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  1. #1
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    On-Off Keying (OOK) receiver/demodulator

    Hello,

    I made a circuit that's output (blue colour (it's asynchronous)) and transmitted this signal by On-Off Keying (OOK) modulation type:


    But I'm trying to find some reveiver or demodulator circuit for more than day, but can't find...

    I have tried to use envelope detector circuit, that looks like this:


    It's ok for ASK, but it won't work for OOK. I have tried to simulate a lot of possibilities, but nothing worked well.

    Maybe some one have tried to do this and can send me some link or approach to solve this problem? Better if there were circuit (schematics) so I could simulate it in PSpice.

    P.S.
    Thanks for any help

    •   Alt1st July 2011, 07:49

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  2. #2
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    Re: On-Off Keying (OOK) receiver/demodulator

    The envelope detector should work. Are you feeding it from the output of a receiver and if so, is is seeing noise between the carrier bursts that makes it think the output is high all the time?

    If yu are using it as the entire receiver, do you have a suitably resonant tuned circuit across its input?

    Brian.



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    Re: On-Off Keying (OOK) receiver/demodulator

    Thanks Brian for your answer.

    Here is my simulation system:


    And here is what I get in bigger picture:




    I Use asynchronous system and there is very imoprtant to know the time when input signal changed his edge. But like you can see in picture after transmison and decoding using enveloped decoder, decoded signal (time events when input signal switched) are incorrect. I what this edge more accurate. Like you can see, for now the error ir about 0.1ms from real signal.

    Any suggestions? circuits? approach?

    Thanks :)



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    Re: On-Off Keying (OOK) receiver/demodulator

    From the graphs I assume the carrier to be 10KHz.

    I'm a little confused over the purpose of this schematic, you say it is a receiver but is V13 or V14 the signal source you show in your first email? It looks almost as though you are trying to reconstruct V13 at the output. Is there a missing cable or radio link which you are not showing?

    Brian.



    •   Alt1st July 2011, 14:07

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    Re: On-Off Keying (OOK) receiver/demodulator

    You're demodulating and filtering using RC components, so it's natural your circuit presents delays t = RC;

    Solution: use smaller RC values. This will inevitably increase the filter's cut off frequency, lowering the ripple attenuation. Therefore you should also increase the filter's order.


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    Re: On-Off Keying (OOK) receiver/demodulator

    Quote Originally Posted by betwixt View Post
    From the graphs I assume the carrier to be 10KHz.

    I'm a little confused over the purpose of this schematic, you say it is a receiver but is V13 or V14 the signal source you show in your first email? It looks almost as though you are trying to reconstruct V13 at the output. Is there a missing cable or radio link which you are not showing?

    Brian.
    V13 - OOK input signal\
    V14 - Sin generator for data transmision

    And after U4A is startig OOK demodulator.

    And Yes, I'm trying to reconstruct V13 signal and its time events where it change its edge from -5 to 5 (input signal is asynchronous).

    Carrier for simulations is 10kHz, but I want to use 433MHz... but there are next problem, If I use so high frequency this circuit doesn't work... why?

    Thanks :)

    ---------- Post added at 16:38 ---------- Previous post was at 16:35 ----------

    Quote Originally Posted by fcfusion View Post
    You're demodulating and filtering using RC components, so it's natural your circuit presents delays t = RC;

    Solution: use smaller RC values. This will inevitably increase the filter's cut off frequency, lowering the ripple attenuation. Therefore you should also increase the filter's order.
    Thanks for your comment, it is very help full, I will try to make new simulations and analys results :)



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    Re: On-Off Keying (OOK) receiver/demodulator

    This doesn't make sense to me. I can understand you using Q4 to clamp the signal from V14 in sympathy with the keying signal from V13 but all you will end up with, even if the time contants are optimal is a copy of the V13 signal but slightly delayed. If it is a receiver as you state, surely you should be recovering the modulation from the transmitter, not applying modulation at the receiver.

    The circuit will only work at low frequencies because of the characteristics of the IC and filter components. At 433MHz it would be better to recover the modulation envelope by monitoring signal level than by rectifying and filtering the modulation output.

    Brian.



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    Re: On-Off Keying (OOK) receiver/demodulator

    Quote Originally Posted by betwixt View Post
    This doesn't make sense to me. I can understand you using Q4 to clamp the signal from V14 in sympathy with the keying signal from V13 but all you will end up with, even if the time contants are optimal is a copy of the V13 signal but slightly delayed. If it is a receiver as you state, surely you should be recovering the modulation from the transmitter, not applying modulation at the receiver.
    Brian.

    Don't you get it? The circuit is the Transmiter + Receiver !!! He connected them both to test the modulation and demodulation at the same time, separating them with a buffer. Surely not the most scientifically correct method, but it's good enough.



  9. #9
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    Re: On-Off Keying (OOK) receiver/demodulator

    That's what I asked in post #4.

    The 'buffer' middle stage will behave quite differently to a 'real life' transmission path and if radio links are used, the residual noise will completely change the output from the detector and filter stages. It's a critical part of the link, that's why I asked for the additional information.

    Brian.



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    Re: On-Off Keying (OOK) receiver/demodulator

    The R and C value in the simple diode detector circuit you first showed have to be carefully adjusted to your data rate in OOK. Also, of course, you will only be getting small voltage swings, and need some sort of comparator circuit to regenerate digital output levels.

    Also, unless you are physically close, there might not be enough of a signal for your receiver to actually rectify and turn into a digital output.

    A lot of actual OOK receivers do not use a detector, but instead use some sort of LOG Amplifier, which can give a big output voltage swing over a very wide range of RF received power levels.



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