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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