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Circuit Ideas?

Discussion in 'Hobby Electronics' started by SFD, Aug 30, 2011.

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  1. SFD

    SFD Guest

    Hello All, I'm looking for circuit ideas/suggestions for the following:

    A receiver to receive a very small 8.5kHz signal using a 470uH choke as the
    pick-up sensor, or any other off the shelf similar small component.
    The transmitted source is a loop 'antenna' of wire ranging between 100m to
    300m total length.
    The avaiable signal level is very low, unfortunately I do not have the exact
    level details available.
    The receiver should be able to detect the signal from a distance of about
    300mm from its source, in other words bringing the receiver within 300mm
    from anywhere of the loop antenna.

    The signal applied to the loop antenna is coded - that is not a problem as I
    have the info to decode.

    So what I am looking for is a front-end preamp that will provide me an
    approximate 2v p-p signal that I can apply to a microP to be
    decoded/processed.

    Ideas comments views criticisms of description etc., ...and so on greatly
    appreciated.
     
  2. Jasen Betts

    Jasen Betts Guest

    Small antennas never work well.

    ideally you want several hundered turns of fine enameled wire on a
    form with the largest cross-section practical, (fit the largest dimensions
    of the case you plan to use.)
    build an 8.5Khz linear oscilator (eg: twin tee) and turn the loop gain down
    until it stops, then couple the antenna to the input

    otoh you could probably connect the antenna to a DAC and do the
    detection in software.
    what sort of coding, and what symbol rate it makes a difference
    to the max Q you can use if you use a tuned circuit.
     
  3. This smells something like the wireless heart-rate and/or bicycle
    sensors and associated receiver watch.
    Make sure the "antenna" inductor is structured like a tuned circuit
    around the transmitted carrier to at least attempt to attenuate other
    unwanted signals.

    From there, treat it like audio, though exactly how it would depend on
    how the signal is carried before you can know how to condition it from
    there.

    I've only observed these signals in the past, I hadn't had to do
    anything with them, so I can't offer too much past there.
     
  4. SFD

    SFD Guest

    It's a receiver that will be used with existing equipment.
    The equipment being for domestic animal containment.
    A length of single core wire is installed around the property perimeter.
    The animal (cat/dog) wears a collar that is activated when brought within
    about 300mm of any point of the wire.
    The wire 'loop antenna' therefore has no fixed regular shape.
    Unfortunately I have no control over the design of the Tx as that already
    exists.
    All I have access to are the pulses (protocol if you like, transmitted by
    the Tx).
    The Tx transmits burst of pulses 187 every 15mS.
    The Tx output is variable. The output comprises a TDA2003 audio amp.
    The signal measured at the output of the Tx is 3v P-P into a 5ohm load,
    about 210mA.
    I have yet to measure the resistance of a typical 300m 'antenna'.
    Tried the audio approach with limited success. Tried opamps MCP6002.
    The other point is power consumption - battery life is important.
    On the micro side its OK, I obtain 80uA standby by placing the micro into
    sleep mode.
    Le Big Mac!

    Thanks for your input.
     
  5. SFD

    SFD Guest

    Agreed, unfortunately these are the parameters in which I have to work, I
    have no choice.
    The phsical size of the choke is important to the construction of the
    design.
    I like this osc idea of yours, I think I will give that a try....
    Will give this one some thought - not enough signal directly from the
    antenna, thats why I need to front end to amplify the received signal and
    use it to provide a clean signal to the micro.
    It's a receiver that will be used with existing equipment.
    The equipment being for domestic animal containment.
    A length of single core wire is installed around the property perimeter.
    The animal (cat/dog) wears a collar that is activated when brought within
    about 300mm of any point of the wire.
    The wire 'loop antenna' therefore has no fixed regular shape.
    Unfortunately I have no control over the design of the Tx as that already
    exists.
    All I have access to are the pulses (protocol if you like, transmitted by
    the Tx).
    The Tx transmits burst of pulses 187 every 15mS.
    The Tx output is variable. The output comprises a TDA2003 audio amp.
    The signal measured at the output of the Tx is 3v P-P into a 5ohm load,
    about 210mA.
    I have yet to measure the resistance of a typical 300m 'antenna'.


    Thank you for you thought and help.
     
