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From: "John Coleman" <jec@pctechref.com>
Date: Thu, 6 Dec 2001 13:00:59 -0600
Subject: [AMRadio] NBFM
Message-ID: <000101c17e88$586bdb00$0200a8c0@WinProxy>


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Thanks Jay:
	Thanks for the signal report Jay.  Jeff and I have been talking about this
for some time now but neither of us has ever done anything about it.  We may
do it some day though just to say we did.  I suspect that if everyone went
to NBFM on 75 meters we would have a big mess of QRM, HIHI.

CUL, 73
John, WA5BXO

-----Original Message-----
From: owner-amradio@qth.net [mailto:owner-amradio@qth.net]On Behalf Of
Jay Bromley
Sent: Thursday, December 06, 2001 11:15 AM
To: John Coleman; 'Bob Bruhns'; AMRadio (E-mail)
Subject: Re: [AMRadio] Narrow Band FM


John hi,
Back in the mid 80s when all the talk about reducing AM power, PEP, etc, a
lot of guys were threatening to use NBFM and did.  I played a little on 75m
and 160m in the daytime.  It was fun talking to guys that were mobile near
Tyler, TX.  They were amaze how well it worked in the daytime.  We tried to
get guys to hang out on 75m NBFM instead of on 2m, but it never did become
popular.  Always we would start out with a couple of guys and end up with a
half a dozen or so in the round table.  I think one of the things that
bugged the new guys checking in was the legality of the mode.  I know, I
know, but the guys wouldn't and still don't check out things on their own.
There are guys now on the bands now the say anything over 3 kHz on SSB is
illegal, go figure.  John your AM signal has always sounded so good and
strong, it would be waste to use NBFM instead of your AM rig.  Maybe less
interference if you had neighbors nearby, well maybe not as noticeable??
Have fun and ---

73 de w5jay/jay..



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From: "Jay Bromley" <w5jay@alltel.net>
Date: Thu, 6 Dec 2001 11:15:23 -0600
Subject: Re: [AMRadio] Narrow Band FM
Message-ID: <006b01c17e79$97e9a960$56adfea9@ibm22209>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

John hi,
Back in the mid 80s when all the talk about reducing AM power, PEP, etc, a
lot of guys were threatening to use NBFM and did.  I played a little on 75m
and 160m in the daytime.  It was fun talking to guys that were mobile near
Tyler, TX.  They were amaze how well it worked in the daytime.  We tried to
get guys to hang out on 75m NBFM instead of on 2m, but it never did become
popular.  Always we would start out with a couple of guys and end up with a
half a dozen or so in the round table.  I think one of the things that
bugged the new guys checking in was the legality of the mode.  I know, I
know, but the guys wouldn't and still don't check out things on their own.
There are guys now on the bands now the say anything over 3 kHz on SSB is
illegal, go figure.  John your AM signal has always sounded so good and
strong, it would be waste to use NBFM instead of your AM rig.  Maybe less
interference if you had neighbors nearby, well maybe not as noticeable??
Have fun and ---

73 de w5jay/jay..


>
> Thanks Bob:
> I guess my suspicions are some what true.  NBFM might be a pleasant
> experiment under ideal or local ground wave conditions but it is probably
> not for every night operations on 75 mtrs.  Then again a rig with 2000
watts
> DC input to generate a carrier level of 1500 watts output, might with the
> 3-4 DB increase help with the interference, but I guess not with the
> distortion caused by selective fade.  BTW, I love your explanation with
the
> clock hand vectors.  Have you looked at my site
> http://www.qsl.net/wa5bxo/amtech.html?  Don, K4KYV helped me with the
> remembering.  With your permission I would like to post some or all of
your
> explanations to this site.  Also, if you see something that is not
accurate
> on the web page please let me know.  If you have a link to a page where
some
> of you explanations or already posted let me know and I will provide a
link
> to it.  If you would prefer to write your own HTML I would be glad to post
> that as well.  After reading some of your writings I feel confidant in
your
> accuracy especially when you find the need to correct it yourself.
>




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From: Mark Foltarz <Foltarz@rocketmail.com>
Date: Thu, 6 Dec 2001 08:28:07 -0800 (PST)
Subject: [AMRadio] A url for Chassis punches
Message-ID: <20011206162807.80345.qmail@web12507.mail.yahoo.com>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi



http://www.eastwoodcompany.com

type in "punch" in the search text box.

