[simpits-tech] Real VOR indicator

[dabigboy at cox.net]

Wayne, that sounds about right. I should point out that Doppler VOR (DVOR) uses frequency modulation on the 30hz variable signal, whereas conventional VOR amplitude-modulates the signal (something I kinda missed in my previous lengthy explanation).

I don't really have a set of good references for you, I just grabbed bits and pieces off the net over a period of several days. I did just come across this link, which goes into even more detail that I had missed before with regard to CVOR vs DVOR:

http://wiki.answers.com/Q/Why_Doppler_VOR_subcarrier_frequency_is_9960_Hz

I did study an excerpt from an avionics book, I believe it was Principles of Avionics by Albert Helfrick. That is probably the single most important resource I found, aside from the program that the ham radio guy sent me (the book is pricey, but GoogleBooks had a few pages as a "sample" which happened to include a sizable portion of the VOR discussion).

Matt

---- wledzian1 at comcast.net wrote: 
> Matt, 
> 
> Can you point me towards your reference material? I can't find anything online more detailed than the Wikipedia article. 
> Are you generating WAVs with two fixed frequencies, or do your samples include an FM-modulated 9960Hz and a phase-shifted 30Hz signal? 
> 
> 
> My understanding of how a VOR works is as follows. If I'm wrong, please correct me. 
> 
> - The VOR does not broadcast two separate signals to provide the omnidirectional and directional components. 
> 
> - The VOR base signal is an AM signal in the 108.00MHz-117.95MHz range. This signal carries a 9960Hz subcarrier frequency-modulated to carry a reference 30Hz sine wave whose phase does not vary with your position. 
> - The antenna is rotated (electronically or mechanically) at 30Hz. This effectively and strongly superimposes a 30HZ AM signal onto the base signal, with phase dependent on your position relative to the station. 
> 
> The receiving equipment demodulates the signal and sends it to the VOR head unit. The head unit splits the signal into the 30Hz directional signal and the 9960Hz subcarrier. The subcarrier is then FM-demodulated to recover the 30Hz reference signal. The reference and directional signals are then compared to determine the relative phase shift. Tuning the OBS knob adjusts a phase-shift circuit on one of the signals - when the signals are in sync, the difference is zero, and the needle is centered. 
> 
> To quote Mike Powell, "I may not be as smart as I think I am". If I'm wrong in the above, please correct me. 
> 
> -Wayne 
> 
> ----- Original Message -----
> From: dabigboy at cox.net 
> To: "Simulator Cockpit Builder's List" <simpits-tech at simpits.org> 
> Sent: Wednesday, February 13, 2013 8:53:56 AM 
> Subject: Re: [simpits-tech] Real VOR indicator 
> 
> 
> ---- geneb <geneb at deltasoft.com> wrote: 
> > On Tue, 12 Feb 2013, dabigboy at cox.net wrote: 
> > 
> > > Thanks! I used my sound card. :) 
> > > 
> > Ok, that all by itself requires a lengthy explanation. :) 
> > 
> > g. 
> 
> LOL! OK, here's more or less what happened: 
> 
> So I had been wanting to get my hands on a real VOR indicator for a while, with the basic plan that I could gut it and use the electrical bits to read OBS setting, and use PWM pins on the Arduino to drive the needle (since, as I understood it, the needle was just a simple D'Arsonval meter). I eventually purchased this KI-201C on ebay for like $30 shipped, mostly just to see what I could do with it. When I got to looking at the instrument and the documentation for it (found on the 'net), I realized that the signal driving the gauge only required three pins, and two of them were actually *output* for an autopilot connection....so only one pin was used for signal input. Intrigued, I started hitting the internet for more info........ 
> 
> Having sat through ground school years ago, I knew that the signal coming from a VOR station was "two different signals that vary based on your orientation to the station"....and that was about as far as a typical ground school instructor would go. It turns out, the composite VOR signal is pretty interesting. Each ground station puts out an omni-directional signal which is composed of a 30hz sine wave, plus the Morse stuff on a different frequency. The "rotating antenna" (actually a circular array of antennae on the ground that is electronically "rotated") puts out another 30hz signal, but is composed of a 9960hz carrier signal. The phasing of the two signals determines what radial you are on, and the 9960hz signal helps keep them "separated", so to speak.The 9960hz signal also allows the signal to be differentiated, if the two 30hz tones are 180* out of phase, for instance (otherwise there would be no difference between the 0* radial and 180*). 
> 
> The complex thing is that the 9960hz carrier signal is frequency-modulated at 30hz, and where the 9960hz signal is at depends on where the 30hz signal is at in its phase. This is the hardest part.......I was getting the right idea, but wasn't sure exactly how to make it work. One thing I knew, that 9960hz was well within the ability of a computer's sound card to produce, so I *should* be able to actually generate this signal myself. 
> 
> Then I come across a post on a message board from 2006 where some folks were discussing VOR signals. One poster was a ham radio operator and mentioned that he had actually written a program that generates all the signals for a VOR gauge. I emailed him and, thankfully, his address was still active. He generously sent me his entire program, source code and all! Unfortunately it was written as a Visual Studio project, and there was so much Microsoft fluff involved, I wasn't sure exactly how to read the code and apply it to my Python/Linux work. So, I fired up my barely-used copy of VS 2010, migrated his project, made some tweaks to get it to run (his program was from 2001, so it was written in an old version of VS), and actually got it to output the tones for each radial. 
> 
> I then used Audacity to record each radial, 0*-359*, and saved them as WAVs. After that, I wrote software that calculates the current radial from X-Plane, then plays the appropriate WAV file. 
> 
> The really tricky thing is that the calibration is funky....depending on what radial I'm at, the error is anywhere from no error at all, to around 30*. So, I had to write a calibration table to send the correct WAV to the sound card. I'm not sure if this is a problem with the instrument, weirdness with my sound card (the phasing is actually messed up by the soundcard, but it's supposed to be a constant error that you can allow for in software), or something funky caused by playing pre-recorded WAVs instead of generating the tone in real time. I've got it pretty well working within 4 degrees accuracy or so, but there are a few radials that are off by more than 20* still..........I think it's a problem with my radial-calculating code (the X-Plane SDK does not seem to have a dataref for current VOR radial, I have to subtract relative bearing to the VOR from current heading, then apply corrections to keep the resulting value within 0-360). But it is working, which is the main thi
>   
> ng. I don't think the calibration errors will be hard to fix. 
> 
> If I wrote my software to generate the 30hz tone and the 9960hz modulated at 30hz, I'm pretty sure I could fill in the gaps of my knowledge through experimentation and study of the other guy's code.....but what I'm doing now with the WAVs appears to work just fine, so I will probably leave it. 
> 
> Now, the astute observer might note that the signal coming from a real VOR station is actually an AMPLITUDE-modulated signal between 108mhz and 119.95mhz, and you would be correct! That's the carrier signal to broadcast to your plane. The NAV receiver in the plane de-modulates that AM signal to yield the "clean" VOR composite signal, which is what goes to the indicator. Theoretically, if I designed a device that takes my current signal and amplitude-modulates it at the above frequencies (basically, half of an AM radio transmitter), I could run the signal right into the antenna input of a real NAV radio and actually be able to use all the tuning and functionality of the real radio! This is overkill for me though, I already have nice Gables control heads doing my nav/com stuff (plus good NAV radios are over $1,000 a piece.........) 
> 
> So what is all this magic inside the VOR indicator itself? It is simply a phase-shifter, and the VOR needle indicates how far out of phase the two signals are....turn the OBS knob until the shifted signals are in phase, and the needle will be centered. 
> 
> Matt 
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