From: williams@qualcomm.com (Paul Williamson)
Subject: TAPR Meeting 92 - 2 of 2
Organization: Qualcomm, Inc., San Diego, CA
Date: Fri, 17 Apr 1992 09:06:38 GMT

[Part 2 of 2]

=====================
Mark Oppenheim, KD6KQ
VITA - Volunteers in Technical Assistance

VITA was founded 30 years ago by a group of scientists in Arlington,
VA, to service as an information conduit to assist developing
countries with their technical infrastructure.  A staff of about 75
people presently answers requests free of charge.  Many VITA
volunteers are also registered with their areas of expertise,
including N6ARE, WA7GXD, NK6K, HB9AQZ.  In times of disaster, like
the earthquake in Armenia, VITA coordinates donors, using their large
communications setup at the office.  HF packet and Pacsat operation
are among the supported modes.

Since 1981, VITA has been working on using packet in developing
countries.  Local telephone systems are often like two tin cans with
a bad string between them, and are unusable for data.  Thanks to the
amateur radio work in reducing the cost of packet radio, packet (on
non-amateur frequencies) can be used to bypass the phone network. VHF
packet was used to coordinate aid during the Ethiopian famine. 
Demonstration networks have been operated in Somalia, Lesuthu, and
Mozambique.  An HF network is operating in Sudan.

VITA first worked with packet satellites using UoSAT-2.  The software
was too manual, and the satellite was so deaf that a large water
buffalo would be needed to power the uplink transmitter.  UoSAT
OSCAR-14 was launched with partial VITA funding.  Software delays and
complexity were problems.  VITA hired a programmer to develop
simple-to-use groundstation software for their application, and they
use Quiktrak for satellite tracking.  The next step is to launch a
dedicated satellite, VITAsat.  There may be room for some amateur
radio payload as well.  A network with up to 500 groundstations in
planned.

UoSAT OSCAR-22 was originally to be mainly used by VITA and Satellife,
a group with similar purposes but oriented toward medical
technology.  The power amplifier on UO-22's non-amateur downlink
failed, so satellite allocations between UO-14 and UO-22 were
reshuffled.  UO-22 is now 100% amateur, and UO-14 is 100% VITA and
Satellife.

Numerous travelogue slides showed people, equipment, and camels.

Question: Are the locals generally able to operate the equipment?
Answer: Sometimes. Luckily, the equipment has been quite reliable so
far, so no maintenance has been required.

Question: How is all this funded?
Answer: The equipment is purchased by the "customer".  Some money is
also available through grants.  VITA does not charge for access to
the satellite.

Question: Do you have difficulties with importing technology?
Answer: Sometimes.

=====================
Masa Sawada, JF2GPF
TASCO

TASCO was founded by JA2AQO, the current president, in 1979.  TASCO
began by selling weather facsimile terminals and RTTY decoder.  In
the mid 1980's, TASCO began selling amateur packet radio into the
Japanese market under license from TAPR.  They are constantly working
on new technologies, and all technical staffers are amateur radio
operators.  Congratulations to TAPR on its 10th Anniversary.

=====================
Bill Henry
Clover II

Several articles about Clover II have appeared, and more are on the
way.  The last two Proceedings of the ARRL Computer Networking
Conference had papers by Ray Petit, W7GHM: 1990 about Clover I, and
1991 about Clover II.  A July 1990 QEX article featured Clover I. 
Several articles have appeared in The RTTY Journal.  A series in
Communications Quarterly and another in QST will lead up to Clover II.

Clover is still experimental - it isn't a product yet.  It was
invented by Ray Petit a year and a half ago.  It was an outgrowth of
earlier coherent CW experiments, which used phase coherent detection
with very stable oscillators (a few parts in 1e8) and detection
bandwidths as narrow as 0.01 Hz.  Such equipment can demodulate
signals that can't be heard by ear.  The early Clover articles in QEX
caught Bill Henry's attention, and they teamed up to make a product.

