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Author Topic: Bw= 2x(D+M)  (Read 2351 times)
KF6GUB
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« on: January 20, 2013, 12:42:45 PM »

First, I don't understand the difference between "D" and "M".  I guess that's my big confusion.  Any clarification would be greatly appreciated.  What are "D" and "M"?  thx   Jim   Grin
« Last Edit: January 20, 2013, 12:50:10 PM by KF6GUB » Logged
G8HQP
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« Reply #1 on: January 20, 2013, 01:53:09 PM »

Some context might help. I don't recognise the equation. Is Bw some sort of bandwidth?
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KF6GUB
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« Reply #2 on: January 20, 2013, 01:58:44 PM »

It's Carson's rule, which is a formula that supposedly gives a good approximation of an FM signal's useful, practical bandwidth.  thx  Jim
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G8HQP
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« Reply #3 on: January 20, 2013, 02:02:57 PM »

OK. I guess one is baseband bandwidth (e.g. 3kHz for us, 15kHz for mono broadcasters, 53 kHz for stereo broadcasters) and the other is maximum deviation (e.g. 75kHz for broadcasters).

This approximation avoids having to study the details of Bessel functions, which give the accurate figures.
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KF6GUB
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« Reply #4 on: January 20, 2013, 02:13:40 PM »

Does the baseband bandwidth refer to the unmodulated RF carrier?  If that is true, then the maximum deviation becomes obvious, I think.   thx   Jim
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KE3WD
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« Reply #5 on: January 20, 2013, 02:38:00 PM »

First, I don't understand the difference between "D" and "M".  I guess that's my big confusion.  Any clarification would be greatly appreciated.  What are "D" and "M"?  thx   Jim   Grin

Deviation and Modulation

73
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N5KNG
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« Reply #6 on: January 20, 2013, 02:43:28 PM »

According to Wikipedia, "Carson's bandwidth rule defines the approximate bandwidth requirements of communications system components for a carrier signal that is frequency modulated by a continuous or broad spectrum of frequencies rather than a single frequency." It does not work well for discontinuous modulating signals (e.g., pulse or square waves). The bandwidth refers to the total bandwidth of the frequency modulated signal.  This rule is a simplified way of determining the RF bandwidth for a system by accounting for most (98%) of the sideband energy.  Again, according to Wikipedia, "setting the arbitrary definition of occupied bandwidth at 98% of the power still means that the power outside the band is only about 17 dB less than the carrier inside, therefore Carson's Rule is of little use in spectrum planning."  

D is the peak frequency deviation, and M is the highest frequency in the modulating signal

Hope that helps.
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N5KNG
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« Reply #7 on: January 20, 2013, 02:48:30 PM »

To see an example of the rule in use, you can refer to: http://www.docstoc.com/docs/78360469/21-Carsons-Rule
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KF6GUB
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« Reply #8 on: January 20, 2013, 02:57:09 PM »

I saw what's written at wikipedia.   The explanation did not enlighten me.   In layman's terms, what do "D" and "M" stand for?  thx  Jim
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AC5UP
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« Reply #9 on: January 20, 2013, 04:22:41 PM »

A frequency modulated carrier changes frequency in step with the audio... The deviation (frequency delta) from the nominal frequency is 5 kHz for 2 Meter FM which is adequate for communications grade audio. Your local FM broadcast stations are spaced 200 kHz apart and allowed a maximum of 150 kHz deviation. Old school NTSC TV ran AM modulation on the video and FM audio at 50 kHz deviation.

From what I gather in this thread the greater the [D]eviation (frequency swing) the better the high frequency response in the [M]odulated audio. You know this is true because FM broadcast sounds way better than your local repeater.
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KB3HG
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« Reply #10 on: January 20, 2013, 04:28:32 PM »

Twice the maximum frequency modulated plus twice the modulation index is possibly what your looking for. It's been years since I thought about Carson's rule.
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N3JBH
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« Reply #11 on: January 20, 2013, 04:45:24 PM »

I saw what's written at wikipedia.   The explanation did not enlighten me.   In layman's terms, what do "D" and "M" stand for?  thx  Jim

D stands for - Deviation
and
M stands for - Modulation
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KF6GUB
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« Reply #12 on: January 20, 2013, 04:47:28 PM »

So...what does the modulated index refer to, in layman's terms?  I'm confusing the maximum frequency modulated with the modulation index.  thx  Jim
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KF6GUB
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« Reply #13 on: January 20, 2013, 04:49:33 PM »

I guess I'm having trouble with modulation and deviation and what they mean to eachother  thx   Jim
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N3JBH
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« Reply #14 on: January 20, 2013, 04:55:56 PM »

http://en.wikipedia.org/wiki/Frequency_modulation

http://en.wikipedia.org/wiki/Frequency_deviation
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