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Author Topic: One more time with the Carson bandwidth rule...please  (Read 3206 times)
KF6GUB
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« on: January 22, 2013, 11:50:54 AM »

Bw=2x(D+M):  I understand the (Bw) bandwidth.  I understand the (D) maximum frequency deviation of the carrier frequency.  I'm guessing that the (M) maximum modulating audio frequency refers to the amplitude modulated wave which is imposed upon the carrier frequency.  Correct?  thx  Jim
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KB4QAA
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« Reply #1 on: January 22, 2013, 12:53:30 PM »

You didn't like the answers you got on QRZed?  Wink
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KE3WD
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« Reply #2 on: January 22, 2013, 01:00:54 PM »

If the Amplitude Modulation "wave" were to be imposed on the Carrier, it would be AM. 

The audio from the microphone is used to *vary the frequency* of the FM transmission and not the Amplitude. 

Your formula describes how there is a bit of nonlinearity between the Audio frequencies being converted to move the actual Carrier frequency back and forth to correspond to the audio information. 


That's a rather basic layman's description, there are other factors to consider, but I'm thinking that you need to first grasp the simple stuff here, judging from all the noise you're making on two different forums. 


73
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WA3SKN
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« Reply #3 on: January 22, 2013, 01:20:48 PM »

I would forget about bandwidth "rules" and get hold of a ARRL Handbook.  Read the chapters on modulation... very educational!  A very good investment when you what to know how things work.
73s.

-Mike.
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KB4QAA
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« Reply #4 on: January 22, 2013, 01:27:59 PM »

Carson's rule is typically used to calculate bandwidth of FM transmitters.

Let's work an example for ham applications.

Peak Deviation:  5khz (typical for repeater FM operations, this is an arbitrary number based on the transmitter design).  This actual number varies depending whether you are whispering or yelling into the mic at any given moment.   [is an equipment setting].

Deviation is caused by the signal being applied to the carrier, whether voice, music, packet/aprs waveform, etc.   Commercial two-way users are switching to 2.5 Khz 'narrowband' deviation to allow more channels.  Commercial FM  stations typically have 75 Khz deviation.  Wideband, indeed!

Peak Modulation:  Alexander Bell determined the optimum frequencies for communications were between 300hz and 3,000hz.  This is typical for telephone and most amateur operations.  So, the Peak Modulation is 3,000Hz/3Khz.

Bandwidth = 2 * (Peak Deviation + Peak Modulation ) = 2*(5,000 Hz +3,000 Hz)
Bandwithch = 2*(8,000 Hz)
Bandwidth = 16,000 Hz or 16 Khz

So if our center/carrier frequency is  146.000 Mhz, we will occupy 8 Khz on either side from 145.992 Mhz to 146.008 Mhz.
« Last Edit: January 22, 2013, 02:02:43 PM by KB4QAA » Logged
KF6GUB
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« Reply #5 on: January 22, 2013, 01:34:38 PM »

You didn't like the answers you got on QRZed?  Wink
I did not dislike the answers; I'm still puzzled on some aspects.  I guess I don't understand about the FM sidebands, for one thing. 
I've got a stack of books; I've spent a hundred and a half to get up to speed on ham. I reviewed the latest technician materials, having got my tech license some 15 years ago.  I've never owned a ham radio.  Maybe that's the issue.  Not looking for an argument.   I'm just looking for illumination.  thx  Jim
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KB4QAA
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« Reply #6 on: January 22, 2013, 01:47:48 PM »

FM sidebands are not something that are commonly discussed among non-engineers, nor typically a problem.  They are a common concern for AM/SSB transmitters.  

Of course, with either type of transmitter, appropriate filters must be included in the design to suppress sidebands to meet FCC specifications.

So, at face value, sidebands of an FM signal are not something addressed in calculating or applying the basic Carson rule.   Typical usage would be evaluating if our signal will remain within allotted bandwidth, and whether we need to adjust the Deviation or Peak Modulation to remain within standards.

[edit]  Practically, we would check the sideband levels with a Spectrum Analyzer.  We might compare this against the transmitter specifications for sideband suppression (measured in dB).   I'm not familiar with a method to mathematically estimate sideband levels or width, not being an engineer.

cheers,  bill
« Last Edit: January 22, 2013, 01:51:51 PM by KB4QAA » Logged
KF6GUB
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« Reply #7 on: January 22, 2013, 02:05:02 PM »

WA3SKN:  I have the ARRL Handbook from '96.  STILL left me puzzled.  That's why I asked here.  

KE3WD:  What's this other forum I'm making "noise" on?   I began on another site; this one seemed more promising.  Believe it or not, these questions I've asked come up in regards to the current examination questions used for the tech and general exams.   I'm for grasping the "simple stuff".  I thought that's what I was doing.  You want to discourage me?   You can.  I have yet to spend any real money on this hobby.   I just have books which provide me with enlightenment.  I'm always good for that.  thx Jim

PS:  Yup, I did pose other questions on another portion of this site.  When I saw the subset "elmer", I thought those would be the people to ask about troublesome concepts.  I meant not to insult anyone. thx   JIm
« Last Edit: January 22, 2013, 02:12:11 PM by KF6GUB » Logged
KB4QAA
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« Reply #8 on: January 22, 2013, 02:17:59 PM »

Ok, lets talk real world application for the ham.