__________________________________________________
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Send your FREE holiday greetings online!
http://greetings.yahoo.com

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From: "George Pritchard" <gpritchard@comtechpst.com>
Date: Thu, 6 Dec 2001 10:49:09 -0500
Subject: RE: [AMRadio] Greenlee type tools
Message-ID: <000801c17e6d$8c22a3e0$1a14a8c0@pritchard.comtech.comtechpst.com>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

The following is a source for may thing concerning us AMer's, Including the
Greenlee tools
George, AB2KC



T&M ELECTRONICS PRODUCTS LISTING
472 EAST MAIN STREET
PATCHOGUE, NEW YORK 11772
631-289-2520 PHONE
631-289-0064 FAX

WEB ADDRESS: WWW.TANDMELECTRONICS.COM
EMAIL:SALES@TANDMELECTRONICS.COM



-----Original Message-----
From: owner-amradio@qth.net [mailto:owner-amradio@qth.net]On Behalf Of
winjones@ix.netcom.com
Sent: Thursday, December 06, 2001 10:33 AM
To: RoadKing
Cc: amradio@qth.net
Subject: Re: [AMRadio] Greenlee type tools


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post
below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

If anyone knows the source of these tools, please post on this reflector. I
think a lot of us would like to have the web address.
73, Winston  K4CWQ

RoadKing <hogtamer@wt.net> wrote:
> http://www.amfone.net - Home of the AM Radio List.
To post or see items for sale or wanted, go to the new AM'ers Trading post
below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

Some very few months ago somebody on this list posted a URL for a source of
Greenlee type  hole makers.  They were British made I think and there were
at least two different Sets depending on the sizes desired.

I would sure like to get that URL or address of the company again as I'm in
need of those Greenlee type hole makers  to begin my HB project.  Please
respond to me via my email address so we don't take up too much space on
the REFLECTOR.  Thanks in advance for the help.

73,
Tony/W5OD
River House Radio


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From: winjones@ix.netcom.com
Date: Thu, 06 Dec 2001 10:32:50 -0500
Subject: Re: [AMRadio] Greenlee type tools
Message-ID: <Springmail.105.1007652770.0.22312400@www.springmail.com>;


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

If anyone knows the source of these tools, please post on this reflector. I think a lot of us would like to have the web address.
73, Winston  K4CWQ

RoadKing <hogtamer@wt.net> wrote:
> http://www.amfone.net - Home of the AM Radio List.
To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

Some very few months ago somebody on this list posted a URL for a source of 
Greenlee type  hole makers.  They were British made I think and there were 
at least two different Sets depending on the sizes desired.

I would sure like to get that URL or address of the company again as I'm in 
need of those Greenlee type hole makers  to begin my HB project.  Please 
respond to me via my email address so we don't take up too much space on 
the REFLECTOR.  Thanks in advance for the help.

73,
Tony/W5OD
River House Radio


______________________________________________________

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with the BODY of the message containing:

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From: "John Coleman" <jec@pctechref.com>
Date: Thu, 6 Dec 2001 09:06:12 -0600
Subject: RE: [AMRadio] Narrow Band FM
Message-ID: <000001c17e67$8bdf8f20$0200a8c0@WinProxy>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

Thanks Bob:
	I guess my suspicions are some what true.  NBFM might be a pleasant
experiment under ideal or local ground wave conditions but it is probably
not for every night operations on 75 mtrs.  Then again a rig with 2000 watts
DC input to generate a carrier level of 1500 watts output, might with the
3-4 DB increase help with the interference, but I guess not with the
distortion caused by selective fade.  BTW, I love your explanation with the
clock hand vectors.  Have you looked at my site
http://www.qsl.net/wa5bxo/amtech.html?  Don, K4KYV helped me with the
remembering.  With your permission I would like to post some or all of your
explanations to this site.  Also, if you see something that is not accurate
on the web page please let me know.  If you have a link to a page where some
of you explanations or already posted let me know and I will provide a link
to it.  If you would prefer to write your own HTML I would be glad to post
that as well.  After reading some of your writings I feel confidant in your
accuracy especially when you find the need to correct it yourself.



