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Quadrature Demodulation at 90 kHz

  1. Feb 19, 2016 #1
    Hello all

    I have a linear frequency modulated (LFM) signal having 90 KHz center frequency and 20 KHz bandwidth around it. My problem is to demodulate it at base band to get I and Q both (phase information required). I have searched a lot at internet but mostly found quadrature demodulator ICs have starting input frequency range in MHz. AD8333 was what i found having lowest input frequency but even it does not work for 90 KHz (as i discussed with analog devices technical staff).

    Can any one kindly guide me how to solve my problem?.

    Thanks in advance

    Best Regards
    Nauman
     
  2. jcsd
  3. Feb 20, 2016 #2
    Use an analog multipier IC as a demodulator.
     
  4. Feb 20, 2016 #3

    tech99

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    My problem is to demodulate it at base band to get I and Q both (phase information required).

    Not sure what you mean by "demodulate it at baseband" when you say it is on a 90kHz carrier?
     
  5. Feb 20, 2016 #4
    Thanks for reply. I require some pure sinusoidal oscillator for generation of sine and cosine signals at 100 KHz and also two identical low pass filters with linear phase.

    Can you kindly suggest some ICs for them?

    Thanks & Regards
    Nauman
     
  6. Feb 20, 2016 #5
    Thanks for reply. I have a LFM signal at 90 KHz carrier with 20KHz bandwidth. After quadrature demodulation, both I and Q should have frequency band from 0 to 10 KHz
     
  7. Feb 20, 2016 #6

    tech99

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    You cannot have quadrature modulation with frequency modulation, only with phase modulation.
    Have you worked out how to obtain or extract the 90 kHz reference carriers?
     
  8. Feb 20, 2016 #7
    Sorry if i am wrong, i think you mean "FM" as used in FM Radio but it is LFM ( linear frequency modulation) where frequency linearly varies with a time. Can you kindly explain further your first point?

    Yes, 90 KHz quadrature sinusoidal oscillator (generating sine and cosine signals) is what i am looking for but could not found some ready made IC for it. Do you have some in mind?

    Thanks.
     
  9. Feb 20, 2016 #8
    Is there a reason you need an IC solution? Discrete components should work at 90kHz. You might even get by with op-amp mixers, etc.

    My guess is the reactive elements in the tank circuit are too big to work well with low frequency ICs.
     
  10. Feb 20, 2016 #9
    In my opinion, a good quadrature sinusoidal Oscillator IC (if it exist for 90 KHz) will cater for noise and phase errors for two sinusoidal. With discrete components, i will have to cater these issues.

    Thanks
     
  11. Feb 20, 2016 #10

    jim hardy

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    even though its datasheet says DC to 50 mhz? hmmm.

    How about good old AD630 ?
     
  12. Feb 20, 2016 #11
    For quadrature RC, Sine wave oscillator ckts see:

    http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf

    If you need higher stability, a crystal oscillator can be used and the quadrature phase obtained with a phase-shift network, as the quadrature RF demodulator chips use.

    If you use a 4-quadrant analog multiplier for demodulation you may get better results overdriving the Local Oscillator inputs to saturation with a squarewave. One minor advantage to using the multiplier is you get both the I and Q outputs from the same package.
     
  13. Feb 20, 2016 #12
    Thanks. According to its data sheet minimum sine wave frequency at 4LOP pin is 2 MHz, however square wave frequency can be lower than 2MHz at this Pin. In your opinion, how much degradation (e.g. in terms of harmonics) in I/Q output will occur if i apply square wave instead of sinusoidal wave?

    yes, I will study AD630 in detail and will bother you again.
     
  14. Feb 21, 2016 #13

    jim hardy

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    i'm in over my head already !
     
  15. Feb 21, 2016 #14

    sophiecentaur

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    I'm not sure what you mean by "cannot" when the only difference between FM and PM is that the PM modulat[Edit ing] signal is the Integral of the FM modulat[ing] signal. I agree that demodulation of a high deviation FM signal may not be convenient with a Phase Demodulator if the demodulator is only looking for phase changes within four quadrants but there is not a fundamental reason. Let's face it, the pre- and de- emphasis that's applied to FM sound signals is actually turning them into wide band PM signals.
    Also, I guess that it's easier to use a quadrature modulator than a frequency modulator to obtain 'QM' signals because the reference phase is less likely to drift around. Nonetheless, in principle . . . . .
     
  16. Feb 21, 2016 #15

    tech99

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    Agree that in principle, yes. But you would have to start with the two data streams being applied to I and Q modulators, then regard the combined signal as FM. At the receiver, a pure FM demodulator, where output is proportional to instantaneous frequency, could not separate the two signals - it would require I and Q demodulation.
    I am not sure how Nauman is going to phase lock the demodulating carriers to the incoming signal, which is essential. For example, with communication systems using 4-PSK modulation, the coding of the data stream is arranged to facilitate carrier recovery.
     
  17. Feb 21, 2016 #16

    sophiecentaur

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    In the context of the most present forms of implementation, you are totally right but the only effect of phase error in the demodulator is to cause crosstalk between the channels, which could be eliminated with the right processing. I'm obviously being tongue in cheek here but, sometimes, there are novel methods of achieving results which seem loopy but which sometimes get taken up in mainstream systems. It's always worth while keeping an open mind.
     
  18. Feb 21, 2016 #17

    tech99

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    I do agree about the effect of phase error, but if the insertion frequency is not locked and is wrong then the subsequent processor stands little chance.
     
  19. Feb 21, 2016 #18

    sophiecentaur

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    Yes. Of course, any conventional receiver needs to phase lock 'explicitly' and the lock has to be to the correct quadrant.
     
  20. Feb 21, 2016 #19

    jim hardy

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  21. Feb 21, 2016 #20

    meBigGuy

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    Last edited: Feb 21, 2016
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