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ADA2200 and WMS modulation for QCL

Question asked by KoRba on Jun 29, 2017
Latest reply on Oct 9, 2017 by brianharrington

Hi

 

I have to build a gas (NO2) sensor based on Quantum Cascade Laser working in infrared.(QCL is a laser with current tuning wavelength capability)  Sensor must be very sensitive (1ppb NO2 detection) so i think i have to incorporate WMS modulation to my system.

(WMS-2F basics -> https://www.princeton.edu/cefrc/Files/2013%20Lecture%20Notes/Hanson/pLecture9.pdf )

 

So a basic principle is that have to tuning wavelength a QCL laser with ramp current (low frequency) and modulating it with sinus current (high frequency). But ADA2200 is generating square signal on RCLK, so I need it to convert it to sine (i.e with DDS - > AD9833  will be ok? ) and multiply it witch ramp current signal (i.e. from DAC) and give it to the laser driver. Then infrared light passing the Herriot cell with NO2 get to the Infrared detector. Detector recieve changed (due to absorption of NO2) signal, then transimpedance opamp will convert I/V and signal will go to the ADA2200.  I have two question.

 

1) It will be working? 

2) Do I need also add another ADA2200 and measure a phase? (as i know QCL will add phase shift, but at constant value). Or Amplitude measurements will be sufficient ?

 

EDIT: I've realised that demodulation witch sine is critical to remove noise, so ADA2200 will be useles? I dont want to go into FPGA so i try to build IQ demodulator (FIG 11 Synchronous Detectors Facilitate Precision Low-Level Measurements | Analog Devices )  witch discrete components.

 

Sine wave generator (in phase and 90 out of phase) -> AD9958

Demodulator - > Two AD630

Low pass filter -. ADRF6520

 

Please replay if you have some ideas

 

Best Regards

KoRba

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