CVOR DVOR, please help me
Thread Starter
Join Date: Oct 2009
Location: Close to you
Posts: 250
Likes: 0
Received 0 Likes
on
0 Posts
CVOR DVOR, please help me
this is giving me a headache!
Let me break this down into what I know at the moment, and please correct me if I'm wrong/missing something.
The CVOR sends out 2 signals. One REF, omnidirectional on the particular freq of the VOR. There is also a VAR signal, which is on a 9960Hz subcarrier? What does that mean? For ex, if the VOR freq is 113.4Mhz, will the subcarrier be on 113.4-9960Hz? Or does it mean that the VAR signal is sent on 9960Hz?
Ok, let's move on..
On a CVOR the VAR is just like the REF a 30Hz signal, but it is amplitude modulated because it is turning?
Where as on DVOR the VAR signal will be frequency modulated due to the doppler effect?
Why does not the doppler effect show up on the CVOR?
Am I missing something here?
Let me break this down into what I know at the moment, and please correct me if I'm wrong/missing something.
The CVOR sends out 2 signals. One REF, omnidirectional on the particular freq of the VOR. There is also a VAR signal, which is on a 9960Hz subcarrier? What does that mean? For ex, if the VOR freq is 113.4Mhz, will the subcarrier be on 113.4-9960Hz? Or does it mean that the VAR signal is sent on 9960Hz?
Ok, let's move on..
On a CVOR the VAR is just like the REF a 30Hz signal, but it is amplitude modulated because it is turning?
Where as on DVOR the VAR signal will be frequency modulated due to the doppler effect?
Why does not the doppler effect show up on the CVOR?
Am I missing something here?
Join Date: Aug 2003
Location: Scotland
Posts: 240
Likes: 0
Received 0 Likes
on
0 Posts
The main radio frequency of VOR transmission - your example is 113.4 MHz - does not change. What the conventional VOR does is transmit a tone on that radio channel, and then modulate the tone. This tone is called a subcarrier, but it's a carrier in the sense that it is carrying information, not in the "radio" sense.
That transmitted tone is frequency modulated with a sine wave, and your VOR receiver can demodulate it and gets back the corresponding sine wave. The tone is sent at 9960 Hertz modulated +/- 480 Hertz, and the sine wave that it carries is at about 30 Hertz. This sine wave is the reference signal.
By the way, since 9960 Hertz is in the audible frequency band (or at least, it was when I was younger) if you tune in a VOR with a ham radio type receiver, you can listen to it going "weeweewee" in a fairly unpleasant chirpy way. Fortunately the VOR receiver in your aircraft has audio filters that remove this. That filter doesn't block lower audio frequencies, and that's how you can get the ident, and in some case audio (such as ATIS weather) transmitted on the same VOR at the same time.
Back to the VOR - The second half of the VOR solution is that the whole signal - meaning the transmitted 113.4 MHz signal (which also contains the reference and the audio stuff) varies in amplitude. In the classic VOR design that's because the VOR antenna itself whirls round with an electric motor (although the reality doesn't involve a huge antenna going round, and actually involves some transformations that are beyond this tutorial, but just imagine an antenna whizzing round and you get the idea). The antenna whirls around at the same 30 Hertz rate as the reference signal. When it is pointing straight at you, the signal is loudest, when it is pointing away, it is quietest. It is therefore amplitude modulated, and we can demodulate that as well.
Since the reference sine wave does NOT come from the modulation of the 113.4 MHz signal, but instead comes from the modulation of the high pitched audio tone we transmitted, it is not affected by this whirling of antennas, and it sounds exactly the same wherever you are. But, the relative peak of the amplitude DOES depend on where you are (because the antenna is going round). By comparing the peak of the reference sine wave with the peak of the overall received signal, we can work out the phase difference between the two. If we measure that in degrees of phase, it will be a number between zero and 359 degrees. Which can very conveniently be correlated with the radial from the VOR transmitter.
