# How does the VOR receiver pick up the signal and how are the needles controlled?

Knowing the basic principle of a VOR and how it works, I am interested to know how the receiver works.

What exactly makes the needle deflect? And how does this deflecting mechanism know how much to deflect the needle?

So here is what I am thinking so far: since the signals are sine waves (I guess) and there are two of them, if you lay them over each other there should be a unique voltage for every direction from the VOR. To keep on guessing, this voltage is probably amplified and put onto a electric magnet that will then control the needle's deflection.

• This might be better in Electronics. I don't know any pilots who know the internals of VOR receivers.
– rbp
Dec 21, 2014 at 2:55
• @rbp: Does it say anywhere that this site is for pilots only? Dec 23, 2014 at 17:43

Your basic understanding of the underlying electronic principles is correct: The VOR receiver detects a phase offset between two signals, which tells the receiver what radial the aircraft is currently on. Wikipedia has a decent explanation, and this article on Digital Flight Instructor has a nice widget you can play with to see the frequency offset.

The underlying electronic principles the receiver uses are generally not useful for pilots: if your VOR receiver dies you're not going to hook up an amplifier to an oscilloscope and try to sort out the phase offset yourself, if for no other reason than your head would be buried in the cockpit staring at the oscilloscope as opposed to looking out the window for traffic or watching your flight instruments.
In addition, the "strobe and sweep" explanation is generally easier for students to understand: Imagine the VOR station has two lights, a white light that rotates, and a red strobe that fires ever time the white light passes the 0 degree (North) radial. By counting the time between when you see the red strobe and when the white light passes you its possible to determine what radial you're on.

The mechanism by which the needle is deflected depends on the type of indicator, but it basically boils down to "The receiver does some magic and converts it into a number of dots of deflection on the indicator". The deflection can displayed by moving needle attached to a meter (applying a specific positive or negative voltage), blinking some LEDs, or showing a graphical representation of the CDI (e.g. on a GPS display) with a mark over the appropriate dot.
How this works for each specific indicator is generally only of interest to an avionics technician, and the information would be contained in the service manual for the radio and indicator.

• Yeah I know it is kind of useless knowledge, but I'd be really interested in it ;) Dec 21, 2014 at 13:47
• Is this forum only for pilots?
– rbp
Dec 23, 2014 at 17:58
• @rbp even if the site were "only for pilots" I'm of the opinion that pilots should have some fundamental understanding of how the equipment in their aircraft works: We don't let people get out of flight training thinking of the engine as a magical noise-making go-machine, but most flight schools (and pilots) are content to think of nav systems as magical black boxes. Knowing how the VOR receiver does its magic probably won't save anyone's life, but it's still not a bad thing to know. Dec 23, 2014 at 18:23
• @rbp I know, I'm just one of those "crusty old guys" - I still make junior sysadmins learn vi and expect drivers to be able to change a flat tire without calling the auto club. Dec 23, 2014 at 19:30
• Here is a great example of why flight crews need some understanding of the electronics. In this case, the ILS reported it was working properly, when it was not. The flight crew knew this was possible, and had a basic understanding of why. Dec 23, 2014 at 20:39