Optical Chopper Measurement

Introduction

The use of lock-in amplifiers in combination with optical choppers is one of the most common low frequency lock-in amplifier applications. In order to make this a bit more interesting, James shows in the following video how to easily set up an experiment that makes use of 2 different choppers simultaneously. This helps to save valuable real estate in the lab and reduce the complexity of the setup.

Application Video

 

Experiment setup

 

Requirements

Tips and Tricks

  • Adjust the input ranges
  • Sometimes it's handy to use the Auxiliary Inputs on the front panel as external reference input
  • Auto-zero the phase
  • Use the Plotter tool to display the chopper frequency, as tracked by the internal PLL, over time - the Math sub-tab offer plenty of statistical function for further signal analysis
  • By adjusting the Harmonic field you can adjust the demodulation frequency to higher harmonics but also sub-multiples of the chopper reference frequency; this can be useful when using the same blade at multiple slot pitches
  • Use the Sinc filter to increase temporal resolution while keeping SNR high, when demodulation bandwidth gets into the same range as demodulation frequency
  • Use the FFT Spectrum Analyzer to determine SNR and see other components in the spectrum
  • For analog outputs to the Auxiliary Output adjust the Offsets, the scaling factor and limits as needed

MFLI Key Features for Optical Chopper applications

  • Works with any chopper model
  • Best noise performance 2.5 nV/√Hz at 1 kHz and above; 7 nV/√Hz at 10 Hz; 40 nV/√Hz at 1 Hz
  • Easy lock to the chopper reference (TTL or analog signals)
  • Demodulate at higher harmonics and sub-multiple frequencies
  • 4 easy to configure analog outputs (pre-offset, scaling, offset, limits)
  • LabOne toolset: Scope, FFT Spectrum Analyzer, Parametric Sweeper, Plotter, etc.

 

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