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Design >> RF Design >> Some questions about I/Q (de)modulation
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Message started by tromeros on Feb 6^th, 2006, 12:13am

Title: Some questions about I/Q (de)modulation
Post by tromeros on Feb 6^th, 2006, 12:13am

Hi, I ve studied the paper "Modeling RF systems" and I would like to ask some basic questions about I/Q modulation and demodulation. I 've found
some references across the internet but I would like to clarify some issues.

First of all we use I/Q demodulation in modulations such as QAM where there are both in-phase and
quadrature components. This is the obvious case.

Can we use I/Q demodulation when we receive a signal that is not I/Q modulated in order to have two
orthogonal paths of it and use Analog to Digital Converters of the same rate as the bandwidth of the
signal?(and not the double rate as suggested by Nycquist)

And finally I would like to ask about the frequency convesions. Supposing a signal of 500 MHz for
example that we I/Q modulate it with a carrier of 4.5 GHz for example, in what range the upconverted signal
will be?

Any further information and suggestion will be welcomed! Thank you for your help!

Title: Re: Some questions about I/Q (de)modulation
Post by Jess Chen on Feb 6^th, 2006, 11:17am

Quote:

Can we use I/Q demodulation when we receive a signal that is not I/Q modulated in order to have two
orthogonal paths of it and use Analog to Digital Converters of the same rate as the bandwidth of the
signal?(and not the double rate as suggested by Nycquist)

If I understand your question, I think the answer is yes and no. Yes, You can IQ-demodulate an arbitrary signal. No, you can not violate the Nyquist criterion. However, if you have an IF carrier that you would like to remove, you could sample at the IF frequency or perhaps some lower frequencies but you still can not violate the Nyquist frequency of the baseband data. I believe the technique is called subsampling.

Quote:

And finally I would like to ask about the frequency convesions. Supposing a signal of 500 MHz for
example that we I/Q modulate it with a carrier of 4.5 GHz for example, in what range the upconverted signal
will be?

If your baseband signal spans +-500KHz, your modulated 4.5GHz carrier will span 4.5GHz +-500KHz, assuming your IQ LO signals are perfect sinusoids and there are no other nonlinearities besides the up conversion.

-Jess Chen