VOR Receiver Theory of Operation

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I have done a lot of searching on the internet for a description of
how (in terms of mixers, limiters, loc osc, and the like) a VOR
reciever determeines course and sets the TO / FROM flag without a lot
of success.

I have formed my own notion based on what I have gleaned form various
sources but I'd love to get a pointer to a definitive answer. If
there is nothing on the net, what book might be reecommended? Or,
could someone can confirm or correct my idea that the ref 30HZ signal
is phase shifted by the OBS then compared to a phase shited signal
sent from the VOR transmitter. This comparator drives the CDI.

I think a second comparitor detects the more-than less-than 180
degrees out of phase state between the OBS signal and the received
variable signal. For more 18O degrees set the TO flag, for less
than... set the FROM flag.


-- mike murphy, October 10, 2005


Take a look here.

-- Tal Reichert, January 17, 2006

Either "Avionics Navigation Systems" by Kayton and Fried or "Principles of Avionics" by Helfrick have satisfactory explanations.

In brief, a conventional VOR site transmits an omnidirectional AM signal with 1040Hz CW ID and 9960Hz FM subcarrier modulated at 30Hz (the reference). In addition, the same carrier, unmodulated, is driven onto an cardioid array which is electrically rotated at 30Hz. The rotating cardioid carrier sums with the omni carrier resulting in a baseband 30Hz AM tone, the phase of which varies with the bearing from the VOR. At radial zero (mag North), the demodulated 30Hz tone from the ref subcarrier and the baseband 30Hz tone appear in phase.

The old KX-170 NAV RX is simply a wideband AM receiver. The AM demod output is called "composite VOR." That feeds a VOR/LOC converter which behaves pretty much as you describe. The 9960Hz carrier is FM demodulated (the old KI-201 uses a Foster-Seely). This ref phase and the OBS knob drive a resolver (sin/cos transformer). This is followed by an RC phase shift network and summer to yield a ref phase delayed by the OBS selection. The two 30Hz tones are multipled; the product drives the meter. A 90deg phase shifter and second multiplier drive the to/from.

The localizer is totally different. See AIM Figure 1-1-7

Since the VOR signal is all audio, it is possible to write a PC sound card application to synthesize it. In practice, however, most audio cards have nonlinear group delay, so the calibration is way off. My laptop is particularly bad in this respect, unfortunately.

-- Dave Page, February 11, 2006

WRT generating VOR audio in software:

I did this a few years ago. Wrote a program in Visual C++, works on any windows machine. It generates both the 30Hz and the 9960 Hz signals algorithmically, using a sine table. The hard part was doing the FM modulation of the 9960Hz. Where are we in this wave? Depends on where we are in THAT wave.

The problem with getting an accurate zero on it was that the sound card was AC coupled, and the coupling cap or caps produced a serious phase shift at 30Hz. And this shift was different for each computer I tried it on. However, the phase shift was constant, and with a "calibration" slider it worked just fine.

It was Way Cool to hook up an indicator to my laptop and have it indicate.

- Jerry Kaidor

-- Jerry Kaidor, November 8, 2006

I have a few questions about some of the responses.

What are the details of how the 90 degree phase shifter is used?

If the OBS adds phase shift to the reference signal and that resulting signal is phase compared to the phase shifted vor signal, how does the phase comparator know which signal should be considered to be leading the other signal rather than lagging it (it seems that one can always consider either signal to be the leading signal or the lagging signal since it is just a relative measurment barring some undisclosed assumptions that may be in play)?

I assume the circuit that detects the phase shift between the reference signal and phase-shifted signal of the vor can detect a full 360 degrees of phase shift and further that it "knows" that the phase shifted signal from the vor always lags the reference vor signal.

But in the case where the OBS adds phase shift to the reference signal, that shifted signal could either lead or lag the vor's phase-shifted signal and so it seems to be that there would be ambiguity in that the indicator circuit would not know which signal at any given setting of the OBS should be considered the lagging signal and which should be considered the leading signal.

How is it that the indicator will zero out its course deflector needle not only when the receiver's OBS is set to the radial the receiver is located at, but also when the OBS is set to the radial that is 180 degrees away?

If someone could clear this up, that would be great. I'm not particularly interested in the modulation/demodulation methods that are used, but rather in just the functional methods used by the circuits to drive the cdi needle and the to/from indicator.

-- Bob Somers, July 6, 2007

hi... exist a application for decoding vor signals via soundcard?

-- edyasia asia, November 12, 2007