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 rates from 4800 to 64000 baud. The slowest speed is suitable for 12.5 kHz channelised 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 necessary to change either of the eproms. If you are going for a higher 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. Component 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 C26 ( 470p 220p 100p 100p 220p ) 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.