Model 197
Precision Light Chopper
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 selector 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 application since
the two frequencies it generates are relative primes.
Using the Model 197 with a Model 7265 to implement a
dual-channel source compensation experiment
Mechanical Dimensions
Blade Dimensions
| Operation | |
| | Frequency | 15 Hz to 3000 Hz |
| | outer sector | 150 Hz to 3000 Hz |
| | inner sector | 15 Hz to 300 Hz |
| | Control | |
| | manual | Digital push-button |
| | external | Application of 0.5 V to 10 V sine or squarewave, 150 Hz to 3000 Hz
to Sync In BNC connector |
| | Internal Frequency | |
| | accuracy | ±20 ppm at 25 °C |
| | stability | ±30 ppm/ °C (range -10 °C to 60 °C) |
| | Jitter (measured pk-pk and presented as a % of a full cycle) | |
| | outer sector | |
| | 150 to 500 Hz | blade only: 0.5%; blade + electronics: 1.5% |
| | 500 to 3000 Hz | blade only: 0.5%; blade + electronics: 1.0% |
| | inner sector | |
| | 15 to 50 Hz | blade only: 0.5%; blade + electronics: 1.5% |
| | 50 to 300 Hz | blade only: 0.5%; blade + electronics: 1.0% |
| | Lock indication | Bicolor LED, showing Red when unlocked and Green when locked |
| | Settling Time | 7 s nominal at 1 kHz from switch-on;
9 s nominal for frequency change from 150 to 3000 Hz;
30 s nominal for frequency change from 3000 to 150 Hz |
| Outputs | 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 kW. 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 Reference Frequency input of any SIGNAL RECOVERY lock-in amplifier without causing problems. |
| Power Requirements | Via power adapter for 110 V AC, 50/60 Hz or 220/240 V AC,
50/60 Hz or external isolated 17 V to 20 V DC or AC rms.
supply. State which voltage is required when ordering |
