fawcett@dalcs.UUCP (Barry Fawcett) (06/13/85)
[bug swatter]
this article is posted for greg roberts @ dalcsug!roberts
This article is in response to a request from Bob Alkire at H.P. Labs
in Palo Alto California. I hope it is not too technical for net.aviation.
Your ground school instructor really did simplify things, 360 separate
antennas? Well, here is a simple explanation to present the concept, and then
a more detailed explanation of what goes on.
Consider a rotating flashlight, turning at a constant speed. Also imagine a
bright strobe light. As the flashlight sweeps out an arc, every time it rotates
through north, the strobe light is triggered. The strobe is visible in all
directions, and the flashlight is only visible when it is pointing exactly in
your direction. Counting the interval from when the strobe flashed, and when
you see the flashlight can be used to determine your bearing from the beacon.
So much for the simple analogy. Now comes the more technical explanation you
requested. An omni ground station has two antennas. One is designed to
electronically rotate a signal, a directional beam, at 1800 revolutions per
minute. This is "visible" electronically as 30 Hz amplitude modulation (AM).
Another antenna broadcasts a signal in all directions simultaneously as a
reference signal. This reference signal also has a 30 Hz component which is
in phase with the rotating signal at exactly north. This produces a different
amount of phase shifting at each of the other 359 compass points.
The 30 Hz rotating AM signal and the 30 Hz AM reference are distinguished by
a 9960 Hz subcarrier, modulated on the 30 Hz reference, thus making the two
identifiable electronically. The VOR in your aircraft receives all the signal
components transmitted by the ground station simultaneously. A filter is used
to separate the 30 Hz AM signal from the rotating signal and its other modulated
information. Another filter selects the 9960 Hz FM component. This FM signal
is sent to a detector, where the 30 Hz reference is recovered. Briefly, these
two 30 Hz signals are compared and a phase difference is detected.
The signal has been 'split' into two parts. Each of the 30 Hz signals is fed to
a phase detector, and the output drives the Left/Right steering needle. The
rotating 30 Hz signal is then phase shifted by 90 degrees, and is fed to still
another phase detector along with the 30 Hz from the reference signal. The
output of this detector drives the To/From flag.
A simple block diagram will perhaps help you understand the technical side a
bit more.
/-\
|T/F|
\_/
|
.---. .---. .---. .-^-.
.---| 1 |---| 2 |---| 3 |----v----| 4 |------.
| `---' `---' `---' | `---' |
| | |
| | |
Audio | .---. /-\ |
from ---->| | 4 |--|L/R| |
receiver | `---' \_/ |
| | |
| | |
| .---. .---. .---. | .---. |
`---| 5 |---| 6 |---| 7 |----^----| 8 |------'
`---' `---' `---' `---'
Legend of Block Diagram
1. 9960 Hz Filter circut
2. Limiter
3. Discriminator
4. Phase Detector
5. 30 Hz Filter circut
6. Resolver
7. Amplifier
8. 90 Degree Phase Shifter
The T/F indicates the To/From flag, and the L/R/ indicates the Left/Right
Steering bar.
Hope this helps you out without making it worse.
Clear Skies,
Gregory Neil Roberts (Avionics Tech Extraordinaire)
Dalhousie University
Department of Mathematics and Engineering