______________________________________________________

To leave AMRadio , send mailto:majordomo@qth.net
with the BODY of the message containing:

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From: RoadKing <hogtamer@wt.net>
Date: Thu, 06 Dec 2001 08:26:16 -0600
Subject: [AMRadio] Greenlee type tools
Message-Id: <5.1.0.14.2.20011206082220.00a27360@mail.wt.net>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

Some very few months ago somebody on this list posted a URL for a source of 
Greenlee type  hole makers.  They were British made I think and there were 
at least two different Sets depending on the sizes desired.

I would sure like to get that URL or address of the company again as I'm in 
need of those Greenlee type hole makers  to begin my HB project.  Please 
respond to me via my email address so we don't take up too much space on 
the REFLECTOR.  Thanks in advance for the help.

73,
Tony/W5OD
River House Radio


______________________________________________________

To leave AMRadio , send mailto:majordomo@qth.net
with the BODY of the message containing:

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From: Bob Bruhns <bbruhns@erols.com>
Date: Thu, 06 Dec 2001 03:06:36 -0500
Subject: Re: [AMRadio] Narrow Band FM
Message-ID: <3C0F270C.B8E08693@erols.com>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

Hello again, John,

On careful review, I found I was 3dB off on some of my figures, so I corrected that, and reworded a line or two.  So... here's one of my famous Corrections.  Disregard the details of sidecurrent levels in my previous message.  Better still, delete that message and keep this one.
---------------------

Generally, FM does not perform as well as AM at frequencies below VHF, because selective fading really tears it up.  Still, many stations have run NBFM with reasonable results on AM receivers, because selective fading often causes a slope effect that gives the FM signal an AM component.  It sounds a little funny on an AM receiver.  It generally doesn't sound as loud as an AM signal, and unlike AM, you hear the station better while it is in a fade than you do when it peaks up.  That is because there is no slope effect when the signal peaks up, so there is no AM to demodulate.  With an FM receiver, the FM signal sounds OK when it peaks up, but it really tears up badly with interference, and when it is in a fade.  Probably advanced demodulation by DSP can improve this situation, and possibly this could make NBFM and PM roughly equivalent to AM.

Here's one of my famous Big Long Explanations.
 
AM, PM AND FM REVEALED!
 
I will try to explain FM and PM by explaining AM first.  Before I begin, let me state that FM and PM are almost equivalent, differing only in frequency response to the modulating signal.  You can simulate PM by passing the modulating signal through a 6 dB per octave rising filter response, from DC to daylight, and then applying that to a frequency modulator.  That will produce true PM.  Likewise you can produce FM with a PM modulator by passing the modulating signal through a 6dB per octave falling response, and applying that to a phase modulator.  That will produce true FM, although it has a well-known dynamic range limit at the lower frequencies.  But with enough frequency multiplication, you can get decent FM that way.  Well, it's not exactly 6 dB per octave, it's closer to 6.021dB per octave.  In fact, it is exactly 20dB per decade (10:1 frequency).  OK, on with the AM explanation.

AM

Imagine you have a clock with three hands.  One hand is long, the other two hands are of equal length, exactly half as long as the long hand.

I will use this diagram to represent AM with 100% modulated with 1 KHz sine wave audio.  The long hand represents the carrier.  The two short hands represent the sidebands.

The hands spin around at enormous speed.  In this example, the carrier hand spins at 3885000 times per second.  One short hand spins at 3884000 times per second (the lower sideband), and the other short hand spins 3886000 time per second (the upper sideband).  Mathematicians like to spin things counter-clockwise, but for this example, I will spin clockwise.

But these hands are spinning too fast to see.  So now, we make the spin relative.  Now the long hand is always pointing up at 12.  This means we are referencing to the carrier frequency and phase.  The two sidebands spin 1000 times per second in opposite directions.  Not only that, but they rotate such that they both point up at the same time, then one rotates back to the 9 o'clock position while the other rotates to the 3 o'clock position, then they rotate so that they both point down, then 3 and 9 o'clock, then straight up again, over and over.