The particulars of Clover II are derived to counteract what the
ionosphere does to corrupt a data signal.  One basic limitation of HF
propagation is signal to noise ratio.  Earlier RTTY designs have
concentrated mainly on this problem, by optimizing bandwidth and
using adaptive AGC or good wideband limiters.

Another difficulty with HF is the existence of multiple propagation
paths of different lengths.  The differential delays result in
selective fading.  On voice, this is merely annoying, but on data it
is a disaster.  Out of phase signals cancel.  On FSK, the mark and
space tones fade separately (a good RTTY demodulator handles this). 
What's worse, the bits can get smeared out in time by up to several
milliseconds.  This is what's wrong with HF packet: 300 baud is too
fast.  Most experts agree that about 100 to 150 baud is the limit for
usual HF conditions.  Now and then, conditions are "like a wire" and
there's no problem with multipath.  Such conditions are quite rare -
and that's the only time HF packet works!  AMTOR users can tell you
that sometimes even 100 baud is too fast.

Clover copes with this problem by using a signaling rate of 31.25
baud.  That's pretty slow.  To get a reasonable data rate with such a
slow baud rate, Clover packs more than one bit in each pulse.  Clover
uses PSK with two, four, eight, or sixteen distinct phases to encode
1 to 4 bits of information on each pulse.  Clover then uses this
scheme in a time-staggered scheme with four different carrier tones,
resulting in a total bandwidth of 500 Hz, which is a good match for
available CW filters.  When conditions are good, Clover goes further
by adding two-level or four-level amplitude modulation, for even
higher maximum data rate, without changing the basic modulation rate
of 31.25 Hz.

PSK modulation can be a problem, because its bandwidth is usually
very wide.  Clover avoids this problem by pulsing the four tones on
and off with a very carefully-chosen pulse shape called a
Dolph-Chebychev function, and performing the phase changes while the
pulse is completely off.  The result is that the energy of a
four-tone Clover signal is very tightly contained within 500 Hz. With
a 60 dB limit imposed by the quantization in the digital-to-analog
converter, the Clover transmitter's sidebands are down 50 dB outside
the 500 Hz.  Two Clover signals can be spaced just 500 Hz apart (edge
to edge) with 55 dB rejection.

Graphs of the spectra of Clover, HF packet, and AMTOR show that Clover
is a *lot* tighter.  The standard rule of thumb says that two AMTOR
signals need to be 1 kHz apart, and HF packet signals need 2 kHz. The
spectra clearly show why: poor sideband suppression.  Clover is
effectively much narrower.  Not only that, but Clover is faster on
real channels.  Ignoring the question of 5-bit characters versus
8-bit characters for now, both HF packet and AMTOR have typical
real-world throughputs of 5 to 7 characters per second.  In tests on
the air, Clover typically achieves 50 to 80 characters per second.

Another weakness of HF packet is the error control scheme used.  With
packet, a long frame of up to 30 seconds is sent, and every single bit
in the frame must be received correctly, or it is discarded.  Because
of this limitation, HF packet operators must run small packet sizes
of 32 or 64 characters.  This makes the packets short enough to get
through (sometimes), but increases the cost of packet headers and
waiting between packets.  That's how a 300 baud mode gets down to 5
or 7 characters per second.  Clover uses a forward error correction
technique called Reed-Solomon coding.  This technique transmits a few
extra bits, and uses the carefully encoded redundancy in the data to
correct the received data without requiring a retransmission.  For
example, a R-S code that is 60% efficient can correct 25 errors in a
block of 255 bits.  Because the R-S code can correct some errors in
each frame, Clover is able to send longer frames without losing too
many to errors.  Of course, sometimes long frames just don't get
through due to fading conditions; in this case Clover can fall back
to shorter frame lengths.

As W3IWI has pointed out, HF calls for adaptive modems. Clover is
about as adaptive as you could want.  There are 8 basic modulation
modes to choose from (different numbers of phases and amplitudes for
each pulse), times 4 frame lengths, plus 4 different Reed-Solomon
codes of varying efficiency and error-correction capability, for a
total of 128 different modulations.  Every one of those 128
modulations has the same 500 Hz spectrum.  The Clover modem also
controls the transmitter output power.  Obviously, mode and power
selection has to be automatic!

