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

Tue Apr 22 14:32:36 EDT 2008

You just reminded me of a curious phenomenon that occurs 
with FM radios. The squelch circuit usually operates on a 
thing called quieting. This is where the noise in the very 
high gain and limiting IF stages is monitored and it only 
takes the smallest signal to bring the noise level down a 
few dB. The drop is used to open the squelch. The receiver 
has a special filter that looks outside of the normal voice 
audio band pass for this noise as obviously your yapping 
voice could be considered by most circuits and people as 
noise so the circuit is usually looking around 20-25 KHz. 
Normally 5KHz of deviation will not have any voice energy to 
speak of in this band, although you might have noticed some 
over deviated folk on the local repeater actually squelching 
out on lowd places in their speech. This may have been the 
process that Sheldon observed.

BTW and all jammers are forbidden to read this...You can 
shut down the local repeater by transmitting a modulating 
tone in this squelch bandpass. You don't need PL or anything 
and as long as you are strong, not capturing but strong 
enough not to be desensed too far, the repeater will remain 
squelched and off the air.

WBob, WB6JPI
\

----- Original Message ----- 
From: "Brooke Clarke" <brooke at pacific.net>
To: "Discussion of Electronic Test Equipment" 
<test-equipment at mailman.qth.net>
Sent: Tuesday, April 22, 2008 10:18 AM
Subject: Re: [Test-Equipment] FS: Heath IM4180 FM deviation 
meter

> 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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