These hands represent "trigonometric vectors" which represent the carrier and the two sidebands of a 100% modulated AM signal.  If you add these vectors to the carrier vector, you see the modulation: a positive peak of twice the carrier voltage when both short hands point straight up; a minimum of zero when both short hands point straight down, carrier level when both short hands point in opposite directions, and intermediate values between those instants.  The combined phase does not shift relative to the carrier phase, because phase differences between the sidebands cancel out, adding to and subtracting from the carrier, leaving only carrier amplitude variations.

We synthesize the sideband signals when we modulate an AM carrier.  It looks like we are raising and lowering the carrier level, and in one instantaneous sense we are, but we can also see the carrier level as constant, and the moment to moment level variations coming from heterodyning action from the sidebands.  We can work with whichever view is more convenient at the moment.

PHASE MODULATION

You can simulate phase modulation by moving the short hands a little differently.  Also, in FM and PM, we call the sidebands "sidecurrents" for some unknown reason.  It is easier to see PM this way if you use shorter hands.  So in the following example, imagine the short hands are 1/4 as long as the big hand.  If you have the big hand (carrier) at 12 o'clock, the small hands will rotate again in opposite directions, and again at 1000 times per second.  But this time, one short hand points toward 12 o'clock while the other short hand points toward 6 o'clock, then both short hands point toward 3 o'clock, then one short hand points toward 12 o'clock while the other short hand points toward 6 o'clock, then both short hands point toward 9 o'clock, then one short hand points toward 12 o'clock while the other short hand points toward 6 o'clock, over and over again.

This time, the amplitude is fairly constant.  When the two short hands are pointing at 12 and 6 o'clock, they cancel out, and you have the carrier level and phase.  When the two short hands point toward 3 o'clock, they add up to 1/2 the carrier length (remember they are extra short in this example), and they add to the carrier vector as a right triangle of carrier side = 1, sideband side = 0.5.  Total signal amplitude is the length of the diagonal or hypotenuse, which is the square root of 1 squared plus 1/2 squared, or the square root of 1.25, or about 1.118, and the phase angle relative to the unmodulated carrier is the angle whose "tangent" is 0.5 / 1, or arctan(0.5), which is about 27.6 degrees.  The same situation exists when both sideband vectors point toward 9 o'clock, but we have -27.6 degrees.  So we have a peak phase modulation of 27.6 degrees, and a little amplitude variation.

In real PM (and FM, too), we do not have any amplitude variation.  This is where the carrier level variations and the 2nd, 3rd, 4th, etc. order sidecurrents come from in PM and FM.   These "higher order" sidecurrents correspond to second, third, etc, harmonic distortion in AM sidebands, and they take up bandwidth.  Fortunately, the higher order sidecurrents have vanishingly low amplitude at low levels of phase modulation.  However, a particular carrier level and an infinite set of harmonically related sidecurrents are created by FM and PM, because the total angle-modulated signal amplitude is constant.

As you apply more phase modulation, a lot more high order sidecurrent energy is created.  This is why we go to narrow FM to reduce bandwidth.  At around a modulation index of 0.5, the 2nd and higher order sidecurrents are pretty low in amplitude, but you still have some reasonable length (amplitude) in the sidecurrents.  It is the energy in these sidecurrents that carries the sounds over the air in PM and FM, very much like AM.

More FM or PM deviation produces stronger sidecurrents, just like AM, corresponding to longer "short" hands in the example, but they also produce higher order sidecurrents that start spreading out in frequency.  This is not an overmodulation effect such as we sometimes see with AM, it is intrinsic to constant-amplitude angle-modulation (FM and PM) in general.  We can avoid it by adding some specific amplitude modulation, but that is complicated, and then we are again dealing with all of the technical difficulties of generating a varying-amplitude signal, as well as angle modulating it.

