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How to build an

ILS/VOR test box, Part 3. * Sales, Service

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Last month’s VOR generator is shown on the left; this month’s work is displayed on the right.

ops—my goof! In the September issue, I told you how to build the first part of the VOR generator but neglected to explain exactly why we needed the signals we needed. My bad. Here ya go:

O

VOR for Dummies While GPS is the darling of the current navigation world and Loran the backup to GPS, there are still a couple of thousand VORs out there chugging out their 24/7 signals on a reliable basis. There is talk of the VOR system going away in 15 or 20 years, but my guess is there will be at least a few dozen that make it into the next century...especially those that have a non-precision approach to a major airport associated with them. Let’s face it, given 100 grand or so, any engineer worth his or her salt could jam the GPS and render it unusable over several thousand square miles. VOR isn’t quite so easy to jam. I’m going to gloss over the intimate details of how the VOR works. If you want to get deep into it, I recommend visiting www.navfltsm.addr.com. The VOR generates a single RF 68 K I T P L A N E S

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frequency and then splits this RF energy into two parts. The first half of this energy is fed to a rotating antenna. The most common form of VOR has an electronic rotation of a horizontally polarized beam antenna going round and round at 1800 rpm. In addition, the second half of the RF energy is broadcast in all horizontal directions by an omnidirectional antenna. Note that the first signal can also be said to be a 30-Hz signal (1800 rpm=30 rps). The omnidirectional signal is amplitude modulated by a 9960Hz signal. This 9960-Hz signal (sometimes referred to as a subcarrier) is itself frequency modulated by a 30-Hz sine wave. The deviation of the 9960 FM signal is +/- 480 Hz. Here’s the deal. The two 30-Hz signals (the one from the rotating antenna and the one from the FM’d subcarrier) are arranged so that they are precisely in phase as the beam passes through 0° (due north). They are then 90° out of phase due east of the VOR, 180° due south of the VOR and so on. By a clever manipulation of a calibrated phase shifter on your instruW W W . K I T P L A N E S . C O M

BY JIM WEIR

ment panel (sometimes called an OBS or OmniBearing Selector), you can shift the phases so that the signals are precisely in phase, zero the omni needle and read your bearing to or from the VOR transmitter.

Creating the Signals OK. Now that we know what we need, let’s get to it and create these signals. We know we need 30 Hz (and

Here’s the schematic of this month’s circuitry.

The composite VOR modulation out shows equal amounts of 9960 subcarrier and 30-Hz variable signals.

The 9960-Hz signal (also called a subcarrier), is FM’d by the 30-Hz reference signal.

Here’s what the 30-Hz variable signal looks like after being filtered. ILLUSTRATIONS: JIM WEIR

designed a little generator in September that lets us pick any one of four phases of this signal), and we know we need 9960 Hz. Because we already have the 30 Hz as a square wave, a simple opamp circuit (U101A and U101B) will let us filter this square wave into the desired sine waves. A simple digital phaselock loop (U102) with an additional filter (U101C) at the output to get a 9960-Hz FM’d sine wave shouldn’t be all that hard, either. The 30-Hz filters need to be matched, and you will see that one of the filters (U101B) has a small tuning potentiometer associated with it. We don’t care about the amplitude of the two filter outputs as much as we do about the relative phase of the two outputs. If the same 30-Hz square wave is fed to both the filter inputs, the outputs are adjusted to bring the sine waves into perfect 0° phase correlation. We will do a heuristic (hammer shut, kick in the edges, file flat, weld closed, paint to match) adjustment after the whole shooting match is done using a known good VOR receiver. The reference subcarrier will also be pretty easy to generate—a digital phaselock loop (U102) locked to the 30Hz reference square wave that we just talked about. If you do the math, you

find that a divide-by-332 in the feedback loop (U103) of the phaselock loop will indeed generate a 9960 square wave...and give us the capability of frequency modulating (R118) it in the bargain. (Just another basic Weir two-fer.) OK…332 is made up of a 256, a 64, an 8 and a 4. (Which, you may recall, is 2(8)+2(6)+2(3)+2(2), or Q8 and Q6 and Q4 and Q2 on our 4040 digital divider U103.) Hey, this is going to be fun!

Part 4 The next installment of this little project (in the January 2005 issue) will take the VOR modulation that we generated this month, make a crystal oscillator on a specified VOR frequency and combine the two into a true VOR tester. Once we get this little gem working, it should be relatively simple to generate the LOC frequencies, an ident tone and keep on proceeding. Then we do the glideslope and the marker tester, and that winds this whole project down. 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.

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