) considers 15 kHz deviation
as 100% modulation.  The FM broadcast band uses 75 kHZ deviation as
the 100% modulation point. 

Unlike AM, where 100% negative modulation cannot be exceeded (and
yes, with asymmetrical modulation, positive peaks over 100% can be 
achieved),
100% modulation for FM is only a point defined by the service of the 
equipment.

I believe FM broadcasters in the states may go above 100% modulation, if 
they
are running stereo and multiple subcarriers, but I haven't looked at the 
rules for
the maximum levels allowed now.

It is not uncommon overseas to find FM broadcasters running upwards of 150%
modulation, by US standards.

73
Sheldon
WA4MZZ


Howard wrote:

>And if your deviation is set too high ,at least in the 2-meter band,you`ll
>actually not put out any modulation.
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