The Mini-Circuits SRA-2H comes to mind...............Level 17, which
requires 50mW of LO injection.

Pete

"Paul Keinanen" wrote in message
...
On Tue, 15 Mar 2005 12:49:01 -0500, "Netgeek"
wrote:

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?

My suggestion of using a DDS for direct conversion in the 10-20 MHz
range is based on the assumption that DDS chips running with 50-60 MHz
clock frequency should be available at a quite a reasonable price,
compared to similar chips running at 400 - 500 MHz, which would be
required for direct synthesis in the VHF band.

However, translating the whole band down to HF requires a strong down
converter, especially due to the nearby strong signal broadcast band.
Some flimsy NE602 type converter will not survive very well in such
environment, but a high current preamplifier followed by a diode ring
mixer might be a better converter.

Paul OH3LWR

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...

wrote in message

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.

SNIP - [lots of good information]

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...



Well, that was certainly a very informative and well thought
out response! But GEEZ, Len, you're starting to take all the
fun out of this by insisting on injecting reality!!!....8-)

Here I was - with soldering iron warmed up and pile of odd
looking components (SA-614, MC1350, NE567, inductors,
IF transformers and all kinds of other "stuff" that's unfamiliar)
and now my hopes are dashed............

I even checked Mouser, Digikey et. al. - and they're fresh out
of goniometers - so there goes *that* approach...8-(

But seriously - let's look at the potential utility of a fairly "mindless"
NAV receiver as it might apply to the non-instrumented-rated,
day-VFR "Sport" or "Recreational" pilot. First off, these guys
believe that GPS coupled to a simple moving map display represents
not only the holy grail - but they'd be willing (foolishly) to bet their
lives on this sole-source nav capability (never mind simple "dead
reckoning" or other elementary - e.g. "follow roads" forms of
navigation). If the batteries run out on the GPS - or the guys at
Cheyenne Mountain pull the big red lever marked "scramble GPS"
for whatever reason - they're in big trouble. Standard VOR-based
equipment would give them a way out - but they don't have it ('cause
it costs too much) and they wouldn't know how to effectively use it
anyway ('cause they aren't instrument rated). Some form of relatively
simple (albeit far from accurate) NAV capability would at least give
them a last chance to drag out the sectional and try to determine roughly
where they are - hopefully close enough to find a place to put down.

My ridiculous little experimental project is to try and come up with a
"poor man's" (and perhaps "stupid man's 8-) nav capability based on
VORs which is inexpensive and SIMPLE. There's no OBS nor any
other "normal" features (e.g. ability to drive a CDI) - but it kicks out
enough info relative to a few nearby VORs so that you can at least
determine what planet you're on 8-).... and provide a few hints as to
*where* you are on that planet...

GPS replacement? Absolutely not. TSO'd NAV receiver replacement?
Nope - not that either. Inexpensive (enough so that you might actually
install one) and simple (enough so that you could derive some useful info
with little training) - that would be the goal.

In the meantime, it's really a personal educational and entertainment
toy to play with, and nothing more ("amateur", "homebrew", etc. - so
it's relevent here, right?)......8-)

I appreciate your thoughts and comments, Len! You obviously have a
wealth of experience to draw upon and I thank you for sharing it. Despite
more than 25 years in product development, most of this is new territory
for me (and I'm enjoying the learning experience!). I've never done an
RF design - well - at least not "deliberately"!!!

Regards,
Bill

Netgeek wrote:

snip

But seriously - let's look at the potential utility of a fairly "mindless"
NAV receiver as it might apply to the non-instrumented-rated,
day-VFR "Sport" or "Recreational" pilot. First off, these guys
believe that GPS coupled to a simple moving map display represents
not only the holy grail - but they'd be willing (foolishly) to bet their
lives on this sole-source nav capability (never mind simple "dead
reckoning" or other elementary - e.g. "follow roads" forms of
navigation). If the batteries run out on the GPS - or the guys at
Cheyenne Mountain pull the big red lever marked "scramble GPS"
for whatever reason - they're in big trouble.

If that happened, and that's a pretty big if given the US government
is forcing GPS as the defacto navigation standard for just about
everything, the accuracy would be reduced to such that it would be
impossible to make a precision approach.

The remaining accuracy would be more than enough to find an airport,
especially since Sport and Recreational are limited to day VFR.

Standard VOR-based
equipment would give them a way out - but they don't have it ('cause
it costs too much) and they wouldn't know how to effectively use it
anyway ('cause they aren't instrument rated). Some form of relatively
simple (albeit far from accurate) NAV capability would at least give
them a last chance to drag out the sectional and try to determine roughly
where they are - hopefully close enough to find a place to put down.

Sporty's sells the SP-200 NAV/COM handheld for $299.00 with a $14.95
rebate if you use your AOPA credit card. It has VOR and LOC with a
digital CDI display and 2,280 channel COM. You still would have to
know what 235 FROM means.

IMHO anyone not flying a big turbine with multiple redundent everything
that doesn't have a handheld just in case is foolish.

My ridiculous little experimental project is to try and come up with a
"poor man's" (and perhaps "stupid man's 8-) nav capability based on
VORs which is inexpensive and SIMPLE. There's no OBS nor any
other "normal" features (e.g. ability to drive a CDI) - but it kicks out
enough info relative to a few nearby VORs so that you can at least
determine what planet you're on 8-).... and provide a few hints as to
*where* you are on that planet...

GPS replacement? Absolutely not. TSO'd NAV receiver replacement?
Nope - not that either. Inexpensive (enough so that you might actually
install one) and simple (enough so that you could derive some useful info
with little training) - that would be the goal.

In the meantime, it's really a personal educational and entertainment
toy to play with, and nothing more ("amateur", "homebrew", etc. - so
it's relevent here, right?)......8-)

I appreciate your thoughts and comments, Len! You obviously have a
wealth of experience to draw upon and I thank you for sharing it. Despite
more than 25 years in product development, most of this is new territory
for me (and I'm enjoying the learning experience!). I've never done an
RF design - well - at least not "deliberately"!!!

Regards,
Bill

Decoding the bearing can be done with a PLL running as a 360X frequency
multiplier on one 30 Hz signal and using the other to gate a counter
which is feed the multiplied signal.

I built such a beast in '75 as a senior project with a NIXIE tube
display. Available compenents have improved a lot since '75.

If I were to do something like this today, I think I would look for
someone's receiver module and use a microcontroller to control the
receiver and do most (maybe all with DSP) the decoding, feeding it all
to a PDA with a database of VOR frequencies and locations and use the PDA
to generate a map display.

It would be a fun project.

--
Jim Pennino

Remove -spam-sux to reply.

On 16 Mar 2005 12:58:44 -0800, wrote:

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.

If you are running an I/Q DDS feeding I and Q demodulators (mixers)
and low-pass filter the quadrature demodulator, producing a DC voltage
proportional to the phase error. This could be used through an ADC to
control the phase modulator input (or the phase accumulator addend) of
the DDS to track the signal. However, DDS chips with only serial input
might be too slow to track the signal. The I demodulator output would
provide the amplitude modulated information.

Paul OH3LWR