Christophe Rhodes  <csr21@cam.ac.uk> wrote:
+---------------
| Let's say that you're interested in transmitting speech-quality data,
| so a modulating frequency of about 22kHz is adequate -- if you want
| music or video, obviously you'll need a higher modulating frequency,
| and probably your transmitter won't lie in the MHz range.  Then your
| band actually lies between 200.083296 MHz and 201.027296 MHz.
| 
| Given this, do you really think it's sensible to require your
| transmitting frequency to be printed and treated as exactly 201.005296
| MHz?  Or would 201.0 perhaps be a better representation?  (If you're
| sending less dense information than speech, then you might be
| interested in one or two more significant figures -- but not very many
| more, because of the limit imposed by your transmitter.)
+---------------

But if you're using narrow-band SSB with 6 kHz channel separation
[which some miltary radios do], then you'd *better* be carrying
6 or 7 digits of precision, 'cuz if you drift from 200.083296 MHz
to 201.027296 MHz, you've just crossed 15 or 16 other conversations!!

Even in amateur radio using (DSB) AM or FM, channel separations
of 25 kHZ at frequencies of hundreds of MHz is not unusual, e.g.:

    <http://solair.eunet.yu/~yu1arl/bp-432.htm>
    2.2(i) On a temporary basis, in those countries where
    433.619-433.781 MHz is the only segment of the 435 MHz band
    available for Digital Communications:

    1. Channels with centre frequencies 433.700, 432.725, 432.750,
       432.775, 434.450, 434.475, 434.500, 434.525, 434.550 and 434.575
       may be used for digital communications.
    2. Use of these channels must nor interfere with linear transponders.
    3. Modulation techniques requiring a channel separation exceeding
       25 kHz must not be used on these channels. 

And for clear separation, the center frequencies must be within +/- 500 Hz.
So, yes, modern radios must use *much* more precise frequencies than
commonly used earlier...


-Rob

-----
Rob Warnock			<rpw3@rpw3.org>
627 26th Avenue			<URL:http://rpw3.org/>
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