Simple Solid-State
Mainline ST-5 RTTY Demodulator


For some time a simple yet effective RTTY demodulator has been needed. The typical newcomer does not want to build a complex "best there is" sort of thing, yet the W2PAT unit in the ARRL HANDBOOK has long since been obsolete.

With the intention of providing something that will give excellent results on normal signals that could readily replace the W2PAT unit, the ST-S was designed.

This unit uses a 709C linear integrated operational amplifier ("op amp") for a limiter, and a second 709C for a slicer (trigger) stage. It uses a Motorola MJE340 300-volt transistor as a keyer to turn the printer from mark to space. It offers the identical "floating loop" power supply that we developed for the TT/L. It has a well—balanced linear discriminator for 850 shift that gives the user the option of normal 2 125—2975 tones for mark and space, or if he insists on using ‘‘low" (non-standard) tones we have included figures for 1275-2125 tones. It can quickly be adapted to 170 shift although this is not shown on the basic diagram -- it will be explained in the text later. A ‘‘plus—plus" takeoff is provided for a tuning meter, and scope points are shown if you wish to use a scope.

The 709C offers gain so fantastic it’s hard to describe the potential performance available. Where something like the TT/L offered perhaps 50 db. of limiting, the 709C offers closer to 90 db. This would be comparable to raising the voltage in the TT/L from say 23O to nearly 7500 volts! There are other advantages as well, since this takes the place of three tubes and two transformers; is all DC coupled; responds up to 10 MHz (which puts the recovery time in the microsecond category); is inexpensive and no larger than many transistors. It has a good output swing of better than plus-minus 10 volts. With the circuit shown, it will start to limit on a signal as low as 200 millivolts input level. A simple one-pole R/C high-pass filter is included in the input to keep the 60 Hz. hum in the receiver audio from reaching the limiter, thus some of the advantages of a bandpass input filter are realized.

With simple one-toroid per channel filters, it is rather difficult to design a proper discriminator. A lot of problems are inherent that most casual observers would not consider. Indeed, when looking at the circuits offered by many designers, no consideration at all appears to have been given some of these areas. "Q" is dependent on frequency, and so is impedance and output voltage. Bandwidth is dependent upon "Q". To simplify the matters we can say that if you merely put a capacitor across a 88 mh toroid, the bandwidth will be too narrow to be useful in RTTY it will not be the same for two different frequencies such as 2125—2975 (it will be worse for 1275— 2125!) and the voltage developed across the filter with a given input will be considerably different for the two frequencies. Hence the designer has to take all these things into account, and at the same time realize that the "total area under the curve" affects the general noise balance as well. Hence it is no simple matter to get a well—designed linear discriminator that exhibits relatively equal bandwidth for mark and space, has equal output voltages good linearity (proper crossover) and reasonable noise immunity.

The detector stage on most demodulators is half-wave rectification, and on some units, voltage doublers are used, making the filtering problem even more difficult. The Mainline ST-series (ST-3, ST-4, etc.) use full—wave detection, which results in much less ripple and easier and more effective filtering.

Most simple demodulators do not offer any low pass filtering at all. The best units have complex 3-pole Butterworth minimum bandwidth filters that usually take a large and expensive inductor plus an isolation stage at either end. The ST-S has a single—pole R/C filter that does an adequate job of removing the audio ripple from the DC keying signal.

Another of the 709C op amps is used. Since we are now dealing with DC signals instead of audio, you will notice different ‘compensating networks" are shown at points 1—8 and 5—6. This slicer has so very much gain that a signal variation as low as 1-2 Hz. will cause the keyer to switch completely from mark to space. Shifts as small as 3—4 Hz. then could easily be copied if the operator had a steady enough hand!

This is the identical keyer stage that will be used in the deluxe ST-6. The MJE 340 is a 300-volt transistor costing approximately $1. Although capable of handling power up to 25 watts, it is used here as a saturated switch that is either "on" or ‘off". Even with a 60 mill, loop, the keyer therefore pulls something like 0.012 watts in mark, due to the very low saturated collector-emitter voltage of only 0.2 volts. Under normal circumstances, it is thus virtually indestructible in RTTY use. It is cut off hard for space, by negative voltage from the slicer. The diode at the base diverts this excessive negative voltage to ground which keeps the base-emitter junction from acting like a Zener diode when the slicer goes to negative 10 volts.

A spike—absorbing network goes from collector-to—ground to reduce the "back EMF" caused by the selector magnet inductance as it switches from space back to mark.