The receiver measures the phase, time, and frequency dispersion of
the received signal and picks a mode.  It sends an order to the
transmitter specifying which mode it wants.  It can change modes
within a second if a short block length is in use.  The modes range
from 2.3 characters per second to 94 characters per second,
theoretical throughput.  The field tests have shown a typical range
of from 28 to 62 characters per second.  Note that the receiver
doesn't just move up to higher speeds when conditions are good and
down to lower speeds when conditions are bad.  It can figure out by
listening to the signal what exactly is wrong with the signal and
request the mode that best fits current conditions.  For instance, if
it notes that phase dispersion is bad, it can fall back to a mode
with fewer phases.  If it notes that it has excess signal to noise
ratio, it can command the other station to reduce power.  (This can
lead to the rather disconcerting situation where the transmitter's
meters are not moving, and the receiver's audio has no audible tones,
yet characters are still moving through the link!)

The implementation is DSP, DSP, DSP.  The input jack goes to a
transformer and a 16-bit A/D converter, and the rest is digital.  The
A/D converter is a 16-bit sigma-delta oversampling converter like the
ones used in digital audio applications.  It doesn't need any
anti-aliasing filter, and it has lots of dynamic range.  It currently
costs $20, but should get cheaper.  The transmit audio is also a 16
bit oversampling audio component, followed by a simple filter to get
rid of the residual 100th harmonic.  A Motorola 56001 DSP processor
supplies the signal processing horsepower.  The original design used
a 6809 microprocessor for general control functions, but it ran out
of gas.  The current prototype now uses a 68EC000 processor at about
30% utilization.  The board contains only bootstrap ROMs; the Clover
code is downloaded from the PC.

Vic Poor is writing a Clover driver for Aplink.  The Clover board has
FIFOs on the input and output to relax realtime requirements, which
is expected to be especially helpful for PCs running Windows.

Two working prototype Clover boards will be displayed at Dayton. On
initial release, the card will do only Clover.  If anybody wants to
write other modems for the board, the door is open.  A more pressing
need is a new protocol suitable for HF work.  Anybody who writes
network code and wants to write drivers for Clover, we can set you up
with hardware and provide assistance.  The command protocol will be
defined by next week.

Question: Isn't frequency accuracy and stability still a problem?
Answer: With the faster CPU, the DSP no longer has to do Reed-Solomon
decoding.  That means it has enough spare horsepower to do more
frequency acquisition and tracking.  Clover can now handle frequency
errors of up to 40 Hz, which is similar to the guidelines for HF
packet.

Question: What about intermodulation distortion in the transmitter?
Answer: Measurements of high-end rigs show very good IMD.  A worse
problem is broadband noise, which is about 40 dB down.  That doesn't
seem to hurt, either.

Question: What's the price?
Answer: The introductory price will be $995.  This is a lower price
than announced before, because it's now a PC plugin board rather than
a box.  The parts are expensive, even the socket for the DSP chip is
expensive.  The board is 4 layer.

Question: What is the peak to average power ratio?
Answer: 3 dB for all modes.

Question: Is it legal?
Answer: Yes.  It's not multiplex because the four tones are sent
serially rather than in parallel.  The emission designator is
500HJ2DEN.  The Chief Engineer and Chief of Enforcement of the FCC
have both agreed verbally that this modulation is legal.

=====================
Gwynn Ready, W1BEL
PacComm Topics

PacComm is currently working on a custom packet protocol for
commercial HF applications.  It runs long frames, but avoids the
problems of AX.25.  It uses a "superframe" containing multiple copies
of the address and multiple checksums.  It uses a selective nack
protocol so receiving stations need only nack missing pieces of each
superframe.

