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300CD Rotating Optical Chopper

300CD Optical ChopperFeatures

 Scitec Instruments Model 300CD is a high stability variable frequency optical chopper. The basic system consists of a control unit, a chopping head and a set of four chemically blacked photo etched discs. This system provides operation over the frequency range 5Hz to 3kHz. A wide selection of additional discs and accessories is available to extend the frequency range and to satisfy individual requirements.

5Hz - 3kHz standard optical chopping frequency range (0.015Hz to 40kHz options are available)

102mm diameter discs

Choice of control units

Compact optical chopper head

Low noise and vibration

Photochemically etched non-magnetic discs

Variable aperture

Range of accessories

300CD Discs Tthe 300CD control unit has a 5-digit display of the chopping frequency. Chopping frequency can be controlled externally by applying an analogue voltage to a "Control" BNC. A "Reference" BNC provides a TTL level output, generated directly from the opto-switch on the chopping head. This signal has constant phase relative to the chopping action and can be used as the reference for other instruments such as lock-in amplifiers.

 The 300H optical chopper chopping head supplied with the standard system may be bench bolted or optical stalk (bench rod) mounted. Mountings are provided to enable parts, such as a high frequency accessory or blade protector, to be attached. For lower frequency operation, the chopper head can be fitted with a reduction gearbox motor.

 The set of four chopping discs included with the system (part number 300D) provides a choice of 2, 5, 10 and 30 slots, each having a 1:1 mark to space ratio. Variable apertures can be achieved by mounting two identical discs simultaneously. Additional discs, with up to 445 slots, are available to extend the chopping frequency up to 40kHz. Although the standard discs are chemically blacked other finishes, such as gold plating, are available.


300CD Control Unit

Output Voltage: 15V DC Maximum.

Stability: ±0.01%/°C

Frequency Control: Internal - Manual Control via 10 turn potentiometer. External - BNC connector for 0 to 15V.

Frequency Read Out: 5 digit 14mm LED display with 0.01Hz resolution, 0.1Hz or 1Hz resolution depending on frequency.

Frequency Stability: Short term - see phase jitter. Long term - ±0.1% of maximum frequency.

Reference output: 5V HCT TTL signal via BNC socket.

Power Requirement: 100-130V or 200-260V AC, 50 or 60Hz, 12 VA.

Dimensions: 254(W) x 76(H) x 178(D)mm

Weight: 2.6kg

Chopper Head300H Chopping Head

Moter: 11 pole DC motor, sleeve bearings with more than 6000 hours lifetime, 0-6000rpm reversible.

Reference pick-up: IR led and phototransistor pair with Schmitt trigger.

Dimensions: 73(H) x 75(L) x 32(W)mm, without disc.

Mounting: Optical stalk - adjustable between 6.5 and 14.0mm diameter. Mounting holes - 2 x 3mm

300D Set of Chopping Discs

Disc types: 2, 5, 10 and 30 slot

Material: Half hard brass, 0.5mm thick

Diameter: 102mm

Surface Finish: Chemically blacked

Mark to Space Ratio: 1 to 1 with one blade

r, a, b & d aperturesFrequency Range:


Frequency (Hz)


Phase Jitter, Max





(2 slots)

5 - 200






(5 slots)

12.5 - 500






(10 slots)

25 - 1000






(30 slots)

75 - 3000






Ordering Information

Model 300CD

 A complete optical chopper system with frequency display includes: 300CD Control Unit, 300H Chopping Head, 300I Interconnecting Cable, 300D Set of Chopping Discs and IEC mains lead.


200 slot disk

200 slot disk which increases range to 20kHz.

200 slot disk

445 slot disk which increases range to 40kHz.

Blade Protector

Blade protector for the chopping head.

High Frequency Accessory

High frequency accessory enabling a 26mm beam to be modulated at 20kHz.

High Frequency Accessory

High frequency accessory (445) enabling a 10mm beam to be modulated at 40kHz.

Full accessory list.

Useful Documents


300CD Datasheet - pdf (128K)


300I Interconnecting Lead Wiring - pdf (15K)


300CD Manual - pdf (82K)


300H Dimensions and Wiring - pdf (25K)

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