This is a similar concept to that we developed for the TI /L. The resistor values are changed somewhat since the 6W6 vacuum tube in the TT/L acts like a switching resistor, while the solid—state keyer in the ST—S acts more like a typical switch. The FSK output point will supply a minus-plus voltage as you switch from mark to space, thus it offers excellent adaptability to various types of transmitters some of which need "conduct-on-mark" instead of the normal "conduct—on— space’’. If you are upside-down" merely reverse the diode in your transmitter’s keyer and it should then be normal. Few FSK driver systems will offer this simple remedy, so don’t expect this trick to work with demodulators other than the Mainline types.

Shorting the Standby switch 51 puts the printer into mark configuration. The voltage across the switch contacts is normally 0.2 volts for mark and perhaps 175 volts for space. This is not alarming, the voltage across switch S2 is of course 120 VAC.

Do not get excited if you find the rating of the Stancor PA-8421 to be "only 50 ma." Once again we should point out this does not apply to our use of the transformer. The high voltage secondary of this transformer is rated on the basis of the current in the primary, which is capable of supplying nearly 20 volt-amperes to the two secondary windings. Since the filament winding is rated at 2 amps at 6.3 VAC, this leaves around 50 mills for the 125 VAC winding. However, if the filament winding is not used the secondary can then take the entire 20 volt—amperes by itself, which would be some 150 mills. So do not be alarmed at the ratings, you’ll never hurt the transformer. As an example, I have had a similar transformer running for six years at 24 hours per day in the TT/L and have never experienced any difficulty nor do I expect to. As long as you can hold your hand on any transformer, it’s usually not too hot!

Practically any 24 volt center—tapped transformer will work fine. The op amps can take up to plus-minus 18 volts on them, so if you get anything from 10-18 volts plus-minus, it’s fine. Regulation is not needed on this unit, and in fact offers very little advantage, since you will be pulling the same amount of current on both the plus and minus supplies. Any change in the transformer will be reflected by an equal change up or down on both supplies at the same time, and cancel out. The voltage at the pin 3 of the limiter will not matter once initially set for the nominal power supply output voltage. It is only a few millivolts and a radical change in the power supply voltage would have negligible effect, if any.

If the voltage is more than 15-16 volts, just increase the size of the 15 ohm resistors until it is what you want. This offers the possibility of any of a number of power transformers being suitable.

This has been discussed before a numher of times. For the most accurate tuning, a counter or accurate audio generator is needed. Otherwise, just put a 0.068 capacitor across a 88 mh toroid and you’ll come out "close enough" to 2125. although the exact right capacitance is 0.06374, assuming no error in the capacitor value. Use Mylar capacitors, such as the Sprague "Orange drop" as an example. The toroids are connected in a normal "series’s manner with the middle connection of the two windings grounded as shown. The values for the capacitors shown in the table are quite accurate, and assuming you have 10% capacitors, you should be close enough to mark and space to be happy. Of course even at 1004, you can miss it 100 cycles easily.

With no input signal or with the input grounded, put a voltmeter at pin six of the limiter, or any place connected directly to pin six, such as the one side of the 5K pot. This is a very low impedance point so you need not use a VTVM for the purpose. Any voltmeter will do. Adjust the 25K pot until you get zero volts at pin six. If you cannot zero this adjustment, you’d better write me a letter, you’ve done something else wrong or ruined the op amp somehow.

Now put the voltmeter at point "A" or refer to the tuning meter which we will talk about a bit later in the text. Go from mark to space on the input and adjust the 5K pot until the meter reads a similar amount of voltage for both signals.

You are finished. Neither adjustment should need to be made again. The only other adjustment would be of the pot in the narrow shift CW identification system on the FSK output.

Fig. 1 shows a suitable 1 ma. meter used for tuning purposes. You can also use any other voltmeter or
VTVM hooked to point A as a tuning indication. If the meter flickers as the station goes from mark to space, you don’t have him tuned correctly. A capacitor may be placed across the meter if desired to dampen its oscillations somewhat. This may be necessary if using an inexpensive imported meter. Although a scope display is preferred by most serious enthusiasts, the meter display is quite adequate, and mare accurate than many might at first think.

If you wish to occasionally copy "narrow shift", add a 0.022 capacitor in series with a toggle switch and put this combination across the space toroid. This automatically will change the 2975 frequency to very close to 2295. However, the balance at point "A" will be upset somewhat, and it is merely an expediency which will give reasonably good 170 shift.

If using the 1275-2125 tones, you need to put two capacitors in parallel -- a 0.068 and a 0.0068, then put the switch in series with these two parallel capacitors and then put this combination across the space toroid. This changes the 2125 space frequency to about 1445. Again this is only an expediency, and does not give optimum filter balance, etc.

