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Doc 12536-D03 Rev D 9/19/2005
Page 14
of
22
Section 8: Operating Instructions
8.1 Internal Frequency Synthesizer
The MC1000A 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.
8.1.1 Setup for Internal 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 lit, press button under the LED to return the unit to the internal frequency mode.
4. Set the chopper operating speed as desired using the ‘
▲
’ or ‘
▼
’ keys. While holding down these keys, the set
speed will be displayed but the motor speed will not change until the key is released.
5. After releasing the keys, 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.
8.1.2 Selecting the REFERENCE OUT signal
In the internal frequency mode, the reference output signal can be set to the actual chopper wheel frequency or the
internal frequency synthesizer. 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 a photo-interrupter on the optical head that senses 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).
1. Press the ‘MODE’ key until the ‘REF SELECT’ LED is lit.
2. Press the ‘
▲
’ or ‘
▼
’ key repeatedly until the desired reference output signal is shown. Once the key is released,
the selected reference signal will be routed to the front panel BNC labeled REF OUT.
Note: the display prompt for the direct chopper wheel frequency is ‘
OUT
’. The internal synthesizer display prompt is
‘
SYN
’.
8.1.3 Why doesn’t the motor speed exactly match the set speed?
The MC1000A internal frequency synthesizer uses a 1.000MHz crystal oscillator, which is divided down by a 32-bit digital
divider to generate the reference signal to lock the chopper wheel to. The MC1000 microprocessor uses an advanced
algorithm to match the set speed as closely as possible. However, there are some frequencies that cannot be achieved
exactly. This is due to the limitations of dividing the 1.000MHz clock by a 32-bit integer divisor, and does not affect the
stability of the chopper frequency.
8.2 External Reference Mode
A major benefit of using a PLL circuit to control the chopper wheel speed is that the chopper can be locked precisely to an
external reference signal. This allows the MC1000A to be used in advanced setups, for example, multiple MC1000A
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 MC1000A will accept a TTL or CMOS logic level input as an external reference. The advanced PLL design used in
the MC1000A 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.
Note: Having an active external reference attached to the ‘EXT REF IN’ during MC1000A power up may cause the unit to
lock up. If this occurs, remove the external reference connection, and recycle the power. When the unit finishes its startup
sequence, attach the external reference signal.
