This is a "quick-and-dirty" page that I set up to describe the
modem that I designed several years ago (in 1993, actually...)
me point out that unless you are going to use this on wires, this modem
is absolutely useless to anyone other than licensed amateur radio
It does NOT intrinsically lend itself to FCC part 15
Terms of usage of the designs referenced on
and my other pages:
You are free to experiment with this design as much as you
All I ask is that you do so legally and, if you make any improvements
refinements, you let me know what you have done so that I can share
with others as well.
Firstly, let me tell you what this modem is NOT:
It is not a real T1 modem, but a T1-Rate
It does not do things like AMI, B8ZS, and so on.
It is not a kit. There are NO plans to make
It is not plug-and-play. What you get are
description. If you are intimidated when it comes to building
from scratch, then maybe you need to pick another project (or find a
to help you build it...)
This modem is not the ultimate in optimized design. This is
refined prototype that DOES work as advertised, as long as it
adjusted correctly by someone who understands what they are
This modem uses easily obtainable, off-the-shelf parts that should be
through only modest effort, anywhere in the world. I am absolutely
confident that this design can be improved upon.
What this modem IS:
This modem, properly built and implemented, will transport data
place to another at up to T1 data rates with a minimum of errors.
This is a baseband modem. That is, given a medium that has
of 1.5 MHz (starting at a few 10's of hertz) and is reasonably
in terms of amplitude and phase, this modem will push data across it.
This is a full-duplex modem (well actually, if you only ever
send data in one direction, that would be fine, too...) Clock
takes a significant fraction of a second, so you will want to have a
that is there all of the time.
Now, having said all of this, I'll describe a typical application of
Suppose you have wideband FM transmit/receive radio set (Amateur
FM transmitter and receiver are almost ideal, except that the
IF bandwidth will be wider than necessary...) As long as you are
on the 33cm Amateur band (or higher) you can legally use this (in the
Now, let's suppose that we want to link two sites that are, say,
miles apart with a full-duplex T1-rate point-to-point link. To
matters, let's go crossband, so on my end I have a 23cm FM
transmitter and a 13cm FM video receiver, and the other end has the
a 13cm FM video transmitter and 23cm FM video receiver. If the
are 10-13 db gain and there is 1 watt of transmit power (overkill,
and a clear line-of-sight path, we'll be fine. Keep in mind that
we've modified the receivers to have a 5 MHz IF bandwidth or so to
match our actual occupied bandwidth. (If you didn't narrow the
it would still work, but you'd have more possibility of
interference issues, plus lower effective receive system sensitivity.)
The baseband output of the modem is simply fed into the
input and the transmit deviation is adjusted so that the occupied
is on the order of 3 MHz or so. On the receive side, the
baseband output is fed into the modem, the VCXO on the clock recovery
centered, the data-sense jumper is set for proper polarity, and we are
ready to go as soon as we do the same for both ends.
The tricky part is to find some sort of box that can handle
data. One such box that we have used is the Gracilis
The modem supplies the transmit clock to the Packeten,
it reconstructs and supplies the receive data clock to
Packeten as well.
At that point, you can set up your routing tables and knock
You make it sound so simple. What's the
Well, you were already pre-warned that all you get are some
The schematic shows an integrated demodulator/slicer system.
that? A demodulator? I thought you said that this was a baseband
modem? Well, it is... except that the radio that we
tested it on (when I drew the schematics) did not have a demodulator,
I used a Motorola MC13055 data receiver IC for its discriminator, and
the built-in comparator/slicer to get a "raw data" stream for clock
If you already have a baseband source (i.e. from a video receiver) then
it's a "no-brainer" to use something like an LM311 as a slicer and some
fast op amp to increase the amplitude of the data if necessary...
Actually, the hardest parts will be:
Finding a set of radios that will do this. We used some
channel Farinon radios and modified them for the closest ham band. NO!!!
OF GUNN TRANSCEIVERS WILL NOT WORK AS A FULL-DUPLEX LINK!
below for the explanation why... As I pointed out,
FM Amateur Television transmitting and receiving equipment can be
I'll provide links to some possible sources later...
Finding some box that can handle T1-rate data. Forget
PC interface cards. Actually, it takes some dedicated
As I mentioned, we use a Gracilis PackeTen box which has a 68302 - a
communications engine with a 68000-like processor and some RISC-type
on it. Since we have those boxes on hand, I can't offer any other
alternatives since I haven't looked. If you find some clever
then please let me know...
Building the thing. I'll offer some advice, but I won't build
one for you! If you go through the trouble of designing a circuit
board, refining the design, and you are of the honest, generous
sort, I'm sure that you'll send an electronic copy of your work
here so I can make it available to others (and your payment? I'll
give you the credit!)
Some thoughts on the design:
I have been completely swamped with other Amateur Radio projects (not
to mention with work...) so I haven't had time to revisit this T1-rate
modem-thing for a few years. However, there are a few aspects of
it that I'll single out for improvement and/or suggestions. Since
I'm writing this off-the-cuff, I'll also throw in some seemingly random
comments as well:
The clock recovery circuit really works quite well. It
fast, but if you have a constant signal from the other end, then that's
no problem as you are always locked on. I have built a variation
of this that uses a 74HC4046 VCO instead of a VCXO (I limited its
range to prevent it from locking onto harmonics, etc.) that worked
well. The 24 MHz VCXO was chosen for this design since it
a reasonable degree of oversampling and was a convienient and commonly
The data recovery uses integrate-and-dump filters as its
of symbol filtering. This isn't particularly ideal and you'll
want to design your own, optimized filter. You'll not doubt
that since the LM311's, the 4066's and the HC logic family used in this
design is operating near its upper frequency design limit, there is a
of integrate-and-dump filters used to help relax timing constraints.