If your receiver does not have a 5OO ohm tap you can hook the ST-5 directly across the speaker impedance. However, you have automatically ‘thrown away about 20-25 db. potential performance in the limiter. A better idea would be to get a voice coil to 500 or 1000 ohm transformer. Inexpensive, imported transformers are available for under $1. If you do hook directly to the speaker tap, just be sure to run the receiver at least a normal room volume.

The 709C op amps are available in a number of brands. The best known is the Fairchild, but Signetics and Motorola have them also. They vary (as of this writing) from $2.62 to $2.80 brand new, depending upon brand selected. Motorola are available through Allied, Newark, etc. The other brands are a little harder to find. Here are two addresses for the Fairchild for mail order.

732 No. Pastoria Avenue
Sunnyvale, California 94086

Hamilton Electro Sales
340 East Middlefield Road
Mountain View, California 94040

The item to ask for is the 709C op amp in the "TO-5" can. This is so much easier to work with than the 14-pin "dual inline" package. However both cost $2.65 currently. Send additional money, approximately $1 to cover packaging and mailing costs, plus sales tax if from California. If buying the Motorola, you need to get the "MC-1709C0" version, they are $2.80.

Several firms dealing with surplus semiconductor items such as advertise in ham magazines are selling the 709C for as low as $1.49 each. The ii mH toroids are available from advertisers in various ham magazines and in RTTY JOURNAL ads. The 4600 MFD. capacitors in the power supply are Sprague 36D462G015AA2A types at $2.31 each, but any large size 15V capacitors will work fine. We recommend at least 2000 Mfd.

The diodes marked: "G" are 1N270 Germanium, those marked ‘Sit" are most any silicon types. The 1N4816 or 1N2069 should be adequate (about 32 cents each) for anything other than the loop supply. There a 400 PIV should be used, or better, such as the 1N2070, etc. The Zener diodes in the limiter input can be replaced with two silicon diodes if cost is essential. In that event, do not put them in series as is shown for the Zeners, but put them in parallel, with one in reverse direction from the other. This is a protective device to keep the input on the op amp under the maximum allowed, which is around plus-or-minus 5 volts peak-to-peak. You can even leave the Zeners off entirely, but it is possible to ruin the op amp if you inadvertently tune the receiver quite loudly. It’s possible on some receivers to get as much as SO volts peak-to-peak at the 5OO ohm tap if the volume control is "wide open."

Looking at the bottom of the op amp where the wires come out, you will see a small tab on the outer circumference. This tab is opposite pin 8 of the op amp. Looking from the bottom, you then go clockwise from there for the other pins. This is similar to an octal plug for a vacuum tube.

This is an elementary demodulator of few parts. Unlike most simple units, it also offers a superb means of keying the transmitter along with narrow shift CW identification.

The limiter section is equal to the very best. The discriminator section is equal to anything published and is comparable to that in the ST-3. The slicer is equal to anything published or likely to be published for some time to come. The keyer section is in the same category.

However, this unit does not have a deluxe low pass L/C 3-pole filter nor does it have a threshold corrector that would allow automatic copy on mark-only, etc. Thus, for something that can be quickly built at low cost and still do a good job as a simple demodulator, it should fit a needed vacancy on the RTTY operator’s table. One could not expect to design a suitable unit for much less money.

The semi—conductors cost $6.36 total. The front end, including semi-conductors, up to the collector of the MJE-340 would cost about $14.50. This is using Mallory 39 cent pots. The loop supply would be around $8, and the power supply around $11. You can thus see that the power supplies are (as always) a disproportionate part of the cost on a simple demodulator.

It is interesting to note, however, that these power supplies may be used to power other solid—state devices as well as the ST-5, and in any event, should you desire to later build a more complex unit, you would use about 95% of the components already used in the ST-6 so it would make an excellent building block for better things to come.

The Mainline ST-6 has already been designed. It is as complex as this unit is "simple". It will be published when we have time to do so and there is room for it. It uses 7 op amps and 9 transistors, including two in the regulated power supply. Practically everything in the ST-5 is used in the ST—6, plus of course a great many more components as well. That unit offers among other things autostart, antispace, an "active" low—pass minimum bandwidth filter, optional limiter-on/off switch, threshold corrector for single channel copy and optional bandpass input filters for 170 and 8S0 shifts. If you were to assemble the parts for the ST—5, it would be a marvelous introduction to the ST-6, later, and almost all the parts would be used for the other unit. The ST-6 will be the long-awaited solid-state replacement for the TT/L or TT/L-2. Schematics are available now from the author for $1.