With narrowband PM and FM, we limit deviation to what we call a modulation index of about 0.5 to 1.  This corresponds to peak FM deviation of 1/2 to 1 times of the modulating frequency, or PM of 1/2 to 1 radian peak.  (A radian is about 57.3 degrees.)  In the example above, the sidebands were 0.25 carrier amplitude (1/16 carrier power) at +/- 27.6 degrees peak, which is about 1/2 radian.  With both of these sidecurrents together, the sidecurrents power is only 1/8 of unmodulated carrier power at that modulation index.  That's only 1/4 of the power in the sidebands of a 100% modulated AM signal of the same carrier power.  Each second order sidecurrent is about 33 dB below unmodulated carrier at a modulation index of 0.5.  Each third order sidecurrent is about -53dB below the unmodulated carrier level, and higher order sidecurrents are far below that. 

At a modulation index of 1 (+/- 57.3 degrees peak phase modulation, and peak frequency deviation equals modulating frequency), you have slightly more total sidecurrent energy than 100% modulated AM of the same carrier strength, because the carrier level is dropping at this modulation index.  But now, some of that energy is in the second order sidecurrents.  The first-order sidecurrents are about 44% of the amplitude of the unmodulated carrier, which means they are about 1.1dB below the strength of the sidebands of a 100% modulated AM signal of the same unmodulated carrier power.  Each second order sidecurrent has about 11.5% of the amplitude of the unmodulated carrier.  This means each 2nd order sidecurrent is about 19 dB down from the unmodulated carrier level, corresponding to a 100% modulated AM signal of the same unmodulated carrier power with about 5% second harmonic distortion.  (This is not actual distortion in the angle-modulated signal, but it is "splatter" intrinsic to the modulation mode.)  Each third order sidecurrent is pretty low at a modulation index of 1, around 34 dB below the unmodulated carrier level, and each fourth order sidecurrent is about 52 dB below unmodulated carrier level.  Narrowband angle modulation is not as clean as AM, although it comes close.

When multiple frequencies (such as speech) are combined to modulate an angle modulated transmitter, the bandwidth of the signal is not as wide as it would have been in the sine wave case, because the modulation index of each frequency component is less than the total.  However, sibilents will not benefit as much from this fact, because they are spectrally concentrated. 

An interesting thing happens with sine-wave modulated FM and PM.  As deviation increases, at first the carrier level decreases as the sidecurrent level increases.  At certain modulation indexes, or indices, the carrier actually nulls out, and all of the signal energy is in the sidecurrents.  These points are called "Bessel nulls," and the first one happens at a modulation index of about 2.405.  This is called the "first Bessel null."  As deviation increases further, the carrier rises, peaks, and then decreases, finally reaching a second Bessel null around a modulation index of 5.52.  More Bessel nulls (the third, fourth, etc) appear around modulation indices of 8.65, 11.79, 14.93, and they go on forever.  When modulation is very linear, and a very clean sine wave is used for modulation, the Bessel nulls are very precise.  Bessel nulls are used to calibrate modulation metering equipment.

  Bacon, WA3WDR

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From: Bob Bruhns <bbruhns@erols.com>
Date: Thu, 06 Dec 2001 01:16:55 -0500
Subject: Re: [AMRadio] Narrow Band FM
Message-ID: <3C0F0D57.D2648771@erols.com>


http://www.amfone.net - Home of the AM Radio List.

To post or see items for sale or wanted, go to the new AM'ers Trading post below.
http://www.amfone.net/cgi-bin/ikonboard/ikonboard.cgi

Hi John,

Generally, FM does not perform as well as AM at frequencies below VHF, because selective fading really tears it up.  Still, many stations have run NBFM with reasonable results on AM receivers, because selective fading often causes a slope effect that gives the FM signal an AM component.  It sounds a little funny on an AM receiver.   It generally doesn't sound as loud as an AM signal, and unlike AM, you hear the station better while it is in a fade than you do when it peaks up.  That is because there is no slope effect when the signal peaks up, so there is no AM to demodulate.  With an FM receiver, the FM signal sounds OK when it peaks up, but it really tears up badly with interference, and when it is in a fade.  Probably advanced demodulation by DSP can improve this situation, and possibly this could make NBFM and PM roughly equivalent to AM.

