Understanding IQ Demodulation in MRI Signals

[likephysics]

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.

 

[waht]

In demodulating IQ, you have to separate the two channels that are out of phase and usually feed them to a microprocessor.

 

[f95toli]

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.

IQ demodulation is very useful if you need both the magnitude and phase of the signal (I suspect this is the reason is is used in this case). There are other methods for achiving this but the nice thing with an IQ demodulator is that you can easily (and quickly) record all the information by just measuring two voltages (as opposed to measuring one voltage and one phase).
Once you have both the I and Q channel data you can either calculate magnitude and phase or if you prefer stay in the "IQ domain" and do your data analysis there; in some cases the latter is actually more convenient.

Note also that there is nothing stopping you from just recording one channel if you are only interested in the magnitude; you can always use a phase shifter to "zero" one channel and then just record the other.

 

[likephysics]

ok thanks. So to get IQ from any signal you just feed the signal into 2 buffers and multiply one by cos and the other by sin. The cos should give the magnitude and the sine part should give the phase?
What happens if I try to IQ demodulate an FM signal or the signal coming out of an op amp oscillator?

 

[f95toli]

No, the magnitude is just given by the norm (sqrt(I^2+Q^2)) and the phase by the phase angle (arctan I/Q) .
I and Q are just representations of the signal in the complex plane ("x+iy") so the usual rules apply.

I don't know what happens if you IQ demodualte a FM signal (I've never tried); I suspect it might get messy and I can't think of a reason why one would do that.
You can obviously use IQ demodulation it for an AM modulated signal (this is typically how it is used when looking for small signals because you can then feed the output of the demodulator directly to a lock-in amplifier).

 

[waht]

ok thanks. So to get IQ from any signal you just feed the signal into 2 buffers and multiply one by cos and the other by sin. The cos should give the magnitude and the sine part should give the phase?
What happens if I try to IQ demodulate an FM signal or the signal coming out of an op amp oscillator?

It's not multiplied by cosines or sine. The two channels carry data in variations of the amplitude. They are fed to two independent ADC (analog to digital converter) where they are turned into 0s and 1s. The microprocessor does some signal processing magic and then it spits out the data.

I don't think it's possible to incorporated FM modulation with IQ. it will change the constant 90 degree phase shift.

here is a constellation diagram representing all possible states of QAM 16.

http://72.232.229.42/thumb/1/1e/16QAM_Gray_Coded.svg/200px-16QAM_Gray_Coded.svg.png

 

[f95toli]

what: I think we are talking about different things here. What you are referring to is the "typical" use of IQ demodulation in telecom for e.g. phase-shift keying.
But I am pretty sure that is not what the OP is asking about since the application was in MRI. In this context IQ demodulators are often used much like you would use a mixer in ordinary homodyning; but -as I wrote above- the added advantage that you get both magnitude and phase of the signal directly.

I use IQ demodulation for nearly the same reason. I e.g. use it to measure the decay shape/time of high-Q resonators; I send a pulse in at the resonance frequency (several GHz) and can then watch the decay of both the I and Q channel on an ordinary oscilloscope (the IF BW of my system is around 10 MHz) meaning I can see how both the magntude and phase changes. I would assume this is why IQ demodulation is also used in MRI, where similar techniques are used.

 

[waht]

what: I think we are talking about different things here. What you are referring to is the "typical" use of IQ demodulation in telecom for e.g. phase-shift keying.
But I am pretty sure that is not what the OP is asking about since the application was in MRI. In this context IQ demodulators are often used much like you would use a mixer in ordinary homodyning; but -as I wrote above- the added advantage that you get both magnitude and phase of the signal directly.

You are right, I was confusing the issue with the actual demodulation of a telecom signal.

I was able to pull up an abstract for a patent of an MRI transceiver. It appears that the IQ scheme is used as an image reject mixer.

http://www.patentstorm.us/patents/6259253/description.html

Image rejection by filtering before mixing has other problems as well. One is that tuning the receiver becomes difficult because the image filter must also be tuned. Thus, an image rejection architecture may be helpful. Image-rejection downconversion schemes (also known as "single sideband" or "SSB" demodulators) are helpful in eliminating image energy over a wide tuning range without the need for tunable image filters. FIG. 1 shows a known image rejection scheme in which in-phase (I) and quadrature (Q) local oscillator (LO) signals are applied to the input signal, resulting in audio-frequency I and Q channels that contain both the desired sideband and the image. The image sideband can be canceled by shifting the phase of the I channel by ±90° (depending on which sideband is desired), and then summing the result to obtain a real signal. The 90° phase shift has the effect of putting the desired sideband in the I and Q channels in phase, and the undesired sideband in the I and Q channels 180° out of phase. Thus, the sum signal contains only the desired sideband.

