[Test-Equipment] FS: Heath IM4180 FM deviation meter
Brooke Clarke
brooke at pacific.net
Tue Apr 22 13:18:51 EDT 2008
Hi Sheldon:
Thanks, the Agilent 7 Hints for signal generator has the table listing the
magic Bessell numbers.
A couple of points on FM.
1) Spread Spectrum can be thought of as the occupied bandwidth exceeding the
base bandwidth. For example: Broadcast FM has plus AND minus 75 kHz deviation
for a total of 150 kHz compared to a music signal of 15 kHz (10:1). This is
similar to C-band satellite TV where the occupied bandwidth is around 36 MHz
and the base band TV is under 5 MHz (8:1).
Military VHF low band (30 to 88 MHz) radios have 25 kHz channel spacing and
when used on the 6 meter ham band (5 kHz channel spacing) the mil radio will
sound weaker.
The single channel Magnavox StealthCom spread spectrum radio occupies the band
of about 30 to 150 MHz for a voice signal with a bandwidth of about 3 kHz
(40,000:1). Conventional radios and television sets are not aware of it's
signal. Another way to think of spread spectrum is process or modulation gain.
In this case the range of a handheld StealthCom radio far exceeds that of
other conventional handheld radios.
The Black Box (still don't know what it's for) has a pulsed signal whose
bandwidth exceeds that of any conventional receiver and so they can't hear it.
http://www.prc68.com/I/BlkBox.shtml
When Howard listened on a scanner that was the problem.
2) Capture Effect
The key difference between AM and FM related to what happens when two signals
on the same channel are received. In an AM receiver you will hear both signals
usually one weaker than the other. But in an FM radio when the weaker signal
is more than a few dB lower than the stronger signal it will not be heard.
This is why aircraft radios are AM, i.e. so that a weak signal will be heard
and not blocked out.
Have Fun,
Brooke Clarke, N6GCE
http://www.prc68.com/P/Prod.html Products I make and sell
http://www.prc68.com/Alpha.shtml All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
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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