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MODEM1.TXT
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1993-07-30
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ANALOG RTTY/AMTOR/PACTOR MODEM
MODEL AN-93
J.B. Forrer
26553 Priceview Drive
Monroe, OR 97456
United States of America
In response to numerous requests for a good performing HF modem
to be used in conjunction with PC-based software TNC's, a modem
based on proven and tested principles was developed. The AN-93
design is a compact, low cost solution for both advanced and
beginning HF-digital enthusiasts.
The AN-93 modem is now available as a kit of parts, or
alternatively in assembled modular form.
Note: The kit is for advanced constructors only. You will be
working with tiny parts that are quite fragile. Please do
not attempt this project if you do not have a pencil-style
low-wattage soldering iron, and a steady hand. You have been
warned.
There are two PCB's - one contains all parts for the demodulator,
the other contains the display unit and A/D convertor. Both
boards are required because connections to the outside world are
routed through them.
The parts kit consists of:
1% resistors, 5% resistors, capacitors
switching, germanium, and zener diodes
transistors, IC's, multiturn potentiometers
LED display, IC sockets
5V voltage regulator, DC power connector
RJ11 phone socket, IDC board interconnect plugs
You will need to supply your own enclosure and wall-mounted 9V
150mA psu.
It is a neat idea to mount the two boards piggyback on a low-cost
serial/parallel IO card and steal the +/- 12V power from the PC.
Connection to the RS232 may be made through a modified header
cable. The connection to your rig could be made through a
connector mounted on a backplate in a spare slot. A 5 pin DIN
sounds like a good connector for this purpose. If you mount the
modem internally and wish to use the LED tuning display, you will
have to fabricate a IDC connector to mount the LED display
remotely.
AN-93 specifications
1. Compact active filter design.
2. 170 Hz shift. Operation at 200 Hz shift is possible
with re-alignment.
3. 2125/2295 Hz tones.
4. Data rates up to 200 baud.
5. Extensive filtering, combined with a well-balanced
discriminator and post-discriminator signal processing,
ensures good performance even under adverse conditions.
6. Computer data interface provides both hard, and "soft"
signals for implementation of advanced error correction
software techniques.
7. Useful bargraph tuning indicator as well as ±1V discriminator
channel output for X-Y oscilloscope displays.
8. Standard RS232 interface for data and PTT.
9. Provides for ±5V fsk keying.
10. Powered by a 9V 150mA wall-mounted supply.
AN-93 description
A brief description of the modem's architecture follows:
┌───────────────────────────────────────────────────────────────┐
│ │
│(A)┌─────┐ ┌───────────────┐ ┌───────────────┐ ┌───────┐ │
│>>─│ AGC ├──┤ PRE-FILTER HP ├──┤ PRE-FILTER LP ├──┤LIMITER│ │
│ └─────┘ └───────────────┘ └───────────────┘ └───┬───┘ │
│ │ │
│ ┌───────────────┐ ┌───────────────┐ │ │
│ │ ACTIVE ├──┤ MARK FILTER ├──────┤ │
│ │ │ └───────────────┘ │ │
│ ┌─────┤ │ ┌───────────────┐ │ │
│ │ │ COMBINER ├──┤ SPACE FILTER ├──────┘ │
│ │ └───────────────┘ └───────────────┘ │
│ │ │
│ │ ┌───────────────┐ ┌───────────────┐ ┌──────┐ (B)│
│ └─────┤ POST DISC LP ├┬─┤ ATC ├──┤SLICER├─>> │
│ └───────────────┘│ └───────────────┘ └──────┘ │
│ ┌──────────────────────┘ │
│ │ ┌───────────────┐ ┌─────────────┐ │
│ ├─────┤BARGRAPH DRIVER├──┤ █▒▒▒▒▒▒▒▒█ │ │
│ │ └───────────────┘ └─────────────┘ │
│ │ ┌───────────────┐ (C) │
│ └─────┤ A/D CONVERTOR ├──>> │
│ └───────────────┘ │
└───────────────────────────────────────────────────────────────┘
AGC
The first stage is an automatic gain control (AGC). This stage is
for coping with an extremely large dynamic input range. The time
constants for AGC action does not interfere with the rhythm of
either AMTOR or PACTOR operation. AGC action starts at roughly
100 mV of signal. The merits of AGC has been debated in the past,
however, without some form of signal level control, severe
distortion could occur in the filter stages.
Pre-filter bandpass
The pre-filter bandpass is implemented with separate third order
highpass and lowpass active filter. The required bandwidth for
pre-filter bandpass filters for RTTY, AMTOR, and PACTOR are
summarized in the table below.
┌───────────────────────┬────────┬─────────────────────┐
│ Signalling rate │ Shift │ Occupied bandwidth │
│ bps │ Hz │ Hz │
├───────────────────────┼────────┼─────────────────────┤
│RTTY 45.45 │ 170 │ 216 │
│ 50 │ 170 │ 220 │
│ 75 │ 170 │ 245 │
│ 110 │ 170 │ 280 │
│AMTOR 100 │ 170 │ 270 │
│PACTOR 100 │ 200 │ 300 │
│ 200 │ 200 │ 400 │
└───────────────────────┴────────┴─────────────────────┘
The effective response of the HP/LP pre-filter is wider than the
required maximum. The measured -6dB (half voltage point)
bandwidth is in excess of 1 Khz.
Limiter
The output from the pre-filter is hard limited before being
applied to the mark and space channel discriminator filters.
Channel filters
A pair of bi-quad active filters are used in the discriminator.
There obviously is a compromise for optimal bandwidth of the
channel filters to accommodate a range of bit rate between 200
and 45.45 bps and 170 Hz shift.
The occupied channel bandwidth is approximately 300 Hz.
Scope output
Outputs from the channel filters are buffered for oscilloscope
tuning purposes.
Detection and post-detection filter
Recovery of the modulation is though rectification. The combined
signal is further low-pass filtered to remove the carrier tones
and thus producing the recovered modulation signal. The post
discriminator low-pass filter, is a third-order Chebycheff active
filter with a cutoff of approximately 200 Hz. This allows for
high speed PACTOR demodulation.
ATC and slicer
To cope with signals that are slightly off frequency, an
automatic threshold corrector (ATC) is used. This circuit will
track unsymmetry in discriminator output signal due to slightly
off frequency signals. Note that the ATC has a relatively long RC
constant in order not to interfere with the rhythm of AMTOR or
PACTOR.
The slicer stage, neatly squares and level shifts the recovered
signal to RS232 level.
AN-93 PRICING
Printed manual ---- $5.00
PCB's AN-93A,AN-93D ---- $50.00
Parts ---- $75.00
Shipping and handling --- $5.00
==========
TOTAL $135.00
==========
Please add $50 for assembled and tested modules.
Checks or money orders accepted. Foreign transactions must be
drawn on US banks.
Assembled and tested modules have a 30 day money-back guarantee.
Disclaimer
Prices and specifications are subject to change without notice.
The author and designer of this product, Johan Forrer, KC7WW is
not responsible for any damage, injury, loss of profit or gain
associated with the use, installation, or application of this
product.
(C) Copyright J.B. Forrer 1992,1993
J.B Forrer
April 1993