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Feasibility study - To design and simulate / build a digital lock-in detection system for a near infrared absorption tomography system.

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Introduction

Feasibility Study


During the Feasibility study I will hopefully go along way to meeting the educational aim as set out in the project outline.

Aims / Objectives

  • To design and simulate / build a digital lock-in detection system for a near infrared absorption tomography system.
  • To further my knowledge of digital lock-in techniques and there applications.
  • Investigate the advantages of a digital approach to lock-in.
  • To fully assess any currently commercially available digital lock-in systems as to there usefulness within our tomography system.

Deliverables

  • Market survey of Analogue Digital Converters and Digital Signal Processors
  • Schematic Design
  • A simulation program for a high frequency digital dual lock-in detection system or a prototype circuit as described
  • Demonstration of the operation of the DDLIDS
  • Oral Presentation and Project report  


The Lock-in Technique

This technique is used to detect and measure very small ac signals. Even when those signals are obscured by noise it is possible to obtain accurate measurements. In our system you would need to detect the signal if,

SNR = 30dB = 20 log (S/N)

1.5 = log (S/N)

30 = S/N

N = S/30

The concept behind the lock-in is a filter with very narrow bandwidth tuned to the frequency of the signal therefore rejecting most unwanted noise.
A good SnR is achieved by frequency rejection. With our system the SnR=anti log 31.6=30.

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Middle

1440



Below is a simple example diagram of a lock in amplifier scheme.


Diagram including (Input, Multiplier, ac Amp, PSD LPF and Oscillator)

Also in this Feasibility study we will want to look at the Dynamic Reserve (DR)

Dynamic Reserve is the amount of reserve that is available in order to preserve the system during a frequency disturbance. Therefore, the amount of Dynamic Reserve should be enough to allow the system to survive the loss of the largest energy contingency.

That is the ratio of the largest noise signal to signal that the lock in can tolerate without overloading. Once the project is undertaken we will then have to match V output to V range as closely as possible. The DR is measured in dB and a Dr5 of 60dB would indicate noise=1000signal (at full scale). In general the lowest DR, which does not cause overload, should be used.

Computer Interface

A computer interface would allow data from the lock-in to be connected directly to the computer. Due to complex nature of the data we are collecting in this experiment it would be useful to have this facility. When I look at other devices currently available I will consider this, it might be one of the main advantages of having a commercially available system modified for this application.

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Conclusion



I have also found sketchy information regarding the NF Corporations LI5640 and 5630 though the data available over the Internet was not of a high enough standard and depth. I have ordered the catalogue for said device.


Conclusion

The Perking Elmer 7200 series shows promise in its ability to be applied to our problem. I will attempt to borrow or loan one of these devices for further analysis. Within this feasibility study I have also come across anecdotal evidence that a digital approach to lock-in detection would not normally be used at the frequencies that are required in the project that I am undertaking, I will over the forcible future be researching this. During the study I have been able to discount the PC based approach and have found that there will not be the possibility of a microprocessor approach, due to not being able to achieve a high enough sampling rate.

References:

PerkinElmer                                                         Technical Note 1000

Physica B                                                         vol.208-209

PerkinElmer                                                         Technical Note 1003

Physics 344                                                         uiuc

NF Corporation                                                 Products 5640

PC-Based Digital Lock-In Detection of small signals                 University of Texas

Notes MSc Lectures ESI                                        Dr w. Yang

Notes Electronic Systems – Instrumentation Systems                Prof. Hugh McCann

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