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help with navtex/medium wave receiver sensitivity and decoding

Discussion in 'Electronic Design' started by bigorangebus, Feb 12, 2007.

  1. bigorangebus

    bigorangebus Guest

    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
     
  2. It seems to me you could perhaps used a tuned circuit there, with
    sufficient bandwidth.
    In fact the active antenne you describe perhaps needs an LC to ground.
    Else indeed any spike will overload the FET, easy with a 'metal plate'.
     
  3. Tim Wescott

    Tim Wescott Guest

    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 Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. In what sense too big? A ferrite rod is not that big.
     
  5. john jardine

    john jardine Guest

    I'm at a different location of course but a speccie analyser fed by a high
    impedance buffer with a 18" telescopic aerial plugged in, just shows noise
    and more noise. Biggest signal was a national radio station about 900kc at
    8mV. 0-1Mc background about 0.5mV. Only starts cooling off above about 10Mc.
    Using 1kc bandwidth gave a few carriers (modulation=?) around the 518kc area
    but they were at the 20uV level and sitting barely above a buzzsaw of noise
    filling all the frequencies and sourced from the aerial, local equipment
    and PC.
    Don't know 'Navtek' but I'd try very strong bandpass filtering at 518kc
    after the FET. (you've probably not really enough wideband signal voltage to
    overload it and may as well take benefit from the increased signal level).
    The mixer is also going to give image frequency 6kc responses, these are
    unfilterable. I'm guessing you're running the LO at 512kc from a 4060 type
    xtal divider. The 3rd 5th 7th and 9th harmonics will easily mix with
    up-spectrum buzzsaw noise and shift it down also to the 6kc slot.
    Commercial stuff will filter, mix up to maybe a 60Mc IF to lose image
    responses, filter and then mix back down again to whatever, (and then filter
    again and once more for luck :).
     
  6. bigorangebus

    bigorangebus Guest

    The active antenna has back to back diodes to protect it in this case
     
  7. bigorangebus

    bigorangebus Guest


    This is brilliant...thanks for taking the time to put in this level of
    detail....I will spend this eve looking through your points..
     
  8. bigorangebus

    bigorangebus Guest


    A ferrite rod is small but very directional...in a boat you never know
    which direction you're going to be facing...and the receiver needs to
    work on a fixed antenna on its own!
     
  9. bigorangebus

    bigorangebus Guest

    Interesting... yes the incoming signal is 518k and I'm mixing it with
    512k generated from an 18.430M crystal divided down. This generates a
    bit of noise, and there is some chance of that getting into the
    system. However, that was the first thing i checked..I disabled it
    and the noise level visible on a scope drop by perhaps 10 to 20%, pre-
    filter, however there is very little evidence of a drop after
    filtering. The signal is a freq shift of 170Hz so detecting it is
    difficult even with a clean signal, and I need to do it digitally
    using the lo generator as a reference, otherwise theres no hope of
    tracking the 170Hz as a crystal is not accurate enough to use absolute
    frequencies.
    Basically you're looking at an intermittant signal (perhaps a 10min
    burst every 3 hours in peak times that are close enough to see), which
    is from a transmitter putting out 100W to 1kW, 100 miles away, which
    is substantially less than a commercial AM station.
    Also, the inverter on my low voltage kitchen lights screws the signal
    audible on my multiband sony recever, as does the TV set, my
    oscilloscope if it gets too close, any PC and the LCD display that
    shows the Navtex information. So its a bit of a harsh environment for
    it.
    The FET front end already has a 60Q bandpass filter, which ive tuned
    for peak (need some lee way as the next step is to dual band and pick
    up 490khz which is the local navtex), i then go into a tuned
    circuit(est Q80) with another can again peaked, and output this into
    the differential inputs of the 612. So two filters pre 612, then band
    pass filter with a Q of 30 before trying to detect the signal.
    I would have run the LO higher to make filtering a little easier, but
    I need a low LO for my software to decode.
    I'd love to try it on an HP, but so far Ive been outbid on ebay! I
    think I'm being a cheap on my bids...! I'll see if I can borrow one to
    check where the noise might be coming from.
    You can hear navtex by tuning near it on SSB, its sounds a lot like
    RTTY.
    Andy
     
