# IQ Demodulation Help - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

antennaant
Post subject: IQ demodulation help Posted: Sat Sep 12, 2009 4:16 am

Lieutenant

Joined: Mon Dec 11, 2006 3:17 pm
Posts: 2
This suddenly came up in a MRI class. I kinda understand IQ modulation (both amplitude and phase are modulated). But how does IQ demodulating a signal help, specially MRI signal.
In case you don't know a MRI signal, its just a exponentially decaying sine.
Also, what is instantaneous phase ?

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Jose Arrina
Post subject: Re: IQ demodulation helpPosted: Wed Sep 16, 2009 12:25 pm

Lieutenant

Joined: Thu Mar 30, 2006 9:01 am
Posts: 4
Hi antennaant:

I'm fairly new at quadrature systems, but basically breaking down a signal into I and Q components allows you to discern angle of arrival, or phase. If you have two detectors orthogonal to each other (90 deg), then mathematically they are independent. So, assuming both antennas are subject to the same signal, the amplitudes of each relative to the other indicates phase. It's a sin vs. cos calculation. This scheme is used in threat detector systems for determining angle of arrival.

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