On Sun, 30 Jan 2000, John Ackermann wrote:
> Thanks for the plug, Jose! The URL is right -- www.febo.com/layer-one/
> will get you there.
My pleasure, it really has been useful, so what is best than guiding others
in need there ?
> Overdeviation on 9600 causes a different problem than 1200 -- the 9600
> issue is that you will run into the skirt of the IF filters if you're
> deviating too wide, and that will cause distortion that makes the modem
> unhappy.
Of course, it is just a different kind of "clipping" and distorting the
data signals. I have had users that have overdeviated so heavily with 1200
baud that the "BUSY" light goes off during modulation !
> FM bandwidth can be "rule of thumbed" as BW = 2*(M+D), where
> M is the maximum modulating frequency and D is the peak deviation.
> Since 9600 baud has a maximum modulating frequency of 4800Hz, 3kHz
> deviation results in a bandwidth of 15.6kHz, which is just about as
> wide as a normal narrowband receiver can handle..
Close enough to 16 Khz so to squash one side to the IF filter skirt if you
are not exactly "on channel".
> Actually, the occupied bandwidth of 9600 baud is somewhat less than
> this formula, which is based on voice and not data characteristics.
> I've heard, but can't track down the reference right now, that a better
> rule for FSK data is BW=2M+D, which would yield about 11.6kHz bandwidth
> for a deviation of 3kHz.
That may account for the brickwall response from the G3RUH modems FIR
filters, which is not what you get when speaking to a microphone.
> In any event, after taking any frequency error between RX and TX into
> account (particularly at UHF), you don't have much slop to work with.
> Overdeviation at 9600 baud is probably a bigger killer than overdriving
> the audio at 1200.
>
> 73,
> John N8UR
> [EMAIL PROTECTED]
> http://www.febo.com
I like this thread ! Thanks, John !
73 de Jose, CO2JA
---
Ing. Jose A. Amador Fundora | Tel : (537) 20-7814
Dept. de Telecomunicaciones | E-Mail : [EMAIL PROTECTED]
Facultad de Ing. Electrica |
ISPJAE |
