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Best way to pick up 8.2 MHz EAS signal

Discussion in 'Electronic Design' started by oopere, Sep 26, 2008.

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  1. oopere

    oopere Guest


    I am working on an EAS (Electronic Article Surveillance) design. The
    objective is to design an active tag that a) beeps (screams) in the
    presence of the 8.2 MHz field generated by the surveillance antennas
    _and_ b) triggers the conventional alarm mechanism of the system.

    To detect the RF field, we have built an LC tank where the inductor is
    made of some turns on a FR4 PCB (half on each layer) spiraled around a
    ferrite bar inserted through the PCB. This also triggers the basic alarm
    of the system.

    The question: Which is the best way to increase sensitivity of this
    setup? [Sensitivity is ok by now, but we may expect that in the future
    someone expects "more"]

    * We can increase the number of turns and decrease line thickness ("y"
    dimension is limited). This will increase inductance by a factor, but
    will increase resistance due to the increased length and the decreased
    cross section. Will there be net gain? Skin depth is roughly 20um at
    this frequency and so does not affect on 35um copper.

    * Widening the loop in the "x" direction keeping the same ferrite? This
    does not make sense, since the effect of the ferrite gets more "diluted".

    Additional constraints: power consumption has to be kept below 10 uA
    (3V) when not beeping.

    Any suggestions/pointers are welcome.

  2. 1. The ultimate limit of the sensitivity is set by the ambient RF noise.
    However it is unlikely that you are approaching this limit with the
    setup that you described; it seems to be self noise limited.

    2. The amount of energy captured by the antenna is coarsely proportional
    to the volume of the ferrite. Without increasing the size of the ferrite
    in either dimension, you can only optimize the parameters of the
    winding, the input amplifier and the matching between the antenna and
    the amplifier for the best noise performance. Depending on the current
    situation, that optimization may or may not provide for the substantial

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
  3. oopere

    oopere Guest

    Thanks! This seems an useful rule of thumb indeed. Is there any
    reference on this so I can try to grasp the reasoning behind this?
    In the current (very crude) configuration the tank is directly connected
    to the BE of an unbiased BFR92. Field strength is now sufficient to turn
    it on. Inductor reactance is aprox. 600R (12uH) and the inductor Q is
    approximately 120. Any guess on the input impedance of the unbiased
    transistor, so that I could try a better match?

  4. Joerg

    Joerg Guest

    Besides antenna/ferrite mods, what is the noise and impedance matching
    situation of your current front end? If it falls into the category of
    "not so great" you might want to consider a switched pre-amplifier.
    Switched in order to make the 10uA because you can't really get a good
    noise figure at quiescent currents this low.

    The feature you need is nowadays called "wake-on-radio" or WOR. It turns
    on, sees if RF is there, and if not goes back to sleep.
  5. oopere

    oopere Guest

    The current configuration could not even be called a "front end" but is
    a direct connection of the tank to the base of an unbiased BFR92! I
    could design a superregenerative receiver as the front-end (however,
    going down to the few uA that are available would be hard) but first I
    would like to exploit the passive possibilities that might be available.

    If, as Vladimir says, there is little optimization possible without
    increasing ferrite volume (which is not possible due to size
    constraints), perhaps there is a matching circuit that might do the job.
    I have no idea what the unbiased BE junction of the BFR92 looks like
    (nothing in datasheet), however I suspect it will not be easy to match
    to the tank. If it's smaller than the tank resistance at resonance, a
    small series capacitor might do the job, however there should also be a
    path for dc... Any comments?

  6. Mark

    Mark Guest

    bias up the base of the BRF92 to about 0.5V DC so it is closer to
    threshold...then it will take less RF to turn it on...

    or add an impedance matching network to step up the RF voltage applied
    to the base...the base is a Hi Z when off...

    or both

  7. Joerg

    Joerg Guest

    Yep. Without any bias it'll be deaf like a door knob. But DC-regulate
    that bias, i.e. via an emitter resistor. Else it'll drift all over the
    place with temperature.

    I'd say "and" add such a network. Sans bias there isn't much hope here.
  8. Joerg

    Joerg Guest

    IME it really never made much of a difference whether a FET or BJT was
    used. Not at those frequencies. For really, really short antenna stubs
    it might though. But it's got to be biased, else you almost need a
    nearby lightning strike to see some signal.
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