Signal Recovery- Precision Light Choppers (Model:197)

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The model 197 is a compact, high performance 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) for dual-path experiments. Frequency control is by a precision internal oscillator set by a 4-digit push-button selector on the unit or by the application of an external AC reference signal. The unit is powered via an external line power supply module.

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 post.

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 signal is divided down to yield the required chopper frequency, and then the motor speed is continuously adjusted to phase lock the actual chopper frequency to this required value. The result is a chopper with an output frequency as stable as any other modern frequency source.

External Frequency Control
Like many other choppers, the frequency can be controlled externally. However, unlike other units the control is via an applied TTL reference rather than an analog voltage. This means that the modulation frequency generated is exactly that required which allows these choppers to be used in conjunction with the dual reference modes offered by our model 7124, 7265, 7270, and 7280 lock-in amplifiers to implement twochannel source compensation experiments.


Dual-aperture self-contained chopper with internal or external refence frequency.  Two sync outputs.
     outer sector

     inter sector

15 Hz to 3000 Hz

150 Hz to 3000 Hz

15 Hz to 300 Hz

Digital push-button
Application of 0.5 v to 10 V pk-pk sine or squarewave, 150 Hz to 3000 Hz to EXT SYNC BNC connector
Internal Frequency



±20 ppm at 25 ºC
±30 ppm/ ºC (range 10 ºC to 60 ºC

Jitter (measured pk-pk and presented as a % of a full cycle)
     outer sector
       150 to 500 Hz

       500 to 3000 Hz
      inter sector
       15 to 50 Hz
       50 to 500 Hz

blade only: 0.5%; blade + electronics: < 1.5%

blade only: 0.5%; blade + electronics: < 1%

blade only: 0.5%; blade + electronics: < 1.5%
blade only: 0.5%; blade + electronics: < 1%

Lock indication Bicolor LED – red when unlocked and green when locked
Setting Time 7 s nomimal at 1 kHz from switch-on; 9 s nominal for frequency change from 150 to 3000 Hz; 30 a nominal for frequency change from 3000 tp 150 Hz
Sync Out 1 10 V pk-pk squarewave at outer sector chopping frequency, 150-3000 Hz
Sync Out 2 10 V pk-pk squarewave at  inner sector chopping frequency, 15-300 Hz
Connectors BNC
Impedance 10 kΩ. Note that although the output voltage is 10 v pk-pk, the high output impedance means that the outputs can be directly connected to the external reference input of any SIGNAL RECOVERY lock-in amplifier without causing problems.
Power Requirements Via separate power adapter for 110 V AC, 50/60 Hz or 220/240 V AC, 50/60 Hz supply.  State which voltage is required when ordering
43/4" (122 mm)
4" (104 mm)
13/4" (44 mm)
Weight 1 lb (0.45 kg) excluding power supply

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