At 12/2/2005 16:36, you wrote:
>I agree. DTMF is two sinusoids. You don't want the limiting action to
>distort the signal. BTW this is also why you should use 3KHz as a baseline
>on linked systems to keep the audio as linear as possible through
>chain-linked RF repeaters.
>
>One thing that bugs me though is that the 2005 ARRL handbook says 5KHz RMS
>deviation (appx 7.07 KHz) for NBFM. I beleive this is wrong.
Absolutely. In fact this is the first time I've ever seen the term "RMS" &
"deviation" together. Some editor oopsed big time on that one.
> I've always used peak-to-peak/2 setting on the HP8920 service monitor.
The (almost) universally accepted method of expressing deviation is peak,
which is equal to peak-to-peak/2.
>
>
>Carson's rule states that channel bandwith is 2 times the sum of the peak
>deviation and highest modulating frequency. If you use 5 KHz RMS deviation
>with a 1KHz sinusoid, the channel bandwidth will be 2*((1.414*5000)+3000)
>= 20.14KHz!
>This ain't 16K0F3E.
Here's the actual "Bessel breakdown" of the FM sidebands in dB, using a
modulation index of 7.05 (closest I have to 7.07 in my spreadsheet):
Carrier -10.46
+/- 1 kHz -39.74
+/- 2 kHz -10.54
+/- 3 kHz -14.95
+/- 4 kHz -16.78
+/- 5 kHz -9.29
+/- 6 kHz -9.32
+/- 7 kHz -12.44
+/- 8 kHz -17.56
+/- 9 kHz -24.21
+/- 10 kHz -32.10
+/- 11 kHz -41.04
+/- 12 kHz -50.90
+/- 13 kHz -61.58
+/- 14 kHz -73.00
So Carson's rule does pretty well here - the 20 dB BW is actually just
under 18 kHz.
Bob NO6B
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