  6. SFD

    SFD Guest

    It's a receiver that will be used with existing equipment.
    The equipment being for domestic animal containment.
    A length of single core wire is installed around the property perimeter.
    The animal (cat/dog) wears a collar that is activated when brought within
    about 300mm of any point of the wire.
    The wire 'loop antenna' therefore has no fixed regular shape.
    Unfortunately I have no control over the design of the Tx as that already
    exists.
    All I have access to are the pulses (protocol if you like, transmitted by
    the Tx).
    The Tx transmits burst of pulses 187 every 15mS.
    The Tx output is variable. The output comprises a TDA2003 audio amp.
    The signal measured at the output of the Tx is 3v P-P into a 5ohm load,
    about 210mA.
    I have yet to measure the resistance of a typical 300m 'antenna'.
    Thanks for you input - looking forward to your comments.
     
  7. SFD

    SFD Guest

  8. Phil Allison

    Phil Allison Guest

    "SFD"

    ** The design of the pick up coil is gonna be crucial - you need high
    inductance and high Q.

    I doubt an inductor sold for use in SMPS is any good at all.

    Best wind your own on a ferrite rod with *lots* of very fine wire - then
    find a cap value that tunes it to 8.5kHz .

    Op amps with large GBW products and low input noise are never low current
    too, so you need to maximise the performance of that coil.



    ...... Phil
     
  9. SFD

    SFD Guest

    Thanks Phil, youre probably well aware of "The Trials and Tribulations of an
    Electronics Bod!"
    Your suggestion for pickup would be the ideal scenario, unfortunately the
    customer (bless their bank account!) almosts insists on using a small
    antenna.
    They have two existing similar devices working using a choke as the pickup.
    The choke they use is virtuall identical in shape and size as the one
    supplied by Electus/Jaycar type LF-1108 470uH - having said that, I have
    yet to remove a choke and measure its inductance.
    Trying to reverse engineer whats already been done is a no go due to epoxy
    encapsulation.
    Thanks for your input.
     
  10. Phil Allison

    Phil Allison Guest

    "SFD"

    ** A coil something like that MIGHT do, but will need a large number of
    turns on it and high inductance.

    Hundreds of mHs, if possible.

    This one from Element14 is getting closer.

    http://au.element14.com/toko/8rb273k/inductor-27mh/dp/1193622?Ntt=1193622

    13nF cap to resonate, Q = 18, suit a high impedance input.


    ..... Phil
     
  11. SFD

    SFD Guest

    Thanks for that info Phil, I have ordered a few pieces. Other similar chokes
    I'd seen in the mH range appeared physically too big - this is a good find!
    The spec indicates its inductance is measured at 79.6kHz, will be
    interesting to see its final in-circuit performance at 8.5kHz.
    Will keep you posted once they arrive and put a test cct put together.
    Cheers.
     
  12. Jasen Betts

    Jasen Betts Guest

    If you need to save power Phil's tuned circuit idea is better
    especially if the frequency is stable.

    do a tuned curcuit and use the B-E junstion if a transistor as the
    detector C-E pulling a microcontroller pin down and a 100K pulling it
    up
    that's a 12.5kHz repetition rate

    Is that 187 bursts every 15 ms ?
    187 pulses every 15ms
    bursts of 187 pulses every 15ms?

    where are you getting 8.5kHz from?

    and what's the coding you wrote about?

    basically I want to know the bandwidth of the signal
     
  13. Phil Allison

    Phil Allison Guest

    "Jasen Betts"
    ** Knock me over with a feather ...

    Jase has actually asked an insightful question.

    Will wonders never cease...............




    ..... Phil
     
  14. SFD

    SFD Guest

    .... quite correct. I should have clarified. The existing Tx is 12.5 kHz.
    The Tx will be modified to 8.5kHz (187 x 20mS). These devices will be
    distributed in the US and apparantly have to be under 10kHz - so I am told.

    The problem has been solved. Using a higher value sensing device (>10mH
    choke) into a FET and opamp to stabalize and clean the signal.
    Thanks for you input, appreciated.
     
  15. SFD

    SFD Guest

    Problem solved. Measured the choke used in existing device and found it to
    be 10mH.
    Made up a test cct, choke into FET, into dual copamp to stabalize and clean
    up the signal - perfect.
    Sensitivity will no doubt (hopefully) be better with the 27mH device.
    I was being over optimistic (stupid comes to mind) trying to get a 470uH
    choke to work, a few simple calculations would have told me otherwise. Oh
    well, when will we ever learn that shortcuts and assumptions often take
    longer!
     
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