Newport New Focus 3502, Руководство пользователя

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16
3
Using the Chopper
3.1
How the Model 3502 Optical Chopper Works
The Model 3502 Optical Chopper is designed to interrupt light paths in optical
experiments at frequencies from 4 Hz to 10.65 kHz. Both single- and dual-beam
experiments can be performed across a broad range of chopping frequencies.
The Chopper has a crystal-controlled frequency synthesizer that serves as an
internal reference frequency for locking the Chopper to a particular chopping
frequency. Reference frequencies can also be provided through the
Sync In
BNC
connection to allow the Chopper to lock to an external source.
Several measures ensure that jitter and drift of the chopping frequency is
reduced to a minimum. Precision photo-etched wheels are mounted on a high
quality DC motor. The Model 3502 Optical Chopper Head has a photo-sensor
for monitoring the chopping frequency of the outer part of the wheel. The
Chopper Controller then actively stabilizes the motor speed to match the desired
chopping frequency. This technique minimizes phase noise at the chopping
frequency and provides for long-term stable chopping with minimal frequency
drift.
Figure 9 shows a block diagram of the Model 3502 Optical Chopper system.
Programmable divide/multiply circuitry allows for harmonic or subharmonic
locking of the Chopper to the reference frequency. In addition, the phase of the
chopping frequency may be varied over a -180 to +179 degree range with
respect to the reference frequency. A variety of TTL-level outputs are available
for use in triggering lock-in amplifiers, oscilloscopes, photon counters, or
boxcar averagers.
The chopping frequency, as well as a number of other operating parameters, can
be viewed on the front panel display. Easy-to-use cursor keys provide easy
adjustment of operating parameters. From the front panel the user can store and
recall up to nine instrument setups. A USB 2.0 interface provides remote
operation of all instrument functions.
The Chopper Head
can be mounted on a 1/2” -diameter post or bolted directly to
a standard optical bench. The Model 3502 Optical Chopper is supplied with six
Chopper wheels and a wheel cover. The Model 3510 Rack Mount Kit, sold
separately, enables the Chopper Controller to be mounted in a rack.