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Quadrature Demodulation & Under Sampling

Discussion in 'Radio and Communications' started by naumank, May 17, 2016.

  1. naumank

    naumank New Member

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    Hi all

    According to my understanding, the basic purpose of quadrature Demodulation is to shift the band pass signal modulated at high frequency carrier to lower frequency region (near zero frequency) in order to reduce burden at ADCs as useful information is only present in pass band. Different types of ICs are used for this purpose e.g. AD8333 from "Analog Devices". FPGA based DDC cores are also there for this purpose.

    Another technique used for demodulation is by "under sampling" i.e. Sampling at ADC is done well below the Nyquist criterai and use aliasing in own advantage by bringing higher frequencies in lower frequency region. My question is that is it not more simple to use this technique instead of using extra electronics (like AD8333) or FPGA resources?

    Can anyone kindly explain what are advantages/disadvantages of both methods i.e. Demodulation using ICs/FPGA and by using "under sampling" principal?
     
  2. nsaspook

    nsaspook Well-Known Member

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  3. Mikebits

    Mikebits Well-Known Member

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    At first glance, one would think that undersampling is easy-peasy, after-all we are eliminating an RF chain, and we all know how RF is somewhat a art gained through experience. Well after reviewing Nyquist-Shannon theory, we learn that undersampling or harmonic sampling is not without trade-offs, and a lack of theory will leave you with a severely distorted output on your A to D. One of the reasons I have not responded to your latest question is that alas, I have less than the required experience in this area to give proper guidance.
    You are not left without resources however, as a quick Google search reveals a good deal of information on this topic. One paper in particular that I found quite good at explaining undersampling can be download from the following link: "What Nyquist Didn’t Say"
    I would suggest you give the above mentioned paper a read.
     
    • Informative Informative x 1
  4. dave

    Dave New Member

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  5. naumank

    naumank New Member

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    Thanks for paper
     
  6. naumank

    naumank New Member

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  7. rameshrai

    rameshrai Member

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    your understanding is lacking the basic so you didn't find your answer(i guess)

    the purpose of quadrature demodulation is recovering quadrature modulated signal and not shift the frequency for the betterment of ADC work-there is a difference in saying that. All demodulator include frequency shifter and demodulator includes not only frequency shifter but also ADC, demultiplexers, filters.

    quadrature modulation is done to increase bandwidth usage squeezing more information into the available channel bandwidth. thus demodulation is shifting down the frequency and demultiplexing the signals that were multiplexed at the transmitter. Thus quadrature demodulation is demultiplexing, followed by frequency shifting, then sampling and finally ADC.

    demux. -> freq.shifter -> filters -> sampler -> ADC
    (i think this is correct!)

    Now, to your question, as you can see there are different components of demodulator. Whether to use IC or FPGA depends on you. They should work equally well. IC are designed by experienced designers and are tested. The IC will have all the component. For FPGA you need to implement the components which can be done.

    In the above sketch, you meant to say to change the sampling interval of the sampler- undersampling, oversampling. The receiver and transmitter agree upon data rate in advance so the sampling rate is fixed. you may fix the sampling rate to under-sampling but notice that you cannot change the components required- demux, frequency shifter, sampler or ADC. hence you shouldn't be able to reduce electronics by using undersampler.

    Undersampling and oversampling have to do with correlation between sampled signals but nothing to do with electronics components required for quadrature mod/demod.
     
  8. naumank

    naumank New Member

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    Dear rameshrai

    Thanks for reply. I think it is not always the "quadrature modulated" signal that we need to "quadrature demodulate". One such example is of a Linear Frequency Modulated (LFM) pulse of some bandwidth (e.g. B KHz) which is frequently used in multi beam echo sounders, bottom imaging or side scan sonar. The center/carrier frequency of such pulses is quite high in range of 100 to 300 KHz or even more for getting high resolution image. In that case, the purpose of quadrature demodulation at receiver end is to remove high frequency carrier and recover the base band signal of B bandwidth as well as extract phase information from I/Q signals for direction finding.

    The other name of this quadrature demodulation is DDC (Digital Down Converter) which can be implemented in analog (electronic) domain or in software domain after digitization (e.g. in FPGAs). Different FPGA Cores for DDC are available in market.

    With this perspective, my question was that theoretically, same results can be obtained by using "under sampling or band pass sampling" as can be obtained by DDC so why not always use cheaper solution i.e. under sampling??? There must be some pros and cons for both methods which i wanted to know.
    Regards
     

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