Topic #1: PACTOR

PacComm is exclusive licensee of PACTOR in the USA (for a few years),
and point of contact for PACTOR.  PACTOR is an ARQ protocol more like
AMTOR than like packet.  The PACTOR controller will also do RTTY and
AMTOR, and automatically falls back when talking to a non-PACTOR
station.  It stores partially-received frames in memory and tries to
combine them to get one good frame.  This technique is called "memory
ARQ".  The box is binary compatible with the original German
version.  Hardware is expensive to build in Germany, so PacComm is
building PACTOR controllers in the USA.  An Aplink driver is on the
way.

Topic #2: Baycom

Baycom (pronounced Bye-Comm) is a packet program for the IBM PC like
DIGICOM>64 for the Commodore 64, from the same German team.  They
have now formed a company, and have licensed PacComm
(non-exclusively) to distribute in the USA (and to work to enforce
their copyright in the USA).  Two modems are to be available: a
serial port modem based on the TCM3105, powered from the port and
physically inside the port connector, and a version based on the
AM7911 in a box with HF capability.  The PC-100 series will also be
upgraded for 4-port Baycom compatibility, 300/1200/9600 baud, with
modem disconnect and mounts for Tekk radios.  These products may be
available by Dayton.

Topic #3: NB-96 Product Line

The NB-96 line is a licensed version of the G3RUH 9600 baud modem. 
The weaknesses of the original design for full duplex use have been
fixed.  The board has more groundplane and more bypass capacitors. 
Receive and transmit circuits have been separated.  The components
that determine the modem speed are now on a header, so you can pick
whatever baud rate your radios can support.

The EM-NB96 external version of the NB-96 has been out of production,
but now it's back in production.  It is intended as a stopgap until
DSP modems become available and affordable, or for the operator who
already has most of the modems he expects to need and just needs to
add 9600 baud.  It is versatile: the user can switch to internal or
external clocks, route the push-to-talk, and choose the TNC's
built-in 1200 baud modem or either of two external modems.  An LED
indicates when you have the settings wrong -- this tends to cut down
on user support calls.

An integrated packet radio system containing the modem, the TNC, and
the RF circuitry all in a single box is in prototyping now.  The
original plan was to mount the radio on the circuit board, but
interference prevented that from working.  So the radio is now
mounted in the chassis, with the side benefit that there is now room
for two separate radios.

Topic #4: TNC Upgrades

When running 9600 baud continuously (like on a satellite downlink),
the TNC needs to support a terminal baud rate greater than 9600.  The
TINY-2 can run 19200 baud continuous, and it's the only TNC that can
do that out of the box.  TINY-2's are now shipping with 10 MHz Z80
microprocessors, which enables them to run at 38400 baud on the
terminal port.

A high speed data controller based on the V53 processor with an 85230
serial chip and a fancy power supply on-board is being marketed
mainly to commercial customers -- it's too expensive for the ham
market.  A PacComm HandiPacket is in use on the Russian space station
MIR, which has generated lots of publicity.  They are also used in
the ground terminals used with the DARPA Microsat project (not to be
confused with AMSAT's more sophisticated Microsats).

A 1200 baud modem about the size of your thumb is shipping 100 to 200
units a month to commercial customers, who are now asking for a tiny
9600 baud modem.

=====================

Pete Eaton made a few announcements, then the meeting broke for the day.
It reconvened at 9:00 AM on Sunday.

=====================
Fred Treasure, KE5CI
A Packet-Controlled Telescope

In 1987, Bill Neely, KC5ZG, purchased a large telescope mirror, with
plans to automate the pointing arrangements.  Since then, the system
has grown a little ...

Block diagram of the observatory system.  A control computer in town
communicates via a 9600 baud packet link, through a dedicated
repeater site, through another 9600 baud packet link to the
communication and storage computer in the "warm room" at the
observatory ranch.  That computer talks via a 2.5 Mbps ARCnet link to
the control computer, which in turn talks via 1200 baud asynchronous
RS-232 to the computer that controls the pointing.  Whew!