Here's one of my famous Big Long Explanations.

AM, PM AND FM REVEALED!

I will try to explain FM and PM by explaining AM first.  Before I begin, let me state that FM and PM are almost equivalent, differing only in frequency response to the modulating signal.  You can simulate PM by passing the modulating signal through a 6 dB per octave rising filter response, from DC to daylight, and then applying that to a frequency modulator.  That will produce true PM.  Likewise you can produce FM with a PM modulator by passing the modulating signal through a 6dB per octave falling response, and applying that to a phase modulator.  That will produce true FM, although it has a well-known dynamic range limit at the lower frequencies.  But with enough frequency multiplication, you can get decent FM that way.  Well, it's not exactly 6 dB per octave, it's closer to 6.021dB per octave.  In fact, it is exactly 20dB per decade (10:1 frequency).  OK, on with the AM explanation.

AM

Imagine you have a clock with three hands.  One hand is long, the other two hands are of equal length, exactly half as long as the long hand.

I will use this diagram to represent AM with 100% modulated with 1 KHz sine wave audio.  The long hand represents the carrier.  The two short hands represent the sidebands.

The hands spin around at enormous speed.  In this example, the carrier hand spins at 3885000 times per second.  One short hand spins at 3884000 times per second (the lower sideband), and the other short hand spins 3886000 time per second (the upper sideband).  Mathematicians like to spin things counter-clockwise, but for this example, I will spin clockwise.
But these hands are spinning too fast to see.  So now, we make the spin relative.  Now the long hand is always pointing up at 12.  This means we are referencing to the carrier frequency and phase.  The two sidebands spin 1000 times per second in opposite directions.  Not only that, but they rotate such that they both point up at the same time, then one rotates back to the 9 o'clock position while the other rotates to the 3 o'clock position, then they rotate so that they both point down, then 3 and 9 o'clock, then straight up again, over and over.

These hands represent "trigonometric vectors" which represent the carrier and the two sidebands of a 100% modulated AM signal.  If you add these vectors to the carrier vector, you see the modulation: a positive peak of twice the carrier voltage when both short hands point straight up; a minimum of zero when both short hands point straight down, carrier level when both short hands point in opposite directions, and intermediate values between those instants.  The combined phase does not shift relative to the carrier phase, because phase differences between the sidebands cancel out, adding to and subtracting from the carrier, leaving only carrier amplitude variations.

We synthesize the sideband signals when we modulate an AM carrier.  It looks like we are raising and lowering the carrier level, and in one instantaneous sense we are, but we can also see the carrier level as constant, and the moment to moment level variations coming from heterodyning action from the sidebands.  We can work with whichever view is more convenient at the moment.

PHASE MODULATION

You can simulate phase modulation by moving the short hands a little differently.  Also, in FM and PM, we call the sidebands "sidecurrents" for some unknown reason.  It is easier to see PM this way if you use shorter hands.  So in the following example, imagine the short hands are 1/4 as long as the big hand.  If you have the big hand (carrier) at 12 o'clock, the small hands will rotate again in opposite directions, and again at 1000 times per second.  But this time, one short hand points toward 12 o'clock while the other short hand points toward 6 o'clock, then both short hands point toward 3 o'clock, then one short hand points toward 12 o'clock while the other short hand points toward 6 o'clock, then both short hands point toward 9 o'clock, then one short hand points toward 12 o'clock while the other short hand points toward 6 o'clock, over and over again.

This time, the amplitude is fairly constant.  When the two short hands are pointing at 12 and 6 o'clock, they cancel out, and you have the carrier level and phase.  When the two short hands point toward 3 o'clock, they add up to 1/2 the carrier length (remember they are extra short in this example), and they add to the carrier vector as a right triangle of carrier side = 1, sideband side = 0.5.  The diagonal or hypotenuse is the square root of 1 squared plus 1/2 squared, or the square root of 1.25, or about 1.118, and the phase angle is the angle whose "tangent" is 0.5 / 1, or arctan(0.5), which is about 27.6 degrees.  The same situation exists when both sideband vectors point toward 9 o'clock, but we have -27.6 degrees.  So we have a peak phase modulation of 27.6 degrees, and a little amplitude variation.

