Bentham 218 Variable Frequency Chopper
Specs at a glance for the 218
- Chopping Frequency: 2-slot disc 5Hz to 200Hz / 5-slot disc 12.5 Hz to 500Hz / 10-slot disc 25 Hz to 1kHz / 30-slot disc 75 Hz to 3KHz
- Phase Jitter: 2-slot disc: 0.6° pk-pk over entire frequency range / 5, 10 & 30-slot disc: 1° pk-pk over entire frequency range
- Reference Output: 800mV positive going wave from 200W. Sensed by IR LED/Photodiode mounted on chopping head
- External Control: The chopping frequency can be externally controlled via a front panel socket / Maximum input is = 15V into 20kW
- Resolution: 1Hz with 10-slot disc.
- Calibration Accuracy: ± 1% for setting between 15% and 100% of full scale.
- Variable Aperture: A variable aperture 2-slot disc is available for non-even mark space modulation.
- Frequency Stability: Versus Temperature: Less than 0.01% change per K / Versus Line Voltage: Less than 0.01% change results from a change in line voltage from 190V to 265(95V to 130V)
- Display: LCD frequency display on free-standing controller turns counting dial on modular unit
- Frequency Selection: By multi-turn potentiometer
- Motor: High quality 11-pole dc motor with precious metal commutator.
The 218 optical chopper consists of a free-standing or modular control unit coupled by a 2m long lead to the chopping head. Four, easily interchangeable, chopping discs are provided as standard which allow chopping frequencies in the range of 0.5Hz to 4kHz (3kHz for the modular control unit). A variable aperture 2-slot disc and higher frequency discs are also available. The free-standing control unit includes LCD display of chopping frequency, while the modular unit uses a digital indicating dial. Both units provide a square wave reference output suitable for use with lock-in amplifiers.