3502
Optical Chopper System
22
Wheel Type
(slots)
Lowest Frequency
(F
outer
)
Highest Frequency
(F
outer
)
100
200 Hz
10.65 kHz
60
120 Hz
6.40 kHz
42/30
84 Hz
4.48 kHz
7/5
14 Hz
746 Hz
2
4 Hz
213 Hz
60/2
See text
See text
F
outer
is the chopping frequency as measured by an optical pick-up on the Model
3502 Optical Chopper Head. If a wheel has two sets of slots, F
outer
refers to the
chopping frequency of the slots on the outer edge of the wheel. To change the
wheel type, push the Wheel button until the LED lights under the desired wheel
type.
For setups requiring very low chopping frequencies and high stability the 60/2-
slot wheel may be used. There is no wheel setting for this particular wheel on the
front panel—the 60-slot wheel setting must be selected. The actual chopping
frequency will differ from that displayed on the front panel by a factor of 2/60 =
1/30. For example, in
NORMAL
mode with the internal synthesizer set to 120 Hz
and with the 60-slot wheel setting selected, the actual chopping frequency through
the 2-slot segment of the wheel will be (1/30)·120 = 4 Hz.
Sync Button
The user may synchronize the chopping frequency to the Chopper's internal
synthesizer (INT), the rising edge (
EXT+
) or falling edge (
EXT-
) of an externally-
supplied signal on
Sync In
. The motor’s rotational speed may be controlled directly
by applying a negative voltage to the
V
ext
input.
When
INT
is selected, the signals available on the
OUT 1
and
OUT 2
outputs (see the
following section on the
Mode
button) on the back panel are phase-locked to the
internal synthesizer. When using the
INT
setting, a signal applied to
Sync In
has no
effect on the device’s operation.
When an external signal is used, either with
EXT+
or
EXT-
, the signals available on
the
OUT 1
and
OUT 2
outputs are instead phase-locked to the external signal. In
EXT+
and
EXT-
modes the output from
Synth Out
(described below) is not locked to, and
does not have the same frequency as, the external signal and remains at the
frequency setpoint set on the front panel.
When
V
ext
is selected, the user may drive the motor directly with an analog voltage
(-10 V to 0 V) applied at
V
ext
on the back panel. When using
V
ext
or
INT,
a signal
applied to the
Sync In
input has no effect on the Chopper. In this setting, the signals
available on
OUT 1
and
OUT 2
are frequency- and phase-locked to the chopping
frequency measured using the photo-interrupt on the Chopper Head, F
outer
. Note