The model 198A is a dual frequency light chopper, using a thin rotating metal blade with an inner set of 11 apertures and an outer set of 18 apertures to simultaneously chop two light beams.
- Dual frequency
- External power supply
- Two apertures provides - inner and outer
Its blade is driven by a precision DC motor whose speed is controlled via a phase locked loop that is referenced to either an internal quartz crystal oscillator or an external reference signal. The chopper has three reference frequency outputs, one at each of the frequencies generated by the inner and outer apertures, and one at a frequency equal to their sum.
- Outer frequency sector: 90Hz to 1500Hz
- Inner frequency sector: 55Hz to 917Hz
- Internal frequency accuracy: ±20ppm at 25°C
- Internal frequency stability: ±30ppm/ °C
- Absorption, reflection and optical transmission