The NF/ Observatory tracks asteroids and other objects of interest,
capturing images using a CCD camera on the 17.5 inch telescope. 
Congress has generated an initiative to shoot down an asteroid rather
than let it collide with Earth.  This observatory has volunteered to
gather data on near-earth orbit asteroids in support of this project.

The name "NF/" stands for "Neely Fraska Bar", the name of the ranch
on which the observatory is located.  The ranch is about 25 miles
from Silver City, NM, at an elevation of 5800 feet.  The packet
repeater is on Baldec Peak.

The 9600 baud links run on Mocom 70 radios with Texnet 9600 baud
modem.  The Texnet network control processor (NCP) has been modified
to run KISS with 2048-byte packets for TCP/IP.  The Silver City to
Baldec link is on 442.5 MHz, line of sight, with 4-element yagis on
each end of the link.  The link from Baldec to the ranch is on 447.5
MHz.  There's no line of sight path on this link, so there's a
16-element yagi on the tower on the mountain and an 11 foot dish with
21 or 22 dB gain at the ranch.

The system can transfer a 512x512 pixel image with 4 bits per pixel
(about a half megabyte of data) in about 10 minutes.  More typically,
it transfers a 320x200x8 VGA image in about 1 minute.  This kind of
image is suitable for previewing the results.  The full results are
stored at the site, and transported using magnetic tape.

The system includes three 80286-based computers and one XT-class
machine.  A request for a series of images is entered into the
computer in Silver City.  It goes through the links to the ranch
house, which turns on the telescope and the rest of the computers. 
Lots of computers made more sense for this job than a larger
multitasking machine, because realtime control was needed.

The XT tracking computer can automatically calibrate the positioners,
starting from the weatherproof "parked" position.  Fixed sensors on
the telescope mount can get it pointed close enough to find a
calibration star.  Then the telescope automatically locks onto the
bright object near the center of the field.  Telemetry about the
weather and control voltages go back over the link to the in-town
computer.

The second computer provides the timing signals to the CCD sensor and
stores the resulting image on its hard disk.  Best performance from
the CCD is obtained by tuning the precise voltages applied to it. 
This telescope achieves about 25 electrons of noise.  Compare this to
the best NASA sensors at 3 electrons of noise; pretty good for an
amateur effort.  About 60 images are stored on the hard disk drive,
then they are dumped in a batch to a magnetic tape drive attached to
the computer in the ranch house.  About once a week, the data is
fetched by car.  Once the tape is verified, the hard disk is erased. 
The telescope dome automatically closes, the telescope parks, and
powers off.  The entire process is completely automated; no human
intervention is not required.  The telescope system has failsafe
interlocks: if moisture is detected, or if the link fails, the system
automatically parks and shuts down.

The system cost about $5000, plus about 5000 man-hours, mostly spent
on the programming.  Bill Neely, KC6ZG, and Lori Neely did most of
the programming; Fred Treasure and Barbara Treasure, N5HJN, provided
most of the hardware.  K2GNR and the Jet Propulsion Laboratory
provided the camera chips.  NASA provided the magnetic tape recorder.

Question: Are there other sites sharing the data format?
Answer: The data is recorded in a standard format.  However, there is
only one other telescope that's as completely radio-controlled as
this one: the Hubble Space Telescope!  The team gets invited to
professional astronomy events.  The professionals couldn't do a
project like this for $100,000.

Question: Is compression used on the links?
Answer: We tried LZW compression, and it gave only about 25%
compression on this type of data.  That's not worth the extra
trouble, since there is plenty of capacity on the links.

Question: What polarization are the links using?  My experience is
that vertical doesn't work in the mountains.
Answer: We chose horizontal, more or less by guess.  It works.

Question: What weather sensors are installed?
Answer: Anemometer, a cloud sensor that works by sensing the
temperature of the sky versus the ground, a moisture-detection grid,
and thermistors to measure temperature.

Question: What kind of cooling is used?
Answer: A three-state thermoelectric cooler and liquid cooling.

Question: Tracking rates?
Answer: Completely variable.