  10. Mark

    Mark Guest

    There are some fundamental differences between the way an E field
    antenna (like a probe) and an H field antenna (like a loop) respond to
    locally generated noise.
    Your may want to investigate the Ryan storm scope that can determine
    the distance of a lighting strike by comparing the relative amplitude
    of the e and H field. You may want to inquire over at
    rec.amateur.antenna. And you may want to consider using two
    perpendicular loops.

    Mark
     
  11. bigorangebus

    bigorangebus Guest

    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
     
  12. bigorangebus

    bigorangebus Guest

    I will ask the question about the antenna in rec.amateur.antenna
    assuming Tim above doesnt get uppity about cross posting again! (I
    posted this question in the radio amateurs site also)
     
  13. Guest

    I found speccies to be unbelievably useful. If you enjoy the radio
    stuff (obviously!) should be No1 on the kit list. ('scope a near 2nd)

    Seems you've covered most stuff worth doing on the Rx. Looks like an
    electrically less harsh environment is the order of the day,
    (something a distance from civilisation, something say like a boat
    trailing a kite flown long wire).
    Failing this, my parlance would say you're buggered :)
    john
     
  14. Tim Wescott

    Tim Wescott Guest

    Demodulating FSK using center crossing really isn't very optimal. If
    that's what you're doing you'll pick up quite a bit of range just using
    the two filters method or a discriminator.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  15. ISTR sets which used a combination of a ferrite rod and a collapsible whip
    but maybe that wouldn't work in your case.

    I also recall two loops set at right angles to each other. Remember that the
    highly directional point is the null signal not the peak signal.
     
  16. Would 2 90 degrees crossed ferrite rods work?
    Sum the signals from the coils?


    |
    |
     
  17. Mark Zenier

    Mark Zenier Guest

    1. Go with a ferrite loopstick antenna. Try pointing it vertically
    so the null is at the zenith, and straight down into the ground.
    A lot of the noise at that band is electrostatic, which your active
    antenna seems to be designed to pick up.

    2. Try for the narrowest bandwidth at the front end. Tune the antenna,
    or have a high Q LC filter, or even build a crystal or ceramic resonator
    filter at 518 (and 490) kHz. It only needs to be 500-1000 Hz wide.
    That applies to your IF bandwidth, too.

    2a. Since your IF is 6 kHz, watch out for images. That could double
    your noise if the front end doesn't reject at the image frequency.

    3. Look at "RTTY Terminal Unit"s, ie. fsk demodulators. The standard
    design was two filters as narrow as possible (determined by the bit
    rate) that were compared. One side could drop out and the other
    half could still compare signal at that shift with the "no signal"
    voltage.

    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
     
  18. bigorangebus

    bigorangebus Guest




    Regarding using a vertical ferrite loopstick antenna (I presume this
    is a ferrite bar would with a coil as usual), if you put it
    vertically, and the transmitter is a vertical mast..does this cause a
    sensitivity issue? Normally ferrite rods are held horizontally. But
    my antenna theory is basic so I'm probably missing something...

    My problem with using the RTTY model is that lo drift will take me
    right off the sweet spot (only have a 170Hz to play with between mark
    and space).

    IThanks Mark, this is very useful
     
  19. jasen

    jasen Guest


    no, by just summing you'd still get two nulls. (at 135 and 215 degrees)

    If you can phase them so that the signal from one is (ideally) 90 degrees
    different from the other signal you'd get 360 degrees range with no nulls
     
  20. OK, should be no problem to make 90 degrees at _one_ frequency,
    either slightly detune it, or just a phase shift network before the add.
    I think the idea from the other poster to put the rod vertical is good too,
    I just tried it with the local radio station on MW 1007kHz, and the directional
    effect is gone (I can zero it horizontally), and the signal does not attenuate
    a lot.
     
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