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Doc 12536-D03 Rev D 9/19/2005
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of
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8.2.1 Setup for External Reference Frequency
1. Follow the setup procedures in the ‘Getting Started Quickly’ section.
2. Turn the MC1000A power on.
3. If the ‘EXT EN’ LED is not lit, press the ‘EXT IN ENABLE’ key under the LED until it lights indicating that the unit is
in the external reference mode.
4. To have the chopper lock to the fundamental of the external reference frequency (i.e. the chopper speed will
equal the external reference frequency, the harmonic multiplier, N, and the sub-harmonic divider, D, must each be
set to 1. To check these values, use the ‘MODE’ key to select the N and D parameters and adjust their values as
necessary.
5. The chopper should adjust its speed and lock onto the reference within a few seconds as indicated by the
chopper speed shown on the digital display.
8.2.2 Selecting the REFERENCE OUT signal
In the external frequency mode, the only signal available for the reference output signal is the actual chopper wheel
frequency. The controller will not allow the ‘REF SELECT’ option to be changed.
8.3 Harmonic Reference Generation
Harmonic generation is a special feature available when operating in the external reference mode. The MC1000A uses a
second PLL circuit to multiply the external reference frequency from 1 to 15 to generate up to the 15
th
harmonic. The PLL
is then followed by a digital divider, which can be programmed from 1 to 15 to divide the reference frequency and create
sub-harmonics. Additionally, both the harmonic, and sub-harmonic modes can be used to create fractional reference
frequencies. The new frequency is used as the chopper reference for chopping at a variety of frequency combinations, all
derived from the external reference.
Note: Harmonic and sub-harmonic modes are not available when using 2-frequency blades.
8.3.1 Setting the Reference Harmonic
1. Press the ‘MODE’ key until the ‘SET N’ LED is lit. The current value for N is displayed. Use the ‘
▲
/ CYCLE’ key
to select the desired harmonic. The display will wrap around to 1 after it reaches 15.
2. Once the ‘
▲
’ key is released, the MC1000 will apply the reference signal to the multiplier circuit and the reference
frequency will increase to N * f
EXT REF
. If the harmonic frequency exceeds the maximum chopping frequency for
the installed blade the unit will eventually time out. In this case, the harmonic value must be lowered.
8.3.2 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 ‘
▲
/ CYCLE’ key
to select the desired harmonic. The display will wrap around to 1 after it reaches 15.
2. Once the ‘
▲
’ 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.
8.3.3 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 features since they are always active.
8.4 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 ratio metric measurements and pump-probe type experiments.
The MC1000A 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:
