The Sciencetech C230X variable frequency optical chopper is typically used with IR detectors that require signal modulation for improved signal to noise detection. The optical chopper is designed for high stability and broad range frequency control from 0.5Hz to 100Hz. When used with a lock-in amplifier, it can filter out significant background noise (of a constant frequency) and thus recover very low signal levels from the detector. The frequency of the chopper is selected by both blade pitch spacing and motor speed. Various blades with different pitch spacing are available to choose from. Motor speed is controlled by software or by using the dip switch jumpers on the base of the chopper assembly. Price includes servo motor controller and Sci-Chopper Active-X software, blades are sold separately.

Blade Selection
The frequency of the chopper is selected by both blade pitch spacing and motor speed. Various blades with different pitch spacing are available to choose from.

Manual and Software Control
The optical chopper blades are mounted onto a spinning DC servo motor. The motor speed which governs frequency can be adjusted through the accompanying software. It has two modes of speed control: through local DIP switches on the chopper itself, or computer controlled through RS-232 serial interface. There are two software programs available for controlling the chopper: Sci-ChopperX is an ActiveX based software for incorporation into Windows based applications such as C++ and Visual Basic, Sci-Chopper-VI is a LabView "VI" Virtual Instrument based version compatible with National Instrument's LabView development environment. Please note that Sci-ChopperX can also be embedded inside LabView as it also supports Active-X controls. The speed can be set using either software, before being relayed to the Chopper Motor via RS-232 cable. The motor shaft speed can be set between 0.5 and 100Hz.

Motor Controller
The Sciencetech MD-100 external Stepper Motor Controller. Instead, it uses its own external Servo Motor Controller which interfaces to a computer via an RS-232 port. Unlike the MD-100 controllers which can share a computer's Serial port with up to 3 other RS-232 devices, this servo motor controller requires its own dedicated serial RS-232 port. Sciencetech SCI-Chopper Active-X software is designed to operate with this motor controller and is available free of charge.

Settle Time
Once you set the rotation speed by software or DIP switches, it takes approximately 3-4 seconds for the chopper to settle into the new rotation speed. The fastest speed is 120 revolutions per second, and the slowest speed is 1 revolution per second (or even stop it).

Absolute Blade Angular Positioning
Since this chopper is also used in Sciencetech SPS-300 Terahertz Spectrometer for Michelson Mode operation, it has a unique feature in that the blade can be set to stop at a particular angular position. This feature is required in the SPS-300 when it operates in Martin Pupplett polarizing mode, this chopper needs to stop such that its blade position does not interfere with the optical beam. With this feature, the blade can be set to always stop at a particular angular position. Please note that the Sci-Chopper Active X software which has this feature has a separate command for stopping the chopper blade at a particular position. This command must be called as stopping the chopper by software without this command will result in the blade stopping at any angular position. The angular position that it stops at is set in the Sci-Chopper-X ActiveX configuration file under the parameter "Stop Delay".

Lock-in Amplifer Interface
The C-230X Chopper outputs analog square pulses over a BNC connector for interfacing to a lock-in amplifier. The square wave pulses, which corresponds to the optical chopping frequency, is obtained through an IR optical sensor that reads each blade that pass through. This assures that the lock-in amplifier receives the actual optical chopping frequency, regardless of the blade density used.

Software
The C230-X comes with its own Active-X Sci-Chopper software that is used to set the speed of the chopper. There is a blade slot parameter that takes into account blade selection and its effect on frequency. Please note that this software is currently unable to determine the open/close position of the blade as such information is feedback through the separate Synchronization Output BNC port. However, this feature could be custom added through the AD board trigger channel of the detector. Please ask an Applications Sales Specialist for more details.

Motor Speed Feedback
The C230-X chopper provides an actual speed feedback to the Sci-Chopper Active-X software indicating when it has reached selected programmed speed. Please note this feedback is done inside the bi-directional RS-232 communications port is and not done through the BNC output opto-sensor.

Technical Specifications

• Chopper Motor Type: DC Servo Motor
• Motor Speed: 0.5 to 100 Hz
• Blade diameter: 4" outer diameter, 3.5" aperture diameter
• Standard Aperture: 2, 4, 10, 30
• Blade Holder Size: Base: 57.2 x 57.2 mm with slots for optical table mounting
• Unit without Blades: 95.3(h)mm
• Center of rotation at 76.2 mm
• Shaft coupler for blade mounting: 0.7"+ diameter
• Input Power: 115VAC @ 60Hz or 230 VAC @ 50 Hz , single phase

Blade Specifications

• Anodized aluminum
• Blade thickness: 0.025"-0.07"
• Mount: three screws to chopper motor shaft coupler
• Outer blade diameter: 4"
• Inner aperture diameter: 3.5"
• Inner circle diameter: 1.5"

BASIC SPECS:

• Motor shaft speed (with blade on) in air: 0.5 to 95 RPS (Hz),
• Motor shaft speed (with blade on) in vacuum: 0.5 to 127 RPS (Hz)
• Stability: blade dependent (TBD)
• Input power requirements: 115Vac, 0.15A, 60Hz (230Vac, 0.075A, 50Hz version available)
• Controller dimensions: 147 x 55 x96 mm