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RC Low pass filter Q

Discussion in 'Electronic Basics' started by thomas, Jul 2, 2004.

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  1. thomas

    thomas Guest

    I have a RCR-433-HP radiotronix receiver which outputs digital hash (random
    high-lows) when there is no transmitter in range. This makes using it as a
    digital input for a microcontroller troublesome.

    I put a RC low-pass filter on the output and was able to smooth the hash to
    a nice 1.5-2.5V ripple, yet maintianed a nice 0V and 5V signal when the
    xmitter is in range. Unfortunately this ripple is right at the 'trip' level
    of the mcu so I still get some noise. How can I reduce the voltage level
    (~1V) to avoid the false trips?

    other info....I'm a newbie so I totally guessed on the RC filter values
    using a scope and a pile of Rs and Cs, ended up with 330 ohms and 94uf. My
    goal is to transmit ON and OFF (I guess this is considered DC), not serial
    data.
    thx
     
  2. Most microprocessor inputs are very high impedance, so a pretty
    significant series resistance will not drop much voltage because of
    the input. Try something like a 10k resistor between signal and micro
    input, with a .47 uf cap to ground. This should give approximately
    the same result as the one you have now. Then you can tie an
    additional 20k resistor between the input and ground, so that the
    average voltage has to go higher to produce the same input voltage.

    If you have a spare output you can tie the 20k resistor to that, and
    every time you read the input, copy the state to that output. This
    will require the signal to have a larger change to swing from a high
    to low or vice versa.
     
  3. thomas

    thomas Guest


    That worked very nicely.
    I did have to keep the 94uf to maintain a nice smooth signal. The hash is a
    square wave with unequal 5V and 0V duties, perhaps that is why.?

    10K and 22K brought me down to around 1V and no more false trips! (I had
    tried something like this but working against my original 330 ohm I wasn't
    able to get it to work). Would there be any hazard in using a smaller "22K"
    to bring the voltage down a little more, as a safety factor? OR is it
    better to go with your suggetion of using an extra output as 'feedback'.

    Thx a bunch
     
  4. You can lower the 22k as much as necessary to prevent false high
    states, until you reach the point that a high is not possible. Keep
    in mind that the logic input does not have a precisely repeatable
    threshold voltage, so if you expect this trick to work with many uP
    units, you need a comparator between the filter and the uP or you need
    ot use the a comparator input function on the pic to increase the
    decision precision. Using the output to reinforce the last measured
    state just increases the ripple that can be tolerated without giving
    false state changes. This can also be done with a comparator. It is
    called hysterisis. You might look up the key word, "schmitt trigger".
     
  5. Dbowey

    Dbowey Guest

    Thomas posted:

    << I have a RCR-433-HP radiotronix receiver which outputs digital hash (random
    high-lows) when there is no transmitter in range. This makes using it as a
    digital input for a microcontroller troublesome.

    I put a RC low-pass filter on the output and was able to smooth the hash to a
    nice 1.5-2.5V ripple, yet maintianed a nice 0V and 5V signal when the xmitter
    is in range. Unfortunately this ripple is right at the 'trip' level of the mcu
    so I still get some noise. How can I reduce the voltage level
    (~1V) to avoid the false trips?

    other info....I'm a newbie so I totally guessed on the RC filter values
    using a scope and a pile of Rs and Cs, ended up with 330 ohms and 94uf. My
    goal is to transmit ON and OFF (I guess this is considered DC), not serial
    data.
    The problem I see is that it uses a super-regenerative receiver, and they
    generate lots of broad-spectrum noise in the absence of a signal. The
    schematic of the receiver is rather poor - you cannot be certain about the
    stage coupling. You don't say what you are using for an antenna. Reducing the
    antenna coupling might reduce the regeneration, giving you some improvement in
    idle noise, but at the expense of range.

    Bottom line is you probably need a better receiver.

    Don
     
  6. thomas

    thomas Guest


    I realize it's a bottom-of-the-barrel receiver but this is more for learning
    than anything else. I'm using a 1/4w LINX rubber duckie. Radiotronix has an
    app note that suggest using a SAW filter on the TX and RX to narrow the
    bandwidth. I think I'll try it. Although after reading your post I don't
    think that is exactly the broad spectrum you are referring to.
    t
     
  7. Dbowey

    Dbowey Guest

    Thomas posted:
    <<

    I realize it's a bottom-of-the-barrel receiver but this is more for learning
    than anything else. I'm using a 1/4w LINX rubber duckie. Radiotronix has an app
    note that suggest using a SAW filter on the TX and RX to narrow the bandwidth.
    I think I'll try it. Although after reading your post I don't think that is
    exactly the broad spectrum you are referring to.

    What happens to the noise if you remove the antenna? Super-regen receivers are
    quite sensitive, but very broad without some filtering such as the saw filter,
    or just an L/C filter. If you remove the antenna and observe the result, you
    will know if a saw filter will help.

    Don
     
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