From: "Netgeek" on Tues, Mar 15 2005 12:49 pm

"Paul Keinanen" wrote in message

One idea would be to use a fixed downconverter e.g. with a 98 MHz
crystal frequency, mixing the VOR band down to 10-20 MHz, filter out
the strong mixing products from the FM broadcast band that is on
frequencies below 10 MHz and use a DDS with I/Q outputs to get I/Q
demodulation of the signal.

Thank you Paul - I'd like to look into this. Can you point me to some
practical examples or reference materials to start with? Quite some
time ago I ran across the articles from the flex-radio.com guys and
was very interested in their approach. It's my understanding (and I
mean a fairly fuzzy understanding) that direct conversion has many
benefits but is limited to lower bands (unless you're the military
with
a big budget)??? What are the trade-offs in doing a downconversion
followed by DDS-based conversion?

Direct conversion (DC) won't be effective on this
application for reasons of the civil aviation band
being AM with no pilot carrier or other reference.
Look into the allowed carrier tolerances and you will
see that, unless you can definitely LOCK onto the
incoming carrier, there will be a great change in the
modulation information, both in frequency and phase.

That is particularly true with VOR. The ground
station antenna pattern is (now) electronically
rotated at 30 Hz and the reference phase
(representing magnetic north) is FM on the 9 KHz
subcarrier. Without a proper phase relationship,
the bearing signal will be very inaccurate.

The VOR system was designed/innovated/invented over
a half century ago and was elegant in simplicity
for simple circuitry in vacuum-state hardware. The
first lightweight VOR receivers in light aircraft
used a (very old technology) small goniometer as
part of the OBS or Omni-Bearing Selector and their
accuracies were dependent on how well the goniometer
was designed and manufactured. [a goniometer is a
coaxial spherical toroid pair, best illustrated in
"Lowfer" or low frequency - below AM BC band -
small handbooks and some websites] More modern
versions use an electronic equivalent of phase
shifting at 30 Hz as part of the OBS subsystem.

A VOR antenna pattern rotation results in about
30% AM at a 30 Hz rate. The magnetic north phase
reference is 30 Hz FM on the 9960 KHz subcarrier.
The FM demodulation will have a limiter stage ahead
of it to effectively wash-out the 30% AM of the
ground station antenna pattern rotation. In
between the 30 Hz of the ground antenna pattern
rotation and about 8 KHz or so of the lower limit
of the 30 Hz FM on subcarrier phase reference is
"empty space" that was reserved for optional AM
from a local Flight Service Station (FSS) or tower
transmission. In short, the elegance of the
concept was ideally suited to vacuum tube circuitry,
that being an almost ultimate simplicity at the
time...and very light weight necessary for aircraft.

The "big bucks" of military electronics doesn't go
wild over fancy schmancy arrangements of the very
"in" modern complications. Those "big bucks" are
spent in making the hardware work over the totally
gargantuan range of temperatures and physical shock
and vibration that would tear apart consumer
electronics style structures. The civil avionics
market is not, nor has it ever been, large compared
to consumer electronics products, hence their costs
appear high.

There IS room for experimentation in ways to
demodulate the VOR information, don't get me wrong.
What you must do is to FIRST concentrate on the
characteristics of how the bearing information is
conveyed...along with all the problems introduced
by multi-path distortion from ground objects around
you. Those problems aren't there in the aircraft
flying a few thousand feet above all those reflecting
objects. [an exception is a VOR in a helicopter and
its own rotors...but that is another story in itself]

Just because the FM BC band upper end is at 108 MHz
doesn't automatically mean there WILL be RFI to the
receiver. That's a matter of checking a local area
to find where all those fixed FM BC carriers are and
how strong they are. Aircraft VOR receivers have
been overflying all sorts of FM BC stations for a
half century all over the world and there aren't
any stories (except invented horror tales) of terrible
interference from FM. Simpler civilian receivers,
not the "big bucks" of military aircraft.

Just offhand, I'd say a simple, even tube-based,
bearing information receiver can be hacked together
to get +/-5 degrees accuracy using the simplest
circuitry with minimum test equipment to check it out.
Anything better is just finesse, bottoming out at the
basic accuracy of whatever VOR ground station is used.
That would be +/-1 degree but worse from any ground
reception multi-path effects.

VOR (Very high frequency Omnidirectional radio Range)
was designed only for aircraft obtaining bearing
information to a fixed ground station. That's a
limited application although extremely important to
pilots. A half century ago it was a quantum leap
above older raw-DF-style radionavigation. GPS it
ain't, nor never was...