=====================
Lyle Johnson, WA7GXD
Bit Regenerative Full-Duplex Repeaters

Why full duplex?  It helps to cure the problems caused by hidden
terminals.  The coverage area can be controlled using antenna
patterns, or to a lesser extent by varying the repeater transmit
power.  The Tucson LAN has a large radius of 150 miles, and the full
duplex repeater helps throughput.  It helps stations at the edges of
the LAN to communicate.

With bit regeneration, the repeater demodulates incoming signals and
remodulates them before retransmission.  This allows the repeater to
control the deviation of the transmitted signal.  By using a bit
regenerator with some FIFO buffering (like the one on the new TAPR
9600 baud modem) to remove clock jitter and clock rate errors, the
transmitted signal can be perfect.  Most TNCs are crystal controlled,
so not much buffer is needed.

The bit regen repeater has some social impact as well: the repeater
operators get some control over the type of traffic that is
transmitted on the channel.  A station that habitually hogs the
channel can be throttled by the node TNC by simply generating a
glitch in the middle of each packet they transmit.  Technical
compliance can also be ensured.

Question: What are the pros and cons of checking the CRC before
regenerating the bits?
Answer: If the TNC waits for the entire frame to arrive so the CRC
can be checked before transmitting, it ends up acting just like a
digipeater.  That cuts the throughput by a factor of at least two. 
The "lid filter" function mentioned above doesn't require checking
the CRC: if the node TNC glitches the wrong frame because of bit
errors in the received frame, the frame wouldn't have been accepted
at the receiving end anyway.  The other aspect is the need for the
output to come on the air quickly as soon as a signal is detected on
the input.  This is required so other users won't start transmitting
and colliding with the first input.  With the FIFO, the repeater only
sends a few bits of garbage data as a busy tone before the real bits
start to come out.

Question: Are the bit regen kits available now?
Answer: Yes.  It's just a couple of parts.  The bit regenerator could
be used on a 1200 baud regenerative repeater, too.

With a central repeater, the users can use directional antennas and
get better performance.  The single-point reliability is about the
same as a single central digipeater.  The digipeater at a good site
probably needs cavities anyway, so the additional cost of going full
duplex is minimal.

=====================
Mike Curtis, WD6EHR
Experiences with 9600 Baud

Working with many other contributors, Mike has created a beginner's
handbook for 9600 baud operation.  9600 isn't as hard as people think
it is.  The only big problem is getting hooked up to the radio.  On
transmit, the radio needs fairly linear response from 100 Hz to 5
kHz, low phase distortion, and a true FM modulator.  On receive, it
needs good linearity and [I missed something here -Ed.].

There are a few radios capable of doing 9600 baud out of the box. 
The Tekk KS900L, the D4-10, and the Ramsey kits are all good.  The
Kantronics DVR2-2 works but isn't recommended where other strong
signals are a problem.  Multimode radios are generally good prospects
for conversion, because they use a separate circuit for FM.

Question: What about removing extra filtering from the discriminator?
Answer: Whatever works.

Question: What test bench setup do you use for bit error rate
measurements?
Answer: The G3RUH modem has a bit error detector, which generates a
click for each error.  Just hook up an audio amplifier and tune for
minimum clicking.

Question: What parameters do you recommend for 9600 baud links?
Answer: TXDELAY as low as 3 usually works, but 6 to 10 is better.
TXTAIL about 2.  FRACK (the T1 timer) at 4 or 5. These parameters
should be slightly more aggressive than at 1200 baud.

Question: Can 9600 be used co-channel with 1200?
Answer: Yes.  In fact, we have a repeater that is dual mode
9600/1200.  It seems to work, though there isn't much 9600 activity.

Question: Is 9600 used on 2m in Los Angeles?
Answer: Yes, on 2m and on 70cm.

Question: What about the Motorola Mostar 800 MHz radio?
Answer: Never heard of it.

Question: Thanks for writing the beginner's notes!
Answer: It was fun, and I had lots of help.  I hope 9600 baud will
take over as people try it and find it so much better that they want
to abandon 1200 baud.

