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Re: AMSAT: 38K4



 (Ulf Kumm) writes:

> So, it makes sense to use it with a G3RUH modem for 38k.

The following two items are old ... but so is the RUH 9K6 modem.


            64000 bps Packet Radio Modem Available Now!
            -------------------------------------------
                       by James Miller G3RUH

                           1993 Feb 01
 
I wonder if experimenters are aware that my 9600 baud Packet Radio Modem
design can (indeed IS) being used at 64 kbps by several groups?

All you have to do is widen some analogue filter bandwidths by a factor of
64000/9600 = 7:1 by scaling down a few resistors and capacitors.  These are
R16/17/18/19/21/22 and C18/20/29/32.

The FM deviation can be some +/-20 kHz to +/-40 kHz, and the RF bandwidth
will be on the order of 100 to 200 kHz.  So you can use typical
cheap broadcast FM radio IF filters at 10.7 MHz in the RX.

The unique advantage offered by my 9600 baud design is the 8-bit long
digital F.I.R. TX waveform filter.  This allows precise matched filtering to
be implemented for the channel (1st Nyquist Criterion).

One or two ostensibly similar offerings encountered recently do not do this.
As a result their ISI (Inter Symbol Interference) properties are poor and
performance degraded accordingly.

The overwhelming advantage of using DFM (Direct FM) is its robustness. 
Radio amateurs are not short of bandwidth or e.i.r.p. at UHF and up.  So in
practice the use of PSK and/or multi-amplitude techniques is unnecessary, as
they are much more complex and comparatively fragile.

End of technical note.
---------------------

73 de James G3RUH



      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

 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


73
Richard W L Limebear G3RWL
g3rwl@amsat.org
FOC # 1188

          So many beautiful girls ..... (sob) so little time
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