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PA2AGA > TCPDIG 09.06.96 20:05l 87 Lines 3206 Bytes #-10988 (0) @ EU
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Subj: TCP-Group Digest 96/121
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Date: Sun, 09 Jun 96 00:33:34 MET
Message-Id: <tcp_96_121>
From: pa2aga
To: tcp_broadcast@pa2aga-10
Subject: TCP-Group Digest 96/121
X-BBS-Msg-Type: B
TCP-Group Digest Sat, 8 Jun 96 Volume 96 : Issue 121
Today's Topics:
9600 Baud Deviation
Send Replies or notes for publication to: <TCP-Group@UCSD.Edu>.
Subscription requests to <TCP-Group-REQUEST@UCSD.Edu>.
Problems you can't solve otherwise to brian@ucsd.edu.
Archives of past issues of the TCP-Group Digest are available
(by FTP only) from ftp.UCSD.Edu in directory "mailarchives".
We trust that readers are intelligent enough to realize that all text
herein consists of personal comments and does not represent the official
policies or positions of any party. Your mileage may vary. So there.
----------------------------------------------------------------------
Date: Fri, 07 Jun 1996 07:53:02 +0100
From: Matthew Phillips <matt@nuthatch.dungeon.com>
Subject: 9600 Baud Deviation
I'm afraid I can't speak about G3RUH modems directly as I don't use one
but I have developed a modem based on similar principles and I would
suggest that the deviation is best set as 1/2 the data rate overall ie.
4800Hz from one peak to the other. In other words +/- 2400 Hz for a
data rate of 9600 b/s.
In most FM voice radios the group delay response of the receiver IF filter
is poor near to the filter skirts. If the deviation is too high the S/N of
the signal will improve but the BER of the modem will be poor because of
the group delay distortion introduced by the IF filter. The effect can be
reduced by keeping the receiver and transmitter carrier oscillators aligned
(to centre the signal in the IF filter) and keeping the deviation low
enough to stay in the less group delay distorted region (the filter centre).
Brian mentioned that most of the 9600 b/s designs require good S/N to work
correctly. My modem design will give good performance in signals of quite
poor S/N ( a BER of 1 in 10000 at 10dB S/N) if you take out the effects of
channel group delay distortion. So its worth taking a bit of a hit on S/N
by keeping the deviation lower than setting it higher and then suffering
group delay distortion in the Rx IF.
---
Best Regards Matt
Internet:------- matt@nuthatch.dungeon.com
Amprnet :------- g6wpj@gb7eip.ampr.org
AX25 BBS:------- G6WPJ@GB7EIP.#31.GBR.EU
GMSK Data Products Radio Modem kit details on:
http://www.dungeon.com/~nuthatch/vfast288.html
------------------------------
End of TCP-Group Digest V96 #121
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