=====================
[ sorry, I didn't catch this ad-hoc speaker's name or call.  -Ed.]
Experience with Mitrek Radios at 9600

We obtained some surplus Mitreks.  A technician from Motorola didn't
think they'd work on 9600.  They do, with both the G3RUH modem and
the K9NG modem.  We intended to support an application for appliance
operators, so we wrote a paper that describes the necessary
modifications in detail.  The Mitrek is fairly easy to modify. 
Several people have successfully followed the paper.

It's important to use shielded wire like RG-174.  The original
instructions called for running the wires through a hole in the case,
but we found it possible to run the wires through the existing I/O
connector, which makes the modified radio much more serviceable. 
About 10 are in use, with paths ranging from a few miles to 40 miles,
vertical and horizontal antennas.  When two of the radios are stacked
on top of each other, some IF crosstalk causes interference between
the two radios.

It's important to get 15 to 20 dB margin on the links.  We did bit
error rate tests, even though that isn't necessarily the best
indicator.  We use Comet and Diamond dualband antennas for
back-to-back VHF/UHF nodes.  Horizontal polarization with beam
antennas seems to work better.

The biggest problem with the Mitrek is bandwidth.  We replace the
4-pole filter in the front end with a wire, which is a little touchy
at 10.7 MHz.  We have avoided preamps because of problems with
turnaround time.  We run TXDELAY of 3 to 5.  We've tried TCP/IP, but
we usually run NET/ROM.  We have a Gracilis packet switch, but it's
not fully installed and working.


Question: Can you really run TXDELAY at 3 to 5 with K9NG modems?
Answer: Yes, even with the Mitrek's antenna relay.

Question: Did you modify the DCD circuit in the K9NG modem?
Answer: No.

Question: Did you make any modifications to the K9NG modem at all?
Answer: Some have done some work on the filters and DCD.

Question: Did you notice any performance loss when the front end filter
was removed from the Mitrek?
Answer: No.  We have three sites that are heavily infested with other
transmitters, and haven't had any real problems.

Question: How many hops work in the network?
Answer: 3, 4, 5.  The link to Chicago works, even though it's not 9600
all the way.

Question: How much difference did the custom EPROM in the G3RUH modem make?
Answer: We didn't make one.  The default modem is good enough.

Question: Do the Mitreks also have a low IF?
Answer: No.

=====================
Eric Gustafson, N7CL
More Experiences with 9600 Baud

The local oscillator the Mitrek generates about 1.5 watts of power,
which is applied to the mixer.  If you follow the Motorola
instructions for tuning the front end filters, you get a strong LO
signal everywhere, especially on the feedlines.  This signal will
leak into any other Mitrek, in both directions.  We had this problem
in a commercial installation in a mine in Nevada, resulting in
crosstalk between vehicles when they were close together.  We were
able to realign the front end filters, and the extra attenuation
helps substantially.


We use Tekk radios for 1200 bps telemetry.  Their front ends can't
take other strong signals.  We have found them reasonably useable;
others have had more luck.

Out of 10 radios, one was 15 kHz off frequency -- with 15 kHz
bandwidth filters!  Filters were misaligned, resulting in over 20 dB
of loss.  The Tekk radios perform very poorly under thermal cycling. 
The transmitters perform pretty well, probably because they have to
pass FCC tests, but the receivers don't.  We went to crystal
manufacturer Hy-Q, sort of a Filters-R-Us house.  They provided
filters and crystals that cured the problems with the Tekk filters
and crystals, including frequency jumps and frequency errors.  So now
we order the Tekk radios without quartz, and get a cheaper price.

For many other radios, we found the filters weren't really quite wide
enough for 9600.  For a given radio, you can find an optimum FSK
shift but low signal to noise ratios.  Often, you have a choice
between high error rate with the best S/N performance or low error
rates but poor S/N performance.

Murata-Erie is now making 450 kHz ceramic filters specifically
designed for data transmission, in the standard package.  Two
bandwidths are available, about right for 9600 and 19200 baud
respectively.  So now the 1st IF in many radios can be opened up by
just replacing the filter with the right bandwidth unit.  The
resulting eye patterns look just like the transmitted signal.