In real PM we do not have any amplitude variation.  This is where the carrier level variations and the 2nd, 3rd, 4th, etc. order sidecurrents come from in PM and FM.   These sidecurrents corespond to second, third, etc harmonic distortion in AM sidebands, and they take up bandwidth.  Fortunately, the higher order sidecurrents have vanishingly low amplitude at low levels of phase modulation.  However, a particular carrier level and an infinite set of harmonically related sidecurrents, at the proper phase angles, are created because the angle-modulated signal amplitude is constant.

As you apply more phase modulation, a lot more high order sidecurrent energy is created.  This is why we go to narrow FM to reduce bandwidth.  At around a modulation index of 0.5, the 2nd and higher order sidecurrents are pretty low in amplitude, but you still have some reasonable length (amplitude) in the sidecurrents.  It is the energy in these sidecurrents that carries the sounds over the air in PM and FM, very much like AM.

More FM or PM deviation produces stronger sidecurrents, just like AM, corresponding to longer "short" hands in the example, but they also produce higher order sidecurrents that start spreading out in frequency.  This is not an overmodulation effect such as we sometimes see with AM, it is intrinsic to constant-amplitude angle-modulation (FM and PM) in general.  We can avoid it by adding some specific amplitude modulation, but that is complicated, and then we are again dealing with all of the technical difficulties of generating a varying-amplitude signal as well as angle modulating it.

With narrowband PM and FM, we limit deviation to what we call a modulation index of about 0.5 to 1.  This corresponds to peak FM deviation of 1/2 to 1 times of the modulating frequency, or PM of 1/2 to 1 radian peak.  A radian is about 57.3 degrees.  In the example above, the sidebands were 0.25 carrier amplitude (1/16 carrier power) at +/- 27.6 degrees peak, which is about 1/2 radian.  With both of these sidecurrents together, the sidecurrents power is only 1/8 of unmodulated carrier power at that modulation index.  Each second order sidecurrent is about 33 dB below unmodulated carrier at a modulation index of 0.5.  Each third order sidecurrent is about -53dB below the unmodulated carrier level, and higher order sidecurrents are far below that. 

At a modulation index of 1 (+/- 57.3 degrees peak phase modulation, peak frequency deviation equals modulating frequency) you have exactly as much sidecurrent energy as 100% modulated AM of the same carrier strength, but now the first-order sidecurrents have about 89% of it, and the second order sidecurents have about 11% of it.  This means each 2nd order sidecurrent is about 22 dB down from the unmodulated carrier level, corresponding to an AM signal with about 2.5% second harmonic distortion.  (This is not actual distortion in the angle-modulated signal, but it is "splatter" intrinsic to the modulation mode.)  Each third order sidecurrent is pretty low at a modulation index of 1, around 37 dB below the unmodulated carrier level, and each fourth order sidecurrent is about 55 dB below unmodulated carrier level.  Narrowband angle modulation is not as clean as AM, although it comes close.

When multiple frequencies (such as speech) are combined to modulate an angle modulated transmitter, bandwidth of the signal is not as wide as it would have been in the sine wave case, because the modulation index of each frequency component is less than the total.  However, sibilents will not benefit as much from this fact, because they are spectrally concentrated. 

An interesting thing that happens with sine-wave modulated FM and PM.  As deviation increases, the carrier level decreases as the sidecurrent level increases.  At certain modulation indexes, or indices, the carrier actually nulls out, and all of the signal energy is in the sidecurrents.  These points are called "Bessel nulls," and the first one happens at a modulation index of about 2.405.  This is called the "first Bessel null."   As deviation increases further, the carrier rises, peaks, and then decreases, finally reaching a second Bessel null around a modulation index of 5.52.   More Bessel nulls (the third, fourth, etc) appear around modulation indices of 8.65, 11.79, 14.93, and they go on forever.  When modulation is very linear, and a very clean sine wave is used for modulation, the Bessel nulls are very precise.  Bessel nulls are used to calibrate modulation metering equipment.

  Bacon, WA3WDR

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This page last updated 18 Jul 2001.