Irl Fsk-1000 Rtty Demodulator

Marc I. Leavey, M.D. WA3AJR 4006 Win lee Road Randallstown MD 21133

Okay, now, how many of you out there have heard of iRL? Come on, raise your hands. That's one, [wo, three... hmmm, not too many. I guess this goes along with what one o) the guys from the company told me. You see, they have a new RTTY demodulator, the FSK 1000, and, in his words, "While we haven't expected the FSK 1000 to stun the free world and enrich us overnight, we still can't quite see why people would want to spend almost two hundred bucks more for something else." Well, after looking at the FSK 1000, f don't know what the problem is either.

Over the pas) few months, we have covered many demodulator designs and considered what makes a good demodulator. A common design point of many of these was the inclusion of a limiter stage in the front end. What the limiter does is boost the signal input to a clipped or limited level so that — in theory —all signals present are processed to the same amplitude. In practice, however, this only works for signals that are in the clear or reasonably noise-free. The ability to work without a limiter, in true limiter-less (often called AM) mode, is a distinct advantage on our often crowded ham bands.

Unfortunately, most of the demodulators marketed to the RTTY amateur have not featured true limiterless operation. While a front-panel switch may be marked "LIMITER ON/OFF" or "AM/FM", there is usually no way to vary the input level to best take advantage of what signal there is. Further, selective fading without adequate logic to allow instantaneous reception on mark or space can be equally disastrous. The FSK 1000 changes all that.

A front-panel input level control and an LED which lights upon clipping, thus exceeding the linear range, make limiterless operation of the FSK 1000 easy. By adjusting the imput so that the LED is just extin guished, maximum capability is ensured. By increasing the input level, any degree of clipping from controlled to hard limiting may be achieved. Clearly, all signals are not alike. Now there is no reason that the demodulator needs to stay the same, either.

Another bugaboo of demodulator design has been the filters. Through the years, filters have ranged, as we have seen, from TV-width coils to toroids to coil-less active filters. The problem has always been to maintain adequate selectivity, gain, and bandwidth at reasonable cost vs. performance trade-off. Well, iRL has come through, again, by using modern, sixth-order active filters in the FSK 1000. This permits selectable bandwidth and tunable peaks to cover any shift from 50 to 1000 Hz, with switch selection of 850 Hz, 425 Hz, and 170 Hz. The shift change is accomplished by tuning a multipole bandpass filter of constant bandwidth, rather than using audio frequency mixers in a heterodyning process. Thus, audio image problems, birdies, and spurious frequencies are minimized.

Nov;, as if the guts were not impressive enough, the boys at iRL have also worked hard to provide a heck of a box. The circuit board is a hefty 3/32-inch glass epoxy number, and the pots and other components are name brands. Full-sized, standard connectors are used on the rear skirt: no scrounging lor molex plugs here. The whole thing is enclosed in an anodized aluminum box that unscrews for service but looks like it will support a TD on top of it. (I said "looks like it will" —I have not done it!)

There are even a bunch of options, as if the basic unit weren't enough. You can get a video board mounted inside and make a full terminal. ASCII-to-Baudot conversions go with that one. Some of the standard features are even more impressive, however. A RS-232 keyboard can be hooked into the back to key the loop, and RS-232 outputs are available also. That means the thing will work with our computer terminal, without a 60-mA

loop at all. There is a keyboard-activated switch (hitting any key turns on your transmitter) and a CW ID key jack. Tuning meters, scope outputs... I even think it makes a pretty good cup of coffee.

There are a few gripes, however. First off, look at the picture. I seriously considered having a contest to see how many of you could tell which push-button on the front panel was pushed. You see, even in real life, it is hard as the devil to tell what shift you are on, whether autostart is on or off, or whatever. Some form of indicator. LED, or whatever, orchanging to toggle switches is needed to clear up that front panel. Speaking of clarity, about midway between the delta-tune and input knobs is a small knob labeled "THRESHOLD." This knob was added to late production runs, and allows you to adjust the autostart threshold (that is, the level at which the autostart will start) from the front panel, Fine, but there is no calibration, scale, or logging on the knob. No way to know where it is set nor return it to a previous setting. Bad news, fellas. And one last note: the autostart, When I first started playing with this thing, about a minute or two into the session, the front lights died and the printer went off. Now, if just the printer and loop had gone off, I would have known the autostart disengaged. But killing the front-panel lights made me think I'd blown a fuse. Only a fortuitous signal brought life to the machine and saved the day. Really, now, why not leave the lights alone? Other wise, when killing equipment at the end of the day, there is no quick way to know the thing is on.

All in all, however, I have to commend the folks at iRL. They have turned out a solid demodulator that well should stun the free world. The FSK 1000 currently sells for $449.00, and you can see their ads here in 73 or write to iRL, 700 Taylor Road, Columbus, Ohio 43230.

I received a letter recently from Tom Waarvik of Indianapolis, Indiana, who related that he was a beginner, with a Teletype Model KSR-35 and a modem, and that he wanted to be able to receive Morse, Baudot, and ASCI! on that setup. He notes that much of the commercial gear is over his budget, and he is looking for cheap ways of code conversion. Well, Tom, this is where the computer in 1 he shack comes in handy. There are reams of published programs for receiving any or all of these modes with just about any of the popular computer chips. You might check back issues of 73 and Microcomputing magazines. In September and December, 1979, I listed many of these articles in this columm. See if you can scrounge a copy in your area, or check with 73's back issue department. Which computer? Well, I am partial to the 6800 and have written some fairly sophisticated programs to work RTTY on one. But whichever you can get within your budget, 6800, 8080, Z-80, Apple, Pet, or TRS-80, they can be made to work on RTTY and Morse. That is probably the best way to go.

1980 Amateur Radio

John Fail, KL7GRF/6 Long Beach, CA

Vernelle "Red" Irwin, K9KUW Kenney, IL

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