general
the model 197 is a compact, high performance, low-cost optical chopper, offering features and benefits that are ideal for use in modern photometric systems.

the unit is self contained, comprising a dual aperture chopper blade, motor and the necessary driving electronics. each aperture provides an independent reference output allowing simultaneous dual frequency operation (10:1 ratio) or dual-path experiments. frequency control is by a precision internal oscillator set by a 4-digit push-button se-lector on the unit or by the application of an external ac reference signal. the unit is powered via a remote line power supply.

mounting holes are provided in the base and right-hand side of the housing (viewed from the front) to allow for mounting the model 197 onto an optical bench or support rod.

quartz crystal frequency accuracy and stability the model 197, in common with all signal recovery light choppers, uses a quartz crystal oscillator as its primary frequency standard. the oscillator signals is divided down to yield the required chopper frequency, and then the motor speed is continuous adjusted to phase lock the actual chopper frequency to this required value. the result is a chopper with an output frequency as stable as other modern frequency sources.

external frequency control
like many other choppers, the frequency can be controlled externally. however, unlike other units the control is via an applied reference signal (ttl levels may be used) rather than an analog voltage. this means that that the modulation frequency generated is exactly that required, and allows the units to be used in conjunction with the dual reference modes offered by our model 7265 and 7280 lock-in amplifiers to implement two-channel source compensation experiments - see diagram below or consult applications note an1000. usually, however, the model 198a is chosen in preference to the model 197 for this applic