Deviation:   
-Over deviation will cause audio distortion
-Under Deviation problems are usually most common when a handheld can hear repeaters well, but the audio signal is clear but weak.

Adjustment:  A audio tone generator (typically 1,000 hz) and a Deviation Meter is needed.  The tone is applied to the mic in\put (or microphone) while keying the radio and the deviation pot is adjusted to show no more than 5 Khz.
 
Modulation:
Modulation values are normally fixed by the radio design. 

-Sub Audible Tones:
The DTMF or PL tones are mostly below normal hearing abilities.  Sometimes the level drifts and either goes low, causing failure to activate repeater functions, or may drift high level, causing the very highest tones to become audible and cause annoyance.  Adjust the tone level pot as necessary.  A Service Monitor is helpful for checking tones.

The repeater is supposed to strip the tones with a high pass filter, but occasionally this fails or needs adjustment.

b.
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G3TXQ
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« Reply #9 on: January 22, 2013, 02:38:51 PM »

I think if you're looking for an "intuitive" explanation of Carson's Rule, you may be disappointed; you really have to dig into the maths to understand it.

Any RF carrier that is modulated produces sidebands. In the case of amplitude modulation those sidebands are easy to figure - not so easy in the case of frequency modulation.

Unlike amplitude modulation, if you frequency modulate an RF carrier with a single audio tone you get an infinite number of sidebands. Carson's Rule is simply an approximation of the bandwidth that contains the major share of the sideband energy.

This may help:
http://www.radio-electronics.com/info/rf-technology-design/fm-frequency-modulation/spectrum-bandwidth-sidebands.php

73,
Steve G3TXQ
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KF6GUB
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Posts: 29




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« Reply #10 on: January 22, 2013, 02:51:11 PM »

In the books, when fm bandwidth is discussed, I seem to understand that fm bandwidth is determined based on peak frequency deviation combined with peak modulating frequency.   Why not just peak frequency deviation and leave modulating frequency out of it?  thx  Jim
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G3TXQ
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« Reply #11 on: January 22, 2013, 03:07:16 PM »

In the books, when fm bandwidth is discussed, I seem to understand that fm bandwidth is determined based on peak frequency deviation combined with peak modulating frequency.   Why not just peak frequency deviation and leave modulating frequency out of it?  thx  Jim

Look at the maths in the article I referenced. The sidebands are spaced by the audio modulating freauency, but their amplitude is a function of the Modulation Index (and therefore the modulating frequency and the deviation)

Like I said, you have to dig into the maths; you can't always expect to find a simple intuitive answer.

73,
Steve G3TXQ
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KB4QAA
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« Reply #12 on: January 22, 2013, 03:47:55 PM »

Ok, I understand your query now, I think. 

You can't separate the effects of Deviation and Modulation. 

Think of dipping your finger into a lake.  this creates ripples.

Let us set the 'Deviation" of your finger as 1 inch or 1 knuckle".   You dip it into the lake 1".

If you dip it slowly in and out over a minute (Modulation), you will have very little disturbance or ripples.

If you dip it every second (1Hz Modulation) you will have quite noticeable ripples.

If you dip it four times a second, 4Hz, you will have many ripples, and probably attract sharks or alligators.  Smiley

You can then change the Deviation, dipping your finger deeper or shallower, but you cannot separate the effect of the Modulation, speed or depth.
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KF6GUB
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Posts: 29




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« Reply #13 on: January 22, 2013, 04:14:22 PM »

I think if you're looking for an "intuitive" explanation of Carson's Rule, you may be disappointed; you really have to dig into the maths to understand it.

Any RF carrier that is modulated produces sidebands. In the case of amplitude modulation those sidebands are easy to figure - not so easy in the case of frequency modulation.

Unlike amplitude modulation, if you frequency modulate an RF carrier with a single audio tone you get an infinite number of sidebands. Carson's Rule is simply an approximation of the bandwidth that contains the major share of the sideband energy.

This may help:
http://www.radio-electronics.com/info/rf-technology-design/fm-frequency-modulation/spectrum-bandwidth-sidebands.php

73,
Steve G3TXQ
Thanks, Steve.  I went to that site (cited above), and that explanation might have done it.  I'm certain I'll have other questions, but those about fm frequencies have bugged me for over 15 years.  I guess getting answers whenever is better than never.  thx  Jim
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KF6GUB
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« Reply #14 on: January 22, 2013, 06:12:45 PM »

Thanks to you all; I think I got it.  Steve (G3TXQ) sent me to radio-electronics.com which seem to widen my thinking.  Yet I ended going back to a site entitled, would you believe, Frequency Modulation within Wikipedia, no less.  I'd looked at that body of information a bunch of times, but I guess this time was the charm.  You wouldn't believe the number of bookmarks I have listed about this stuff.  I guess I need to organize them, because I don't want to throw them away...yet.  Thanks, again, for your patience.  Jim Grin
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