Re: help with navtex/medium wave receiver sensitivity and decoding

On 12 Feb, 19:13, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
bigorangebus wrote:
Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! I can't use a loop
antenna because its just too big and too directional for use on a
boat. So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). I'm driving the 612 differentially, as its a slight
improvement over single ended. From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

Many Thanks
Andy

In spite of the rich detail you've given us, this is still a problem
with many interesting dimensions.

Atmospheric noise is very strong at MF, so even with a capacitive probe
antenna such as yours it should still dominate. I would start by
measuring your signal strength at IF or baseband with the antenna
connected, then with the antenna replaced by an equivalent impedance.
Ideally you'd see an increase in noise level from 3 to 6dB -- much less
than that and your system's inherent noise is your limitation, much
higher than that and you've got too much gain ahead of the mixer, which
will make it susceptible to distortion.

If its practical I'd do a similar test with the antenna. I'd calculate
the antenna's effective capacitance to ground, then replace it with a
capacitor of that value from ground to the FET gate. You should see the
same 3-6dB increase in noise with the antenna vs. the noise with just
the cap connected -- if you don't, then your performance is being
limited by the antenna.

After noise, the next thing you need to worry about is distortion. If
your active antenna or your mixer are being overwhelmed by strong
signals (such as AM broadcast signals) then your desired signal will
either be blacked out, or the strong signals could combine in your
electronics to generate a signal at your desired frequency. Building
low distortion front ends is worth several pages if not a whole book,
but the gist of it is that you want to limit your signal's bandwidth as
much as you can, and you want to use intelligent design practices. Your
active antenna can't do much before the signal hits the FET gate, but
you can take Jan's suggestion and put a filter between the active
antenna and the mixer. I'd put in an elliptical filter that had good
attenuation in the AM broadcast band, to give your little Navtex signal
a fighting chance.

Assuming that you're limited by atmospheric noise at the output of your
IF, the only thing you can do is to improve your signal processing. I'm
not familiar with Navtex signaling, so I can't give you more than
general guidance, but here goes:

The nature of the atmospheric noise at MF is _not_ Gaussian. Noise in
that band is predominantly from electrostatic discharges both near and
far. It has a noise density that is more like a Cauer density, in that
for all practical purposes it has an infinite variance. Any sort of
signal processing scheme which is based on an assumption of Gaussian
noise will be compromised by that assumption.

The best way to deal with MF atmospheric noise is to take advantage of
the fact that it tends to occur as 'crackles' or 'hits' that have a
high-amplitude, wide spectrum characteristic. In general what one does
is to use 'side information' to detect when a discharge event has
happened, then discount the data that's collected during that event.
This is what noise blankers in communications radios do.

If the Communications Gods are smiling on you then there are clear
channels adjacent to your signal, or Navtex uses forward error
correction. In the case of the clear adjacent channels, you can use an
IF that's wider than your signal and use a noise blanker. Your signal
will go quiet during the crashes, but that non-information is
demonstrably better than having huge signals get into your demodulator's
filters to mess things up. In the case of FEC, you can demodulate the
signal as usual, then look for unusually large filter outputs. When
this happens you replace the demodulated bit with an erasure and forward
the whole shebang to an error correction algorithm that can benefit from
the erasures (and some do, to a significant degree).

I hope this helps.

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Posting from Google? Seehttp://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details athttp://www.wescottdesign.com/actfes/actfes.html
F
Further to this...I think I will need to get my hands on a spectrum
analyser. My scope is just not set up to read the small signals pre-
mixer and is probably just showing its own interaction. Its on max
sensitivity reading the output of the mixer, so pretty sure its not
being overwealmed. I did once try amping pre-mixer as Ive heard the
612 is not the best noise performer, but i just ended up creating a
feedback oscillation randomly! Navtex does have error correction , in
that it interleaves the transmitted bytes to detect burst errors, and
the spec requires you to receive both before you commit that character
to the screen. The problem is the shift is so small 170Hz that you
have to have your detection in synch with your local oscillation,
which means trying to measure the waveform period (as I do) or using
some heavy DSP (which has a theoretical solution for this solution Ive
heard). All you need is a little noise, or even a miniscule bit of
mains humm to raise and lower the signal and the period measurement
goes out of the window. This easily detects burst noise, by presenting
periods outside of the expected measurement window.

What is interesting is that the noise as appearing on the scope in
time domain at any rate looks the same but bigger when I hold the
plate antenna. Of course that could be a red herring as I have no
idea what other frequency components there are in there, but the
general shape of the background noise in the time domain remains the
same.

Ive heard of someone using a large hoop antenna to clear up the
signal, though thats too directional for my application.
Presumably with the dominance of the background noise making the
antenna bigger wouldnt work, but narrowing the bandwidth could.
However if I narrow it any more I wont be able to pick up the adjacent
490kHz local navtex signal (the active antenna is supposed to be dual
frequency, but only by virtue of having a loose enough Q).

I wonder if there is a better active antenna circuit than the simple
fet circuit in the active antenna.

Really appreciate your input
Andy

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