-- o ICOM

•
RTTYLOOP
Marc I. Leavey, M.D. WAJAJR
6 Jenny Lane
Baftimore MD2120B
Well, I heard you, With the responses
to the March 1989 RTTY
Loop Reader Survey pouring in
(I mean it- you really did respond
to this one!), many of you made
one thing quite clear. You are anxious
to anger your spouse, parent,
or significant other and bum little
holes in the kitchen table with a
construction project or two. Glad
to oblige.
The most-asked-tor items by far
were small, one- or two-chip, or
transistor projects useful to the
RTTYer, new or old, Simple demodulators
and AFSK units led
the pack, with other types of test
or interfacing circuitry following.
With these requests ringing in
my ears , let's see what I can
mu ster up.
The TU,
Demodulato r, and Modem
Receiving RTTY is the first item
of business, and that requires a
terminal unit of some kind. Early in
this hobby, the device that decoded
the bloOOle-bleeps of RTTY into
the clunky-chunks of TTY was
called a " Terminal Unit ," probably
because it was the electronic
unit used at the terminal. Commonly
abbreviated TU, this term is
still quite widely used. A bitlater,
the moniker " demodulator" surfaced
, derived from the demodulation
of frequency shift keying in-
Demodulator Parts List
Number 23 on your FeedDacl< card
Amateur Radio Teletype
to TTY loop vohages. We also still
hear the phrase " truncate to demod."
Recently, an interface that
both sends and receives, modul
ates and demodulates so to
speak, is called a " modem," from
the initial syllables of these two
words. Originally a computer
term, it has crept into our vocabulary
as well.
However you wish to call it, our
goal is to make a simple device
Integrated Circu it XR·2211 Jim-Paks or mail order
· Reslstors 51000 RS271 ·1330 5ISO.39
Y. or v.z Watl isoooo RS271·1337 5ISO.39
iooso RS27H347 51$0,39
200kO RS271·1350 51$0.39
510kO AS 271-1354 5f$0.39
" MODEM MGR . . .
supports split-screen
operation or full screen at
speeds to more than 19KB, and
it will run under either ProDOS
or Apple DOS 3.3."
which will convert frequency-shifted
audio tones into a keyed
voltage. The heart of this circuit is
a demodulator on a Chip, the XR­
2211 phase-locked loop demodulator.
AlthOugh not in the latest
Rad io Shack catalog, it used to be
carried on their parts wall, and it
may be in the clearance bin at
many stores. If not, Radio Shack
dealers can order one for you . Also,
you can bUy it at many parts
stores and by mail order; the Jim­
Paks line carries it.
All other components in this
important to present a clean signal,
either a good VHF RTTY signal
or a strong, interference-free
HF signal.
A phase-locked lOOp (PLL) demodulator
is normally tuned for
the frequency and bandwidth desired.
Here, the 0.022 lJF capacitor
from pin 13 to pin 14 of the
integrated circuit and the 20 k resistor
(an 18 k liKed and 5 k variable)
on pin 12 set the center frequency
to about 2125 Hz, midway
between a 2025 Hz mark and
2225 Hz space frequency.
Set the bandwidth to allow 300
baud transmission with the 200 k
resistor between pins 11 and 12.
The fitter of the 0.0051-lF capacitor
and 100 k resistor com ing from pin
8 provide additional trimming.
Naturally, with a circuit operatproject
are common parts. You
can bUy them at Radio Shack
or other parts houses. I recommend
building the device on a
small piece of perf board, with
point-to-point wiring. Clubs may
wish to etch a circuit board; this
could be a useful introductory
project.
The Demodulator
Now, to the matter at hand. Fig.
ure 1 is a schematic diagram of
the demodulator, The audio output
of your receiver, either HF or
VHF , is coupled to the demodulator
through a 0.1 I-lF capacitor to
pin 2 of the XR·2211. With no m­
put filtering on this device, it is
ing at TTL levels, the output from
this one-chip wonder is at TTL leY.
ets. Don't try to drive a Model 15
with it-at least, not directly. Future
columns will include some interfacing
schemes that will let you
do this.
Ap ple News
In the March issue of RTTY
loop, I mentioned Byron Schulten's
Apple woes, but somewhere
between my data transmission
and the typesetter, his computer
mutated from an Apple IIgs to an
Apple lie, Well, Bruce Klutchko,
M.D. WB2HLX, Hastings-on-Hudson,
New York , offers his advice
regard ing a piece of software
called MODEM MGA. Bruce says
that he has used this program for
several years, and has found it
one of the most comprehensive
and well-supported programs
available for the Apple II + , Ilc, lie,
or IIgs. It supports split-screen 01>
eretrcn or full screen at speeds to
more than 19KB, and it will run
under either ProDOS or Apple
DOS 3.3.
Bruce notes that Apple users
sometimes feel orphaned by the
amateur industry, but they 're reluctant
to give up their machines.
He fe els th at MODEM MGR
speaks to that need precisely, It is
available from MGR Software,
Suite 101, 305 So. State CD/lege
Blvd, Anaheim CA 92806. Contact
them for current pricing and
availability, and be sure to mentio
n 73's RTTY Loop when you
write.
Next- TransmiUing
Next, we'll cover the other side,
transmining, with a one-chip AT·
TY modulator, As always, I welcome
your questions and comments,
either by mail or E·m ail.
Send paper to the above address,
e-man to me on CompuServe
(75036,2501) or Delphi (MAR.
CWA3~R) with your comments,
Questions, suggestions, or criticism
, All of it is appreciated.III
Potentiometer 50000 RS 271-217 SO.69
Miniature PC mount
Capacito rs O.022I-lF RS 272·1066 21$0.69
Precision disk O. OO5 I-lF RS 272-130 21$0.49
0 .00471-lF RS 272·130 21$0.40
0.11-lFx3 RS 272-135 21$0.59
Perl board 0.1" grid RS276-1394 $1.99
"Resistor values are nominally within 10% . For all practical purposes,
the evenaeie Rad io Shack values are clOse enough for th is
project to the specified values. If you can get exact values, fine. If
not, don't jose any sleep over it.
'0
L~
., , rrr
.eae
rsl Aud io ."-J'"• . ..Jir,
' "~' "i~ .. ~
9
" •
, -'! llll
.-
'"
E~BJ i'""
, • ,
t
x/t "U tl . 8 9