I use IQ demodulation for nearly the same reason. I e.g. use it to measure the decay shape/time of high-Q resonators; I send a pulse in at the resonance frequency (several GHz) and can then watch the decay of both the I and Q channel on an ordinary oscilloscope (the IF BW of my system is around 10 MHz) meaning I can see how both the magntude and phase changes. I would assume this is why IQ demodulation is also used in MRI, where similar techniques are used.

This is really cool. I'm a rf/microwave junky.

 

[likephysics]

I use IQ demodulation for nearly the same reason. I e.g. use it to measure the decay shape/time of high-Q resonators; I send a pulse in at the resonance frequency (several GHz) and can then watch the decay of both the I and Q channel on an ordinary oscilloscope (the IF BW of my system is around 10 MHz) meaning I can see how both the magntude and phase changes. I would assume this is why IQ demodulation is also used in MRI, where similar techniques are used.

When I first saw the MRI decay, it reminded me of ringing and the way it decays(some times).
Anyway, by measuring the decay of the pulse I can understand you see the amplitude decay. But phase also decays/changes?
For example in a exponentially decaying sine, the amplitude decay is obvious. But where is the phase decay? The frequency of the sine does not change.

what,

Another term I don't understand is image reject mixer. Are they really rejecting an image using a mixer?

 

[f95toli]

Anyway, by measuring the decay of the pulse I can understand you see the amplitude decay. But phase also decays/changes?
For example in a exponentially decaying sine, the amplitude decay is obvious. But where is the phase decay? The frequency of the sine does not change.

No, but real systems are never 100% ideal meaning while the decay of most real resonant systems (incuding MRI) looks very much like the decaying sine you would expect from theory there are always other factors that will affect it (various types of non-linearities); it is those factors that I am interested in (I am using the resonators to probe the properties of various materials).

Another term I don't understand is image reject mixer. Are they really rejecting an image using a mixer?

No, it is a mixer that reject the (mirror) image of the signal, i.e. it rejects one sideband from the IF.

A good place to read about microwave/RF techniques and equipment is www.microwaves101.com

 

[skeptic2]

When I first saw the MRI decay, it reminded me of ringing and the way it decays(some times).
Anyway, by measuring the decay of the pulse I can understand you see the amplitude decay. But phase also decays/changes?
For example in a exponentially decaying sine, the amplitude decay is obvious. But where is the phase decay? The frequency of the sine does not change.

what,

Another term I don't understand is image reject mixer. Are they really rejecting an image using a mixer?

What is it that you're trying to measure with the decaying sine wave? Is it important to know how fast the sine wave is decaying?

What are the two frequencies that are being mixed in the image reject mixer? How will the product be used? For instance is the frequency or amplitude of the mixer product more important?

 

[f95toli]

Well, since it is MRI we are talking about I would assume they are interested in parameters like T1 and T2.
This means that in general a homodyne measurement in used and what you measure is the amplitude of the signal as a function of time; meaning the signal of interest is the decay envelope of the decaying sine wavefom.

 

[kirubanithi]

Could you please explain the different between IQ and IF signals

 

[f95toli]

Both I and Q are IF signals. An IQ demodulator will givbe you both the in-phase(I) and quadrature(Q) IF signals whereas an ordinary mixer will only give you one signal out.

 

[waht]

Could you please explain the different between IQ and IF signals

In IQ, there are two independent signals at the same frequency, but are offset by 90 degrees in phase.

IF (intermediate frequency) is a result of multiplication of two other signals. It is composed of the sum and difference of frequencies being multiplied. The electronic circuit that does the multiplication is called a mixer. The mixer is often used when you want to down or up convert a signal in frequency.

 

[f95toli]

It is perhaps worth pointing out that an IQ modulator/demodulator is just a couple of mixers and hybrids in a single package. Hence, the I and Q outputs of a demodulator are really just the IF outputs of its internal mixers.

 

[kirubanithi]

what is the main advantage of splitting the signal into I and Q signals.

 

[f95toli]

Did you read my first post?

 

[kirubanithi]

In set top box, two types of tuners are available. One is IF tuner and another one is IQ tuner.Could you please explain about the two tuners.

 

[waht]

In set top box, two types of tuners are available. One is IF tuner and another one is IQ tuner.Could you please explain about the two tuners.

What is the set top box?

IF could be input or output of a converter such as IQ

 

[kirubanithi]

set top box is a channel decoder, IF is input signal

 

[waht]

set top box is a channel decoder, IF is input signal

The IQ tuner is used to pick up digital channels modulated in QAM. It's unclear what the IF port is used in this case. Could be output from the analog tuner.