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High Speed Modem for Packet Radio
Designed and Manufactured by GMSK Data Products - UK
Reviewed by
Roger J. Cooke, G3LDI
GMSK Data Products
is a partnership company set up by two professional electronic design
engineers who are also keen radio amateurs, Matt, G6WPJ, and John,
G8STW. They are the sysops of the TCP/IP packet stations GB7WPJ and
GB7STW respectively. Matt and John are keen members of their local TCP/IP
group, the Essex IP Group or EIPG.
The first project
was to design a high speed modem for use Radio Amateurs.
From the desire to see if better than 9600 b/s was possible in normal
25kHz channels and also to modernise on the excellent design some years
ago by James Miller, G3RUH. The result was the Vfast28.8 modem adapter
for a TNC2. This proved very effective and as a result of an approach
from the Essex group and also the East Anglian Data Group (EADG), Matt
and Oh decided to develop a real time bit repeater for speeds up to 38,400
b/s. In fact they managed 57,600 b/s and two further products resulted,
the AX384 and the AX576.
These new TNC designs
have generated a lot of interest and are now being looked at by a number
of packet groups around the UK as the basis for the new High Speed Network.
The Vfast28.8 modem adapter also developed into a full RS232 driven radio
modem for transparent data transfer not using the overhead of AX25 - the
RSM192, latterly a smaller more compact version, the RSM192S has been
produced as a custom project for a company in the USA.
These well engineered
products use modern design techniques including the PIC family of micro-controllers
for high speed bit processing. In the RSM192, the PIC micro is used to
manage the RS232 comms, handle a complex 2-speed fast frame acquisition
sequence, generate a very effective Data Carrier Detector and provide
real time bit randomising. The PCB's are very high quality and the layout
done to ensure good analogue to digital separation. Quality in design
and production is taken very seriously by both Matt and John.
GMSK.
A method of data modulation
known as Gaussian Minimum Shift Keying (GMSK) is used to pass data
as fast as practicable in the radio channel. GMSK modulation and demodulation
and the modems' control and data coding are all performed in the latest
integrated circuit devices. This gives a compact design and layout which
is easy to build with high performance.
The following is a
list of features:
- Radio Port speeds
of 4800, 9600, 19200 and 38400 b/s with AX384 or 7200, 14400, 28800,
57600 b/s with AX576.
- High Speed Radio
Modem operates full duplex at all the radio speeds above with no component
changes.
- RS232 port speeds
from 9600 to 38400 b/s (AX384) or 115200 b/s (AX576).
- TAPR TNC-2 EPROM
compatible including NET/ROM & 64K EPROM's such as TheNet X1J, and
ROSE.
- A Real Time Bit
Repeater can be enabled from software. Provides a contention free LAN
in its coverage area. An intelligent FIFO buffer allows transmission
of long frames without bit under/over -runs. Other TNC functions are
unaffected allowing simultaneous use, e.g. as a Node - TheNet X1J.
- "Set-up"
Software in ROM allows modem configuration to be modified on screen.
- Live Link Bit error
Rate Measurements enable easy set-up of data links.
- Full Morse Ident
as per the UK license regulations, regardless of TNC software fitted.
This can be disabled for use elsewhere.
- 96k ROM Space allows
in addition to "Set-up and Kiss ROM", 1 x 32k EPROM, 1 x 64k
EPROM or 2 x 32k EPROM images to be fitted.
- 128k RAM can be
fitted in place of the normal 32k if required.
- Radio Control Signals
PTT and Mute can be set active high or low from Set-up software
- 10 Mhz Z80 Processor
ensures no lost or missed frames due to software errors.
- Choice of data
randomizer allows G3RUH or GRAPES method to be used.
- Bi-Phase Data Coding
can be selected in place of data randomizer to allow simple interfacing
to most types of voice radios (Includes FM / PM crystalled or synthesized)
- RS485 Interface
Option allows multiple TNC's to be connected in "Node Stack"
with simple 4-wire cable. No more diode matrices!
Theory of Operation.
