Aero ‘Lectrics

BY JIM WEIR

How to build an

ILS/VOR test box, Part 2. oomph to generate a signal in the milliwatt range. U101A is connected in a conventional non-inverting amplifier configuration with Q101 as the emitter follower so that we can draw as many milliamps as we might need. D101 sets a reference 5 volts on the non-inverting input of U101A, and resistors R101 and R102 set the gain of the opamp at 1.70, which multiplies the 5-volt reference to an output of 8.5 volts. This is a good starting point, because this will back-bias D102 and keep the little battery from powering the circuit when we have 12 volts available from either a wall-wart supply or the aircraft battery.

Crystal Clear Schematic. If you need pinouts for any of the ICs in this article, simply Google on the part number and CMOS. For example, search for 4040 and CMOS for the 4040 divider. You will get several hundred hits to pick from. As far as I know, they are all correct.

f you take a two-month backstep to the July 2004 issue of KITPLANES®, you can see that to produce the signals needed for ramp testing the VOR and ILS receivers, there are a number of audio-type frequencies that you will have to generate. The most difficult signals to generate are the precise 30-Hz waves for the VOR tester. Not only do you have to generate the frequency precisely, but you also have to find a way to vary the phase of the 30-Hz waves to test at various bearings around the VOR dial.

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The Power Supply I generally begin any design with ILLUSTRATIONS: JIM WEIR

the requirements of the power supply. I took a few things into consideration when determining these requirements: (1) I’m going to use CMOS (complementary metal oxide semiconductor) digital logic in the design, and (2) I want this little rascal to be able to run from an ordinary household wall outlet, from a 12-volt aircraft battery or from a 9-volt transistor radio battery. Those considerations taken into account, I think an operating voltage somewhere around 8.5 volts should be just fine. CMOS will run anywhere between 3-18 volts, and when we go to do the little RF oscillators later, 8 volts should give us more than enough

With the power supply in hand (at least for the moment), we can turn our attention to the generation of our 30-Hz tones. Some radios depend on these tones to be exactly 30 Hz and will give you errors if the tones are off, even by just a little. The only way I know to generate a stable, precise frequency is with a crystal oscillator. And since you can’t buy 30-Hz crystals, we are going to have to use some other frequency and manipulate our way to 30 Hz. The cheapest source of crystals I know of are manufactured by the billions every year—watches. As the tuning fork crystals used in watches, these little rascals are quite precise and are available for half a buck or so from the usual providers such as Mouser, Digikey and surplus stores. But a crystal by itself does not oscillate. Just like a bell by itself does not ring, we need a little hammer to

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ding our crystal and make it ring also. U102A is such a hammer. In reality, it is an inverting buffer amplifier, and it starts out life hitting the crystal with a burst of noise when the power supply is first turned on. Shortly thereafter (nanoseconds) the crystal will start to ring, feeding this ring signal into the input of U102A. The output hits the crystal with its own frequency (32,768 Hz), and we are off to the races with our Pierce-type oscillator circuit. The nice thing about a Pierce is that it is rock stable, changes practically none with voltage or temperature and is inherently self-starting. The device is actually called a 4049 and is of the CMOS family that I mentioned before. Here’s the result of this month’s work.

The Details CMOS is a popular logic family, and one of the great virtues of CMOS is that it draws only microamperes of current to do its job. Actually, the 4049 is what is called a HEX inverting buffer because there are six individual buffers inside of each IC package. Let’s use one of those other five buffers (U102B) to provide some isolation from the rest of the circuit to the crystal. We will call the output of this buffer the 32-kHz Clock. And just because I suspect that we may need it somewhere else down the line, let’s use one of the remaining four buffers to invert the clock. We will call this output the /32 kHz Clock (the / symbol means inverted or not). For reasons that I will explain later, the first thing I want to do with this accurate clock signal is divide it in frequency by 273. This will give us an output signal of 120 Hz. U103 is such a divider, and again, it is a CMOS device—a 4040. Each of the Q outputs is a power of two. Q8, for example, is a divide by of 28 or 256. Similarly, Q4 is a divide by 16, and Q0 is a divide by one. D103-D105 act as an adder so that 60 K I T P L A N E S

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the IC is reset to start counting all over when 256+16+1 (273) happens. The signal at the cathodes of these diodes is the input frequency (32 kHz) divided by 273, or 120 Hz. R107 and C104 simply make a 10-microsecond pulse stretcher so that I can see the 120-Hz signal on my oscilloscope. Make sure you understand what I am doing here, because to get the localizer, glideslope and marker beacon tones in future articles I am going to use the 32-kHz clock and simply choose the divide-by ratio to get the audio tones for these tests. I am also going to use this technique to get the 9960-Hz subcarrier for the VOR reference channel. So, what we’ve got feeding the Cl (Clock) inputs of the next part of this circuit (U104A and U104B) is a pulse 10 microseconds wide with a frequency of 120 Hz. U104 is (once more) a CMOS device called a 4013 flip-flop. There are two of them in a single package, and they are arranged in what is called a quadrature generator, which is some2 0 0 4

times named a two-bit shift register. The function of this quadrature generator is to divide the input frequency by four and simultaneously give out the four cardinal angles (bearings) of 0°, 90°, 180° and 270°. Thus, we wind up with a 30-Hz square wave at these four angles. Now the engineer in my right brain fights with the pilot in my left brain. The engineer says that you can never have too much test equipment capability. And wouldn’t it be nice to have the angles in 1° steps? The pilot says that to make the circuit that complex would require at least as much circuitry as we already have, and the odds of needing to check our VOR at anything other than these four headings is slim to none. The pilot wins. We can have any of the four bearings that we just generated.

The Saga Continues I’m going to stop here and let you cobble up this little circuit and test it. In the upcoming November issue, we will make the 9960-Hz reference channel, a couple of filters to make our square waves into sine waves and probably the RF generator/modulator as well. Then in January, we’ll go for the localizer and glideslope generator. We’ll wrap the whole thing up next March with the marker beacon generator. During the “in-between” months, we will busy ourselves with a little antenna work, perhaps some more work on the Ferry Box from last month and a few more goodies I’ve got rattling around in my head. Jim Weir is the chief avioniker at RST Engineering. He answers avionics questions in the Internet newsgroup rec. aviation.homebuilt. Check out his web site at www.rst-engr.com/kitplanes for previous articles and supplements. W W W . K I T P L A N E S . C O M