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

Tue Apr 22 00:12:11 EDT 2008

I think Richards answer was appropriate for the venue at hand. However if we
must get into it more we should be a little more correct.
With an FM signal the carrier does indeed change in frequency with
modulation. However the AVERAGE frequency remains constant with a sin wave
modulating it as the frequency swings an equal amount up as it does down so
the average change is zero over a given amount of time. 
The composite of the signal does appear not to change in frequency. The
amplitude of the composite signal is constant. The sum of all the side band
amplitudes with modulation are equal to the level of the carrier without
modulation. 
The side band frequencies of an FM signal are produced when the main carrier
is displaced from its resting frequency. Energy is taken away from the
carrier frequency and goes into the side band frequencies thus the side band
energy increases in amplitude as the amplitude of the carrier decreases. The
energy just swaps places. This energy transfer is dependent on the
modulating frequency and amplitude of the modulating frequency.

To see it in action modulate an FM transmitter with a low frequency
modulating signal such as a 1 Hz signal and watch it on a spectrum analyzer.
You will see the carrier move back and forth in frequency at the modulation
(1Hz) rate. Where it moves to at any given instant in time is equivalent to
the side band energy at that point in time.

In an AM signal it appears that the amplitude of the carrier varies in
amplitude with modulation when looking at it in the time domain (on an
oscilloscope). 
The composite of the signal does indeed vary in amplitude with modulation.
But looking at it in the frequency domain, with a spectrum analyzer, the
carrier is constant and unchanging. The side bands vary in amplitude and
frequency depending on the modulation frequency and amplitude of the
modulating signal. Energy is added to the composite signal in the form of
side band power derived from the modulating signal. None of the increase in
energy in the side bands or total energy of the composite AM signal comes
from the carrier. 
An FM signals total energy remains constant and its side band energy comes
from its carrier.

73
Gary  K4FMX

> -----Original Message-----
> From: test-equipment-bounces at mailman.qth.net [mailto:test-equipment-
> bounces at mailman.qth.net] On Behalf Of James Duffer
> Sent: Monday, April 21, 2008 9:15 PM
> To: Discussion of Electronic Test Equipment; Richard Knoppow
> Subject: RE: [Test-Equipment] FS: Heath IM4180 FM deviation meter
> 
> 
> 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.
> 
> Keep asking questions about things that interest you, and follow up with
> reference material.
> 
> 73, Jim, wd4air> From: kc7hkp at comcast.net> To: dickburk at ix.netcom.com;
> test-equipment at mailman.qth.net> Subject: Re: [Test-Equipment] FS: Heath
> IM4180 FM deviation meter> Date: Tue, 22 Apr 2008 00:40:01 +0000> CC: > >
> Thanks You very Much Richard for answering Me !, I did not know what it
> was, I can only learn from books since elmer are not around much any
> more.> being handicapped in a wheelchair and stuck in my bedroom, I only
> have books or the net to learn> So Thanks again for taking the time to
> answer.> Instead of degrading me because i did not know.> George> -->
> KC7HKP > George Yazzolino > 14801 NE 20th Circle > Vancouver, Wa. USA
> ,98684 > Grid CN-85 > kc7hkp at arrl.net> > -------------- Original message -
> ------------- > From: "Richard Knoppow" <dickburk at ix.netcom.com> > > > > >
> ----- Original Message ----- > > From: > > To: "Discussion of Electronic
> Test Equipment" > > > > Sent: Monday, April 21, 2008 1:25 PM > > Subject:
> Re: [Test-Equipment] FS: Heath IM4180 FM deviation > > meter > > > > > > >
> Hi Dick > > > What is FM Deviation ? > > > I have not heard this word
> before in ham radio . > > > Sorry to bother you > > > George > > > > > In
> case this is a serious question... > > Deviation is the measure of the
> amount of frequency > > modulation in an FM signal. It is superficially
> the amount > > the carrier is changed in frequency when modulated. There >
> > are several ways to measure the amount of FM but its > > possible to
> make a fairly simple device to indicate it on a > > meter although other
> methods are more accurate. > > FM transmission became popular for the ham
> bands after > > WW-2. It has pretty much disappeared on the HF bands but
> is > > quite widely used on 6 meters and above. > > The other reaction to
> the question is because FM should > > be elementary knowledge. OTOH, no
> one should be afraid to > > ask a question for fear of being humiliated. >
> > May I suggest that a Google search is a good place to > > start for
> definitions. > > > > --- > > Richard Knoppow > > Los Angeles, CA, USA > >
> dickburk at ix.netcom.com > > > >
> ______________________________________________________________ > > Test-
> Equipment mailing list > > Home:
> http://mailman.qth.net/mailman/listinfo/test-equipment > > Help:
> http://mailman.qth.net/mmfaq.html > > Post: mailto:Test-
> Equipment at mailman.qth.net >
> ______________________________________________________________> Test-
> Equipment mailing list> Home:
> http://mailman.qth.net/mailman/listinfo/test-equipment> Help:
> http://mailman.qth.net/mmfaq.html> Post: mailto:Test-
> Equipment at mailman.qth.net_________________________________________________
> _____________
> Test-Equipment mailing list
> Home: http://mailman.qth.net/mailman/listinfo/test-equipment
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Test-Equipment at mailman.qth.net