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    About RF receiver I/Q modulation/demodulation

    Hi all,

    I know that I/Q modulation is for cancelling mirror images, but is I/Q modulation needed under all circumstances? For example, for a direct-conversion RF receiver using FMCW, is I/Q modulation necessary? Why? Also, to get deeper knowledge on this topic, do you have a reference to recommend?

    Thank you very much!

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    •   Alt19th May 2017, 20:45

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    Re: About RF receiver I/Q modulation/demodulation

    Quadrature detection is even more necessary for direct conversion than IF conversion, unless the receiver and transmitter are synchronous. Definitely necessary for FM schemes, otherwise you can't distinguish positive offset frequencies from negative.



    •   Alt19th May 2017, 21:46

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    Re: About RF receiver I/Q modulation/demodulation

    Quote Originally Posted by mtwieg View Post
    Quadrature detection is even more necessary for direct conversion than IF conversion, unless the receiver and transmitter are synchronous. Definitely necessary for FM schemes, otherwise you can't distinguish positive offset frequencies from negative.
    --Thank you very much for the reply, mtwieg. I'm not quite familiar with communication theory. Could you explain what do you mean by "unless the receiver and transmitter are synchronous"? Do you mean the receiver is synchronized with the transmitter using a pilot clock in the received signal? How does this help the image problem?



    •   Alt19th May 2017, 23:21

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    Re: About RF receiver I/Q modulation/demodulation

    Basically it means the LO in the receiver's mixer is somehow frequency and phase locked with the carrier from the transmitter. This is often accomplished by a pilot tone in FM.

    Consider an QAM scheme, where the transmitter is sending a message which can be decomposed into I and Q carriers. A direct conversion receiver with IQ demodulation will detect both I and Q components, though with some phase offset which can be compensated so long as the phase offset does not drift too quickly.

    Now consider if the receiver only has one mixer. It can only detect the projection of the transmitted IQ vector along its own LO phase. If the transmitter only uses AM instead of QAM, then this can still be fine, so long as that LO phase always tracks the transmitter phase. If that phase drifts, then you'll get fading.


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    Re: About RF receiver I/Q modulation/demodulation

    Thanks again for your detailed explanation! I can understand your QAM example. But for the case like FMCW used for vehicle distance and speed detection, I have read in papers that they usually generate the RF transmitted signal using a Sigma-Delta (which generates the ramp frequency sweeping signal) frequency synthesizer, which does not have I/Q carriers. In this case, does the receiver still need I/Q demodulation?



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