Doc. No. 1693-D03 Revision B 6/6/00
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frequency for the installed blade, the unit will eventually time out. In this case, the harmonic value must be
lowered.
Setting the Sub-Harmonic
1. Press the MODE key until the SET D LED is lit. The current value for D is displayed. Use the FREQ UP /
CYCLE key to select the desired harmonic. The display will wrap around to 1 after it reaches 15.
2. Once the FREQ UP key is released, the MC1000 will apply the reference harmonic multiplier output to the
divider circuit and the reference frequency will decrease to f
EXT REF
/ D. If the harmonic frequency is lower than
the minimum chopping frequency for the installed blade, the unit will eventually time out. In this case, the sub-
harmonic value must be lowered.
Combining the Harmonic and Sub-Harmonic Modes
The sub-harmonic divider follows the harmonic multiplier. Therefore, the generated reference frequency will always
be f
EXT REF
* N / D. There are no special steps to combine these two feature since they are alwya active.
2-Frequency Chopping
A special two-frequency blade is available from Thorlabs which has seven slots on the outer portion of the wheel
and 5 slots on the inner part of the wheel. This unique prime number combination allows the same chopper to
discriminately chop two different light paths. This can be used in ratiometric measurements and pump-probe type
experiments.
The MC1000 provides two additional reference outputs in this mode. The SUM, and the DIFFERENCE reference
frequencies track the combined signal paths when they are frequency mixed together. The example below
illustrates this:
Example:
The MC1000 is set to run the outer blade set at 70Hz. The inner blade, by virtue of the ratio of inner to
outer slots will be running at 50Hz. If a light path is common to both the inner and outer blades, the
signal will see a sum frequency of 120Hz and a difference frequency of 20Hz.
Setting the Chopping Speed in the 2-Frequency Mode
The chopping frequency for the 2-frequency blade is set the same way as described above for the single frequency
blades. The only exception is that the harmonic mode (both the harmonic multiplier and sub-harmonic divider) is not
available with 2-Frequency operation.
Note: the MC1000 synchronizes the outer portion of the 2-frequency blade to the internal or external synthesizer.
The inner portion is chopping at a rate of 5/7 * reference.
Example:
Internal Reference (or external reference) frequency = 100Hz
The outer portion of the blade is chopping at 100Hz
The inner portion is chopping at 100Hz * 5 / 7 = 71.42Hz
Setting the Chopper Blade