Question: What's a Tekk radio?
Answer: A very small crystal-controlled transceiver that's both cheap
and inexpensive.  They are available for amateur frequencies as well
as the commercial units we use.  The transmitter tuning is very
broad.  Unfortunately, so is the receiver tuning.

Question: What application did Tekk originally intend for these radios?
Answer: Not sure.  They are advertised for telemetry applications.

Question: Do you know of a source for manuals for a 2m Mitrek radio?
Answer: If all Mitrek manuals are the same, I have one.

Question: There is a radio by Maxim (sp?) that's similar to the Tekk.
And the WA4ONG commercial product uses a Motorola Radius board set. 
Do you have any experience with these?
Answer: We tried the Radius, and it worked well except for the
Motorola local oscillator problem.  It uses a MMIC mixer with no RF
stage, with the result that every harmonic of the LO out to at least
2 GHz is coming out strong.  They also have a spurious response 900
kHz away from the primary response (on 8 out of 8 units).

Question: Maxars?
Answer: Not tried.  We have used Maxtors successfully.

Question: Kantronics D4-10?
Answer: Not tried.  The Kantronics 2m version is just wretched.

Question: What is the part number of the filter you use in the Tekk
radio?
Answer: Custom part number Hy-Q QMF21MB30 roofing filter.  This mod
requires other component changes too.  The 450 filter is SFG or SFH
line.  The SFH450F works at 19200 or 9600 without compensation.

Question: What bandwidth?
Answer: +/- 12 kHz, which is a bit wide for 9600, but that allows for
some frequency error.

Question: So you would need a channel wider than 25 kHz?
Answer: It wouldn't interfere with, but might get some interference
from, adjacent channels at 25 kHz.  With the G filter, you get +/- 9
kHz, which is about right for 9600.  The skirts are a bit high, but
that's how they get flat group delay for data.

Question: What's a source for Murata filters is small quantities?
Answer: Maybe TAPR should provide this service.

Question: If we use a 455 kHz model instead of a 450 kHz model, it
would work in other radios than the Tekk.
Answer: So would the 450 kHz model, but it would be more painful.

Question: Was the 2nd LO frequency changed because of stability
problems?
Answer: The stock crystals were just on the wrong frequency.

Question: How much power does the Tekk radio generate?
Answer: Just under 2 watts.

Question: What quantity do you order to get prices of $40 to $50
each?
Answer: 8's.

Question: Any experience with Repco or Ramsey radios?
Answer: Not with Repco.  With Ramsey, my only experience is with
their test equipment, which was junk.
WA7GXD: Built a Ramsey kit.  The Ramsey people were cooperative about
filling shortages in the received kit.  Got it working for about an
hour, and then it died.  Haven't had a chance to work on it since.
KE3Z: Has a Ramsey radio working the ARRL Lab.  The coil in the
transmitter was way off, but could be tweaked.  The spectral purity
didn't meet the FCC requirements; it wasn't even close.  Ramsey has
said there is a new model coming out, and not to bother publishing a
review of the current model.  A user could provide postfiltering to
meet the rules.  The receive side wasn't great either.
N7CL: I prefer receivers that have a double-balanced mixer in the
front end.
Somebody: I just built a new 220 MHz Ramsey kit, and it's hot.  I have
it transmitting on 9600, still working on receive.
WA7GXD: Most radios work fine on frequency -- the question is how well
they work where they aren't supposed to work.

Question: What about the Alinco data radio?
W1BEL: It doesn't pass direct FM signal without modification.
W3IWI: I've heard that the only difference is the front panel and the
fact that it doesn't come with a microphone.
Somebody: The manual on my Alinco is the worst I've seen.
N7CL: The manual always comes last.

=====================

Pete Eaton said thanks to everybody who came.  If you're going to be
at Dayton, stop by the booth and say hi.

He then closed the TAPR annual meeting at 10:40 AM.

[End of file]