Frequency Shift Keying
The simplest method of sending data signals over an FM radio is to use
a frequency shift in one direction to represent a logic 1 and a frequency
shift in the other direction to represent a logic 0. A an example, a transmitter
may operate on a carrier frequency of 144.500Mhz na dmove its' carrier
down by 3kHz to 144.497 to signal a logic 0, and then up 3kHz to 144.503Mhz
to represent a logic 1. Note that no audio sub-carrier is used. This is
shown in Fig. 1.
This method, known as FSK, in fact works well and is used widely at present
on the UK network with the usual 2 meter and 70cm rigs. However, it has
been shown that an efficient form of FSK occurs when the frequency shift
(dF) is half the data rate (DR) of transmission. This form of FSK is known
as Minimum Shift Keying (MSK). Expressed in mathematical terms, the definition
of MSK becomes:
dF= DR/2
In the example in
Fig 1, the total frequency shift is 6kHz. It can therefore beinferred
that is MSK is being used with this deviation the data rate must be 1200
b/s. A further requirement of MSK is that the change from signaling frequency
to the other must be done with a continuous waveform and with no phase
discontinuities. This may not be true when using the Vfast28.8 modem but
has no practical effect on its' actual performance.
It is apparent that although the Vfast28.8 modem is described as a (G)MSK
modem, this will only be true if the radio peak deviation is set up to
be half the data signaling rate, e.g. 7200Hz for 14400 b/s transmission
speed.
Gaussian Filters
The main reduction in bandwidth comes from the use of frequency shaping
before FM modulation takes place. This is done by taking the square waves
of the serial data signal and passing them through a low pass filter.
One type of suitable filter is the Raised Cosine Filter which is used
in some radio modem designs.
The Vfast28.8 modem uses a so-called Gaussian Filter. A Gaussian low-pass
filter is a filter which, when excited by a single impulse at its' input,
gives a Gaussian shaped output response. The shape known as a Gaussian
curve is also sometimes known as a Normal Distribution Curve.
The Gaussian filter provides an excellent shape for radio data applications.
If the basic FSK transmitter is modified by the use of MSK and the addition
of a Gaussian input filter to give a GMSK system, the resultant bandwidth
of the data signal is now much reduced and is suitable for transmission
by a narrow band FM radio.
The frequency at which the low pass Gaussian filter starts to filter has
a direct influence on the bandwidth used for transmission. The lower the
frequency relative to the data rate of the signal, the narrower the transmission
spectrum, but the higher the likelihood of bit errors during transmission.
The ratio of the -3db point of the low pass filter (F1) to the data rate
is known as the BT value. This can be shown by:
BT=F1/DR
The Vfast28.8 design
provides the ability to select two BT values (0.5 and 0.3) for each data
rate. As a guide, the following figures can be regarded as the maximum
data rates which can be achieved in different channel bandwidths using
BT values of 0.5 and 0.3
BT = 0.5 BT = 0.3
4800 b/s in 12.5kHz 8000 b/s in 12.5kHz
9600 b/s in 25.0kHz 1600 b/s in 25.0kHz
19200 bb/s in 50.0kHz 32000 b/s in 50kHz
GMSK as a method of transmission is simple and effective. It does, however,
require care with some points. These mainly concern the transmission of
long strings of "1's and 0's".
A string of 1000 logic "1's" will appear as a single frequency
shift at the beginning of the string and a further frequency shift at
the end of the string. This can lead to problems with the receiver not
being able to track the incoming signal correctly. Both bit timing and
lever information can be lost. When a BT value of 0.3 is being used, problems
also occur with single bits bounded by their complement being sent repeatedly.
For example, the bytes 00010000 or 11111110 sent many times over can cause
the GMSK demodulator to give out bit errors.
All problems of this type can be avoided by the use of a data scrambler
or randomizer in the modem design. The Vfast28.8 modem includes this function
in the modem controller device. As the data will still contain some low
frequency content, the radios must be able to carry, without distortion,
frequencies as low as 30Hz in transmit and 40Hz in receive. The upper
frequency limit required is dependent on the data rate used.