A few years ago I built a 128-256 kbaud modem prototype based
to be used as a subcarrier on video (using the 74HC4046 for the VCO as
mentioned above) and used only one integrate-and-dump chain.
I have since lost the schematic and I don't have any idea what happened
to the prototype... One of these days I'll re-create it I
Now 128-256 kbaud is a data rate that is high enough to be useful, and
still can be interfaced to more commonly available PC-type interface
(like the PackeTwin card from Pac-comm, or the Ottawa PI or PI2 card.)
A completely different modem design that could be implemented as a
subcarrier rather than a baseband signal. If it were to be
spectrally efficient, it would have to be much more complicated than
one. However, if you are talking about only a 128 kbaud or so
on video, then the aforementioned design would be quite workable.
Ok, I was planning to just use a pair of Gunn Transceivers, which
are full-duplex, right? But you said that these won't work.
What's the problem?
A well-publicized high-speed data link is the "2
mbps microwave data link" article that first appeared in Ham Radio
and was later published in the ARRL Radio Amateur's Handbook.
article describes what appears to be a full-duplex microwave data link
that operates on the 24 GHz ham band using Gunn transceivers. Just like
coax- based ethernet, this links is NOT full duplex, but half-duplex!
Perhaps you have used Gunn transceivers and thought
"hey, these are neato! They're full duplex like a phone!"
That's the problem! Like a telephone, you can hear
Gunn transceivers receive at the same time that
they transmit by virtue of the fact that the transmit oscillator is
the local oscillator for the receiver. Because they modulate that
oscillator for transmit, what you are transmitting also appears on your
receive signal. So, if you are transmitting data, the receive
from the other end will also have your own transmitted
on top of it. The result will likely be interpreted as garbage by
There are three obvious solutions:
Use a different set of Gunn units for receive and
is that it will take two antennas and twice as many Gunn
devices. This is quite do-able, but it certainly adds to the hardware.
Perhaps one could use an orthagonal feed and utilize opposite
for the two directions, thus allowing the use of a single dish...
Modulate the data on one (or two) subcarriers. One direction
(from 0-1.5 MHz or so) and the other could be modulated on a subcarrier
to occupy baseband from, say, 2-5 MHz. If you were clever, this could
done (maybe some 74HC4046's may be modulatable at the data rate, but I
don't know for sure - they do work up through at least 256
so I can at least vouch for that... but you'll probably have to
the subcarrier to a higher frequency to get a decent demodulator, or
good filtering and use, say, a UA733 and some inverters for the demod,
but I digress...) The obvious disadvantage is that you've increased
occupied bandwidth tremendously and will suffer the obvious effects
doing that (i.e. less range, more susceptible to nonlinearities,
etc.) By doing this, they two signals, while they will be present at
receiver, will be spectrally different and can be (relatively) easily
out. Hey, you might want to do this anyway for lower-speed channels, or
some analog audio links, anyway!
Subtract out your own transmit modulation from your received
This is the most likely to be doable. Since you know what YOU are
sending, just use an inverse copy of it to cancel out your own transmit
signal from the received signal. This assumes, of course, that
your modulator and demodulator are adequately linear to allow good
If you do a simple subtraction, keep in mind that your receive
will be approxmately doubled since you now are trying to receive two
signals and that will increase the system's noise bandwidth. To
this, do a broadband conversion of the Gunn's mixer diode output to an
IF, then, using a local oscillator modulated with your own transmitted
waveform (but in the oppposite sense) cancel out your transmitted
After mixing down to a new IF the resulting signal will now be
free of your transmitted signal and you can do your optimal IF
Enough with the talk already, so where's the modem?
Originally, there was an archived file that contained the
and PCL4 format. Since those formats aren't particularly easy to
use these days, they have been converted to 2-color .GIF bitmap images
instead. Not being able to find the original .DXF files from
these images were orignally created, I was forced to convert the GL
to the aforementioned raster format at a fairly high resolution.
Note that these bitmap images are quite large and they may
not print properly directly from your browser. When you
print them, please do so in landscape mode for best results. Errata
previous drawing/documation package has been used to update the
drawings. If you have the previous versions of drawings and
please discard them and use those in the links below.
The images are as follows:
t1mdmrxa.gif - A sample
using the MC13055 chip. This shows how the "raw data" (for clock
recovery) as well as the slicing levels and baseband signals are
prior to handing them off to the modem. (Ver. E.1, 154102 bytes)
t1mdmtxd.gif - The transmit
the modem. (Ver, E.1, 158611 bytes)
t1mdmcdd.gif - The
recovery portion of the modem. (Ver. E.1, 212845 bytes)
There is also a text file that describes the theory of
and adjustment of the modem that may be found here.
1.4, 25754 bytes)
I would expect that you will read this page AND the documentation
before you start asking questions. If the answer isn't in the
(possible...) or you can't make sense out of what I have written
then I can be reached via
Ok, I'm willing to actually plug in a soldering iron and jump in
and build something. Where do I get some of the radio things?
Perhaps I'll get some time and add links to this page, but for
information about Amateur FM ATV transmitters, go to the Utah
ATV Home Page and follow the link to the Houston Amateur Television
Society (HATS). These people sell some FM ATV transmitter kits. You can
also look at some of the vendors listed under the Where do you go
parts/kits/information on VHF/UHF/Microwave projects? section. In
go to the Mini-Circuits page and look at their VCOs: These VCOs can be
used as the basis of transmitters and LOs for receivers - keeping in
that they have an intrinsic limit on the highest frequency that can be
modulated on them (translation: You probably can't push them at T1