Phase Synchronization for Demodulation

[jemazter]

Hello. Currently i'm designing  a respiration rate measurement device based on Impedance Pneumography for a university project (http://www.ti.com/lit/an/sbaa181/sbaa181.pdf > similar to that)

I have already defined the first part of the circuit (modulation signal will be generate by a DDS IC, applied to a Howland current pump then applied to patients body), now i working on the second part of the circuit (measuring the sinal from patient, demodulation, filtering and sampling).

My questions is, how can i phase align the modulated signal coming from a human body to the demodulation clock, and then demodulate that to extract the information needed? I was reading about PLL, but couldn't figure out exactly how it works and how i can (if i can) use it in this case.

Thanks in advance.

[Kleinstein]

It depends how demodulation is supposed to be done:

 If in hardware one needs a square wave of the same frequency with suitable phase, possible two with a 90 deg phase shift. For a phase shift one might use an PLL, but with fixed frequency an other method might be easier.
The DDS chip might have an extra sync / square wave output, but not all DDS chips have one.
I doubt one would really need a pure sine for excitation - it might be easier to crate the drive signal from a filtered square wave. This would make is easier to derive the demodulation signal(s). The filter circuit could also be used for phase adjustment (if needed).

The alternative would be demodulation in software: using a faster ADC and doing it in math. This would also need a start signal, but one could  use something like an µC do generate a square signal for the driving signal and read the ADC and do demodulation - so no external demodulation signal needed, it is just virtual in software.

[jbb]

... using a faster ADC and doing it in math. This would also need a start signal... so no external demodulation signal needed, it is just virtual in software.

This is a very powerful technique.  You would need an ADC to run at several times the excitation frequency; maybe a divided copy of the DDS clock would do it.  Then you can do the sampling function in software and have easily adjustable delay.  Or you can set the delay to zero and extract in-phase (0 degrees) and out-of-phase (90 degrees) components, and do some trigonometry to get info about both magitude and phase which could be useful.

Depending on your excitation frequency, the AD5933 might be suitable http://www.analog.com/en/products/rf-microwave/direct-digital-synthesis/ad5933.html#product-overview.

They also do the ADUCM350 with a Cortex M3 inside but it's lower bandwidth and BGA package. http://www.analog.com/en/products/processors-dsp/microcontrollers/precision-microcontrollers/aducm350.html#product-overview