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Receiver bandpass filter questions.

Discussion in 'Electronic Basics' started by Rheilly Phoull, Aug 16, 2004.

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  1. Dunno about the filter stuff but most of the comparators that "Sink" their
    output will need a pullup resistor if you need a :high".
  2. Ban

    Ban Guest

    Your bandpass filter will latch at negative rail if you keep the resistors
    R8 and R7. C1 is useless and can be omitted. We have posted the same
    bandpass here under ugly transfer function. Look at the design formula and
    determine your Q, it is not 14.
    Maybe you put the part-B stage in front of the bandpass. I doubt that this
    bandpass filter is needed at all. If you have the transmitting signal at
    hand it will be better to make a synchonous rectifier with the delayed
    transmitter signal. What do you want to measure?
  3. I don't know much about filter design, but I've had really good luck
    using LM567 chips in 40kHz ultrasonic applications such as this. Its a
    'tone decoder', basically a PLL with a logic output when the thing
    locks. It can be a bit persnickety to get going, but does a very good
    job once its tuned up. It is VERY sensitive.

    LM567s are out of production, at least at national, but you can
    occasionally get them surplus. Also, I think NTE still sells them.

    However, I'm guessing you could use any PLL, such as a 4046, using the
    phase comparator output as an indication of lock. Here is a lock
    indication circuit I lifted from a Forest Mims book:

    Vcc ------------------------------------+----------------+
    | |
    - LED - LED
    | Unlocked | LCKD
    .-. .-.
    |1| |1|
    4001 4001 |k| 4001 |k|
    __ 100k __ '-' __ '-'
    PC1-----|>=| ___ +-|>=| | +-|>=| |
    |1 |o--+--|___|--+-----+ |1 |o----+----+ |1 |o-----+
    PC2-----|__| | | +-|__| +-|__|
    1N4148 |
    --- 0.05 uF

    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Its meant to indicate lock of a 4046; PC1 goes to pin 1, and PC2 goes
    to pin 2. Thus, it works no matter what phase comparator you are
    using. The PC1 or PC2 input goes high, and the little circuit between
    the first two NOR gates acts like a filter, filtering out the
    transistions caused by the phase comparators. I'm guessing that the
    4046 + digital output is similar to an LM567. The 4046 + 4001 probably
    uses far less power, if thats an issue.

    Bob Monsen
  4. Rubicon

    Rubicon Guest


    I have some questions about my first basic bandpass filter for an
    ultrasonic receiver circuit. It's a Mupltiple feedback second order
    bandpass II with Q=13.97, f0=40kHz, Apb=0.5dB, Bpb=1kHz and a gain of
    32. All resistors are 1% with 2% ceramic caps. As long as R7 is exact
    then the filters gain/frequency graph remains O.K. with the other
    resistors as standard values.

    I have put a schematic on the net at:

    Should I have the bandpass filter on both parts of the dual opamp? It
    seems logical that with the filter on the first half the second half
    wouldn't need one.

    Have I connected it correctly to the second half - C4 and Pin#5?

    In addition a problem I also have is without any load I can adjust the
    trimmer TR1 down to 0.3V and still keep the comparators output high.
    The comparator is attached to the trigger pin of a 555 astable timer
    circuit which switches a load. As soon as I attach a load to the 555
    things change. If for example I attach a 3V motor or even an LED via a
    transistor switch then I have to readjust the trimmer or the load
    won't turn off. The trimmer has to be adjusted to about 3.5V out for
    the motor and 1.4V out for the LED. This reduces the sensitivity of
    the receiver. My next step after the filter is to look into a stable
    voltage reference for the LM393 but will that solve this particular

    Also is it wise to use a pullup resistor on the comparators output?


  5. Andrew, A phase comparator is part of a phase-locked loop (PLL), which
    is what a 4046 is called.

    A PLL consists of a voltage controlled oscillator (VCO), and a phase
    comparator circuit. The phase comparator takes as input the input
    frequency and the output of the VCO, and outputs a voltage. That
    voltage is used to control the VCO. Thus, you have a feedback system
    where the input frequency is tracked by the VCO. The circuit above is
    used to indicate whether the VCO is tracking the input freqency. The
    first LED is on if its not, and the second LED is on if it is...

    Thus, the way you set it up is to configure the 4046 to lock near
    40kHz (using information in the datasheet), and to feed the amplified
    output of your receiver transducer into the PLL input. When the input
    is near 40kHz, it'll lock, and the green LED will turn on.

    You can use this to turn on your switch.

    The advantage of the LM567 mentioned before is that its designed with
    this application in mind, so you don't need the output filter circuit
    given above. Here is a link where you can get 5 of these for 99 cents
    + shipping.

    Bob Monsen
  6. Rubicon

    Rubicon Guest

    Thanyou for your reply.

    I'm not trying to actually measure anything but create a sensitive and
    reliable battery powered ultrasonic switch and I'm having trouble
    doing it.


  7. Rubicon

    Rubicon Guest


    Thankyou for all the info there. Phase comparators are components I
    haven't yet had to deal with but I'll look into them now.


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