The Model 3501 optical chopper provides rock-solid stability and ﬂexibility because its chopping frequency is phase-locked to its own internal frequency synthesizer. It comes complete with four wheels, a wheel cover, and a GPIB (IEEE-488) interface.You can chop from 4 Hz to 6.4 kHz, using either the internal frequency synthesizer or an external source as the reference. And you can chop at the fundamental, a harmonic (2 to 15), or even a sub-harmonic (1/2 to 1/15) of the reference. Other choppers dont give you nearly as much ﬂexibility for the price.Unique features of the Model 3501 include outputs at the sum and difference of the inner and outer wheels chopping frequencies for nonlinear-optics experiments, and a high-stability phase shifter that can lock two choppers together for long ﬂuorescence-decay measurements. In addition, variable duty-cycle chopping is easily achieved by mounting two of the same wheels on the chopper head.To mount the chopper controller in a standard 19" rack, order the Model 3510 rack-mount kit. The kit includes hardware for mounting one controller in a rack or two controllers side by side in a rack.
For stable chopping with minimum jitter and drift, the Model 3501 optical chopper uses precision photo-etched wheels mounted on a high-quality DC motor. In addition, the chopper controller has its own internal crystal-controlled frequency synthesizer and phase-locked loop for precisely locking the chopping frequency. A photo-interrupter on the chopper head monitors the chopping frequency, and the chopper controller actively stabilizes the motor speed to ensure stable chopping with a minimum of frequency drift.In internal-reference mode, the chopper controller uses its internal frequency synthesizer as the reference. When the Model 3501 is operated in external-reference mode, the chopping rate is locked to the user-provided reference from another chopper or signal source. The chopper may also be run asynchronously with the rear-panel analog-control-voltage input. In all cases, the frequency-synthesizer output is available on the front panel for use in your measurement setup. The reference signal can be divided and multiplied by the sub-harmonic and harmonic generators before being processed by the phase shifter. The microprocessor-controlled system provides ﬂexibility and performance never before available in an optical chopper.The Model 3501 optical chopper also has especially 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. So phase noise far away from the chopping frequency that contributes to the rms-phase deviation does not contribute to the noise ﬂoor in a lock-in experiment.To measure chopper performance in the frequency domain, we compared the phase-noise spectrum of a typical voltage-controlled chopper with that of the Model 3501 optical chopper (see graphs). Both devices have the same time-domain jitter speciﬁcation, dominated by large frequency-modulation sidebands (not shown). However, as the ﬁgures show, phase noise close to the chopping frequency is drastically reduced in our optical chopper. In other words, the Model 3501 , with its frequency-locking circuitry, is much more stable than a typical voltage-controlled chopper. What this means to you is increased sensitivity and a lower noise ﬂoor with the Model 3501 and a lock-in ampliﬁer.