What differentiates the Model 3501 optical chopper from others on the market today?
What makes the Model 3501 optical chopper special is its low phase noise very near the chopping frequency. Most choppers specify their phase noise in the time domain as a root-mean-square (rms) deviation in degrees or as a peak-to-peak value in microseconds. In the case of the Model 3501 chopper, the internal frequency synthesizer gives you <10-ppm/°C frequency stability and ±100-ppm frequency drift with 0.4° phase jitter at 1 kHz. However, for applications using lock-in amplifiers, the frequency-domain characteristics are more important and this time-domain specification is misleading. A more revealing way to characterize chopper performance in the frequency domain is by measuring the phase noise with a spectrum analyzer. While a time-domain experiment is sensitive to the integrated effects of residual phase modulation, a frequency-domain experiment like lock-in detection is only sensitive to phase noise within a detection bandwidth (typically a few hertz) of the chopping frequency. S
