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Antenna over ground plane -

Discussion in 'Electronic Basics' started by Dummy, Apr 14, 2004.

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  1. Dummy

    Dummy Guest

    In a two way radio (frequency range from 403-470MHz), when a 100pF
    capacitor was being placed on the microcontroller reset line, the
    radio was having problem to transmit at 436.025MHz. There was actually
    no signal being transmitted out when the antenna was radiating over a
    ground plane. No power could be seen from spectrum analyzer. If the
    cap was replaced with other values, no problems were seen at that
    frequency. Why? It only happened when transmitting over the ground
    plane. Did the frequency transmitted from antenna cause somekind of
    resonance with 100pF at reset line? But RF resonance would not affect
    the DC level of the reset line right? Reset line is active low. High
    voltage is 3.3V.

    If 436.025MHz is the resonance frequency, then from the resonance
    formula, we could derive the parasitic inductance of the cap. And
    assuming the resonance was the culprit, we would see the same problem
    if other cap values were used, depending on the self resonance
    frequency. Let's say another cap cause a self-resonance frequency at
    416.025MHz. But there's no problem seen at this transmission freq.
     
  2. If the cap is resonant at that frequency in combination with a circuit
    parasitic inductance, that will result in an r.f. a.c. voltage on the uP
    reset line. If the negative peaks of this voltage, superimposed on the
    d.c. level, result in a low enough level, this might cause a reset.

    That shouldn't be difficult to diagnose. If the system goes through its
    power-on boot up sequence, suspect the reset line. Throw a scope on that
    line and see if a signal is present. You might have to be pretty quick
    with the trigger, since it seems that the side effect of the reset is
    the transmitter being shut down.
    The resonant frequency is dependent on circuit parasitics in addition to
    your cap and might not move to exactly the point you calculated. You
    might have to inject r.f. into the circuit and sweep the frequency
    around to hit the new resonant point.
     
  3. Tam/WB2TT

    Tam/WB2TT Guest

    Another thing he could do is to bring the power up slowly, while looking at
    the reset line with a 'scope. Of course, there is the problem of the probe
    adding to the capacitor value.

    Tam
     
  4. budgie

    budgie Guest

    Another approach might be to scope the reset pin with an attenuator in the Tx
    line to the antenna. Operating it below the threshold of interaction - if there
    is a threshold - may well expose the mechanism.
     
  5. Robert Baer

    Robert Baer Guest

    Something else to consider: Excessive RF level on any IC pin can cause
    damage and/or destruction.
    The equivalent of a parallel resonant circuit being connected to an IC
    pin can do this nasty (and maybe perplexing) damage.
     
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