OPERATING INSTRUCTIONS
6
INTRODUCTION
The Model SR540 Optical Chopper is used to
square-wave modulate the intensity of optical
signals. The unit can chop light sources at rates
from 4 Hz to 3.7 kHz. Versatile, low jitter
reference outputs provide the synchronizing
signals required for several operating modes:
single or dual beam; sum & difference
frequency; and synthesized chopping to 20 kHz.
FREQUENCY OF OPERATION
The choice of operating frequency is influenced
by several factors:
1. Avoid low frequency operation (below
100Hz). In general, phase jitter,
background noise and lock-in amplifier
noise all degrade at low frequencies.
2. Avoid high frequency operation. Motor
lifetime will be reduced if the unit is
operated for extended periods above
2KHz.
3. Avoid known noise frequencies. For
example, the line frequency and all its
even and odd harmonics should be
carefully avoided.
4. Avoid using the bottom 10% of the
frequency control dial. The phase jitter
of the reference outputs will degrade at
the low end of the range, so switch to the
next lower MAX FREQ / SLOTS range.
5. Always use the correct wheel as
indicated by the MAX FREQ / SLOTS
switch. Using the wrong wheel will
seriously degrade the performance of the
synthesized reference outputs (left
BNC). For frequencies from 4Hz to 400
Hz use the 6/5 slot blade; from 400Hz to
3.7kHz use the 30/25 slot blade.
CHANGING BLADES
The chopper blade (figure 4) may be replaced
by removing the three flat head screws, which
hold the outside hub to the wheel. When
replacing the blade, be sure to insert the blade
between the slotted optical detectors before
placing it on the wheel. Secure the blade by
sandwiching it between the wheel and outer
hub. Install and loosely tighten the three flat
head screws, which hold the hub to the wheel.
Then firmly tighten all three screws. Handle
blades carefully as they are easily bent.
BASEPLATE REMOVAL
The baseplate of the unit may be removed to
allow for pole mounting. Use an allen wrench to
remove the 6-32 bolt from the bottom of the
baseplate.
