Higher speed with G3RUH 9600bd modem

Date: 24-Aug-93 01:22 UTC+9
From: James Miller >INTERNET:james.miller@mrc-apu.xxxxxxxxx
Subj: G3RUH modem 9600++ = 38400

I am frequently asked how to make my 9600 baud packet radio modem, as used
by UOSAT and packet trunks, go faster (and slower!).

The prospect of a 38400 baud device on a new UOSAT prompts the following
notes. That neither your TNCs go this fast, nor will your radios have the
bandwidth is not my fault. Onward!  Upward!  Enjoy!

If you inject this onto your packet network you may change the "9600" or the
"WW" but do not alter the BID (bulletin identifier) which is $RUH93235.
This helps prevents duplicates, although some helpful BBSs generate their own
fresh BID which makes a nonsense of the system.

---------------------------------- cut here --------------------------------
SB 9600 @ WW $RUH93235
Use 9600bd modem at higher speed

         Higher Speeds with the G3RUH 9600 baud Packet Radio Modem
                         by James Miller G3RUH

                              1993 Aug 23

The modem is capable of speeds up to 64000 baud. This limit is set by the
maximum rate that the DAC chips can operate. This note describes how to
achieve retes from 4800 to 64000 baud. The slowest speed is suitable for
12.5 kHz channellised radios. The highest speed suits radios that have
broadcast FM bandwidth filters.

To implement a higher speed you need to:

 1. Increase your TXData rate (!)
 2. Increase the associated TXClock
 3. Change some analogue filter components proportional to
    the speed increase.

It is not nessessary to change either of the eproms. If you are going
for a higer speed, it is likely that the radios involved are "specials"
and you will already have wide bandwidth and flattish group delay, so the
loopback selection 0 from the standard ROM will be OK.

The table below suggests the best conditions for different speeds.
Components references are for my own PCB card. Clones are different.

                     Data rate - Baud
 Comp           4800    9600   19200    38400   64000
 R6             220k    100k     47k      22k     15k

 R16            100k    100k    100k      47k     15k
 R17             82k     82k     82k      39k     12k
 R18             39k     39k     39k      18k      5k6
 R19             27k     27k     27k      15k      3k9
 R21            100k    100k    100k      47k     15k
 R22             56k     56k     56k      27k      8k2
 C18             4n7      4n7     4n7      1n    680p
 C20           220p     100p     47p      22p     12p C27             2n2     1n     470p     470p      1n        )
 C28             2n2     1n     470p     470p      1n        )
 C29             6n8     3n3      1n5    470p    470p        ) 2% or
 C30           220p    100p      47p      47p    100p        ) better
 C31             1n    470p     220p     220p    470p        )
 C32             2n2     1n     470p     220p    150p        )
 Deviation +/-   1.5      3       6       12      20 kHz        ) In FM
 IF Bandwidth      8     15      30       60     100 kHz        ) service

These modifications have been tested in both amateur and commercial
service.  All comments gratefully received, and added to the database.

73 de James G3RUH @ GB7DDX.#22.GBR.EU        1993 Aug 23 [Mon] 0917 utc


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