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

Tue Apr 22 22:19:12 EDT 2008

Yes.  I think some hams know about the Bessel function of FM mod index.
In fact, one can modulate an FM xmtr effectively in CW by using a properly
selected sine modulation freq and deviation.  If the mod freq is greater
than the receiver bandwidth, then the carrier will disappear when the
modulation signal is applied.  This has been used to generate CW with
Gunn diode microwave oscillators equipped with tuning varactors.
This lets a CW or SSB receiver copy a sig that it otherwise couldn't.

But despite that, the total output power of the oscillator remains
constant over it's full output bandwidth. The power that appears one
the carrier freq with no modulation gets spread out into the sidebands
when the modulation is applied.

But again, that's an arcane aspect of FM that even goes beyond the
scope of the Extra ham exam.

73 de Mike W5VSI

Sheldon Daitch wrote:
> Yes,
> 
> the indicated power does remain constant, but if one is specifically 
> looking
> at the power in the envelope at the carrier frequency, that power level 
> does
> change.
> 
> Take a look at:
> 
> http://www.cliftonlaboratories.com/bessel_nulls.htm
> 
> for one discussion on carrier null.
> 
> Shamelessly llifted from the website:
> 
> The principle behind Bessel Nulls is that in a phase or frequency
> modulation system, the relationship of the carrier level, the modulating
> frequency and the deviation is defined by a mathematical relationship
> known as a Bessel function, named after their discoverer, the German
> mathematician Friedrich Bessel. Bessel lived nearly 100 years before
> FM was invented but his mathematical techniques have proven useful
> in describing many physical phenomena.  For more details, see Agilent's
> Application Note 1390. At the moment, it may be found at
> http://cp.literature.agilent.com/litweb/pdf/5988-5677EN.pdf,
> but Agilent periodically rearranges its web site so this address may
> not last indefinitely.
> 
> For our purpose, it is sufficient to understand that for certain values
> of deviation and modulating frequencies, the carrier will reduce to zero.
> More practically, a deep null, on the order of 30 or 40 dB can be seen
> as these values are approached.
> 
> 
> 73
> 
> Sheldon
> 
> 
> 
> Mike Manes wrote:
> 
>> How arcane.  Yes, the math works, but try looking at it with just an
>> RF power instrument: FM envelope amplitude is constant, and that's why
>> one can use a non-linear PA, like Class C.
>> 73 de Mike W5VSI
>>
>> James Duffer wrote:
>>
>>> 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 deviatio
>>
>> n > > 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 > > > > 
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-- 
Mike Manes    mrmanes at gmail.com     Tel: 303-979-4899
"Things should be made as simple as possible, but not more so."
A. Einstein