Optical Chopper System
18488-D02 Rev G, September 24, 2012 Page 11 www.thorlabs.com
Part 5. Advanced Operating Instructions
5.1. Internal Frequency Synthesizer
The MC2000 is most frequently used in this mode where the chopper wheel frequency is locked to an
internal crystal stabilized frequency reference. The PLL motor speed control circuit maintains a precise
lock to the frequency and phase of the internal reference frequency creating an ultra-stable chopping
signal. The chopper wheel phase can be continuously adjusted.
Setup for Internal Reference Frequency
Follow the setup procedures for internal reference operating mode in Part 4: Basic Operation on page 9.
Selecting the REFERENCE OUT signal
In the internal frequency mode, the reference output signal can be set to the actual chopper wheel
frequency (ACTUAL) or the internal frequency synthesizer (TARGET). In most cases, such as when
selecting a reference for a lock-in amplifier, the chopper wheel frequency will be used since it provides a
direct measurement of the chopper phase and frequency. The chopper wheel reference is derived from
two photo-interrupters on the optical head that sense the motion of the chopper blade.
The internal frequency synthesizer has slightly less phase jitter than the chopper wheel reference since it
is not affected by external disturbances. Therefore, there may be cases where using the internal
synthesizer as the reference output yields better performance (i.e. when synchronizing multiple
choppers).
5.2. External Reference Mode
A major benefit of using a PLL circuit to control the chopper wheel speed is the chopper can be locked
precisely to an external reference signal. This allows the MC2000 to be used in advanced setups, for
example, multiple MC2000 choppers can be synchronized to a single reference signal, or master-slave
combinations where one chopper is the master reference and a second chopper is slaved off the reference
output of the first. The latter example provides a convenient way to measure long decay time
fluorescence and other similar types of experiments.
The MC2000 will accept a TTL or CMOS logic level input as an external reference. The advanced PLL
design used in the MC2000 even accepts reference signals that do not have a 50% duty cycle. A special
feature of the external reference mode, locking to harmonics and sub-harmonics of the reference signal,
is described in the following section.
Setup for External Reference Frequency
Follow the setup procedures for external operating mode in Part 4: Basic Operation on page 9.
Selecting the REFERENCE OUT signal
In the External frequency mode, the reference output signal (ACTUAL) can be set to the actual chopper
wheel frequency or the internal frequency synthesizer (TARGET). Most cases in this mode, only the
actual chopper wheel frequency will ever be used.
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