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• 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
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