That transmitted tone is frequency modulated with a sine wave, and your VOR receiver can demodulate it and gets back the corresponding sine wave. The tone is sent at 9960 Hertz modulated +/- 480 Hertz, and the sine wave that it carries is at about 30 Hertz. This sine wave is the reference signal.
By the way, since 9960 Hertz is in the audible frequency band (or at least, it was when I was younger) if you tune in a VOR with a ham radio type receiver, you can listen to it going "weeweewee" in a fairly unpleasant chirpy way. Fortunately the VOR receiver in your aircraft has audio filters that remove this. That filter doesn't block lower audio frequencies, and that's how you can get the ident, and in some case audio (such as ATIS weather) transmitted on the same VOR at the same time.
Back to the VOR - The second half of the VOR solution is that the whole signal - meaning the transmitted 113.4 MHz signal (which also contains the reference and the audio stuff) varies in amplitude. In the classic VOR design that's because the VOR antenna itself whirls round with an electric motor (although the reality doesn't involve a huge antenna going round, and actually involves some transformations that are beyond this tutorial, but just imagine an antenna whizzing round and you get the idea). The antenna whirls around at the same 30 Hertz rate as the reference signal. When it is pointing straight at you, the signal is loudest, when it is pointing away, it is quietest. It is therefore amplitude modulated, and we can demodulate that as well.
Since the reference sine wave does NOT come from the modulation of the 113.4 MHz signal, but instead comes from the modulation of the high pitched audio tone we transmitted, it is not affected by this whirling of antennas, and it sounds exactly the same wherever you are. But, the relative peak of the amplitude DOES depend on where you are (because the antenna is going round). By comparing the peak of the reference sine wave with the peak of the overall received signal, we can work out the phase difference between the two. If we measure that in degrees of phase, it will be a number between zero and 359 degrees. Which can very conveniently be correlated with the radial from the VOR transmitter.
Thread Starter
Join Date: Oct 2009
Location: Close to you
Posts: 250
Likes: 0
Received 0 Likes
on
0 Posts
"Back to the VOR - The second half of the VOR solution is that the whole signal - meaning the transmitted 113.4 MHz signal (which also contains the reference and the audio stuff) varies in amplitude. In the classic VOR design that's because the VOR antenna itself whirls round with an electric motor (although the reality doesn't involve a huge antenna going round, and actually involves some transformations that are beyond this tutorial, but just imagine an antenna whizzing round and you get the idea). The antenna whirls around at the same 30 Hertz rate as the reference signal. When it is pointing straight at you, the signal is loudest, when it is pointing away, it is quietest. It is therefore amplitude modulated, and we can demodulate that as well."
This is something that gives me an headache.
Why will the VAR signald be amplitude modulated?
In my mind something moving towards and away from you will have a shift in frequency.
As for the DVOR, the VAR is frequency modulated,right?
Can you see what I mean?
Thanks for your answer!
This is something that gives me an headache.
Why will the VAR signald be amplitude modulated?
In my mind something moving towards and away from you will have a shift in frequency.
As for the DVOR, the VAR is frequency modulated,right?
Can you see what I mean?
Thanks for your answer!
Join Date: Oct 2004
Location: Southern England
Posts: 483
Likes: 0
Received 0 Likes
on
0 Posts
Why will the VAR signal be amplitude modulated?
In a DVOR they swop around. The REF signal is the AM one and the ground station transmits the modulated carrier from an omni directional aerial. The VAR signal is the FM modulated one. In this case the sideband signal is rotated and the doppler effect means that it appears to the receiver as having a frequency modulation.
Join Date: Oct 2011
Location: Right Here
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
Deviation (DVOR)
Dont know whether this question suits here 
.. Can somebody explain the relationship between 9960Hz Modulation, Azimuth Index and 30 Hz FM ... Is it true Deviation (16) cannot be adjusted and it is the result of Carrier to Sideband Phasing... Please verify..
.. Can somebody explain the relationship between 9960Hz Modulation, Azimuth Index and 30 Hz FM ... Is it true Deviation (16) cannot be adjusted and it is the result of Carrier to Sideband Phasing... Please verify..