Considerations
The Vfast28.8 full duplex radio modem provides excellent performance in
the most demanding conditions. When used at 9600 b/s it is compatible
with other modem designs. However, it is capable of much more. Both versions,
the AX384 and the AX576 include the K9NG data-randomizer as a default
option. The GRAPES randomizer is also included. However, the user can
also select the alternative method of data coding, known as "Bi-phase
coding", as described above. This makes interfacing to typical voice
radios much easier.
A unique feature in both modems is the inclusion of a "Real Time
Bit Repeater". This allows the user to install a contention free
LAN in an area. The operation is as follows. The repeating station is
equipped with an AX384 or AX576 TNC and a Full Duplex radio. This radio
has split transmit and receive frequencies. The stations wishing to use
the repeater use split frequency half-duplex radios. This is the same
situation as with as voice repeater. A soon as one of the user stations
starts to transmit data the repeater keys its' transmitter and starts
to relay the input data. As soon as the other user stations detect that
the repeater is sending they are inhibited from transmitting. Thus all
contention to access the repeater is avoided. Since the data is resent
in real-time this is unlike a normal Node, where the packet must be fully
received before it is forwarded to its' end destination. The bit repeater
in the AX384 and AX576 includes full data bit rate clock regeneration
to avoid excessive clock jitter on the repeated data. It also include
an extending FIFO buffer to ensure bits cannot over-run or under-run where
the bit rate clocks of the user stations and the repeater are slightly
different.
Software and set-up
The main operating software ROM supplied includes a KISS mode driver for
the TNC and a "Set-up" program. For many users such as those
running G8BPQ Node, KA9Q TCP/IP, and Linux AX25 software this is all that
is required. The "Set-up" program allows the user to simply
configure some TNC and modem functions. The "Set-up" program
may be driven by a simple terminal program or a Windows based program
(supplied with the TNC's). The "Set-up" mode is activated by
holding in the "Set-up" button while switching on the TNC. It
also allows various test signals to be generated by the radio modem to
enable the user to get the best performance from the connected radio equipment.
When used on a radio link with a AX384 or AX576 at each end the users
may do bit error rate measurements on the link to allow fine-tuning of
the RF equipment for the best performance.
 As
can be seen, the AX384 and AX576 are state-of-the-art TNC's, yet are available
in both kit and built versions.
Full user documentation
is included in electronic form along with a Windows based program to control
the "Set-up" mode.
The construction is
from high quality materials including an RFI screen coated stylish grey
casing. The styling is designed to match well with modern office and computer
equipment. The PSU is extensively filtered and smoothed to ensure correct
operation of the TNC even in the presence of strong RF fields.
This TNC is being used in the local East Anglia area as the basis of our
High-Speed Backbone, hopefully operating at 19200 b/s full duplex on 23cms.
Some problems are being experienced with the RF gear for 23cms at present,
but hopefully these will be resolved soon. For anybody looking for a High-speed
TNC, at up to 57600 b/s then this would be an ideal answer.
For further information
contact GMSK at:
GMSK DATA PRODUCTS,
80, CLONE ROAD,
HALSTEAD,
ESSEX,
CO9 2HP,
UK
Email: sales@nuthatch.dungeon.com
or info@gmskdata.co.uk
They also have a WEB site on: http://www.gmskdata.co.uk
Operating Specifications
Power Supply +9 to
=125 volts via power connector
Current Consumption 200mA (Typical maximum)
Radio Bit Rates
AX384 4800, 9600, 19200, 38400 b/s
AX576 7200, 14400, 28800, 57600 b/s
Radio interface Receive audio
Transmit audio
Press to Talk (PTT)
Mute input
Transmit audio output impedance 10k Ohms (Max)
Transmit audio output level 0 Volts (min) to 5 Volts (max)
Receive audio input level 50 mVolts (min) to 5 Volts (max)
Receive audio input impedance 100k Ohms (min)
Computer Interface
RS232 interface with the following speeds
AX384 9600, 19200, 38400 b/s
AX576 9600, 19200, 57600, 115200 b/s
RS485 interface also capable of working at the above speeds (optional)
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