Frequently Asked Questions about Signal Processing
Optical Choppers and Shutters
Can the 300CD be used with a 6 watt argon laser?
A: Yes. In a 1988 test at a university laboratory, the stationary
spokes of the chemically blackened 200-slot blade incorporated in the
Scitec Instruments 300HF-200 accessory showed a damage threshold (beginning
of discoloration) of 3 watts when irradiated by an Argon Ion Laser (UV,
351 and 363 mm) in a 2.25mm diameter spot for 2 minutes.
Three watts in a 2.25mm spot is about 75 watts/cm2. If the
beam were being chopped by a rotating chopper blade, the heating of the
blade by the laser would be only about 1% as great since the 2.25mm laser
spot traces out a circle on the blade equal to about 220mm long based
on the spot impinging on our 100m diameter blade at a radius of 35mm (typical).
Any irradiated spot on the rotating blade sees an average power of 0.75
watts/cm2 due to this duty cycle effect, under the same circumstances
in which a stationary blade encounters 75 watts/cm2 in one
For the rotating blade to encounter 75 watts/cm2 average power
at any point, the instantaneous power would have to be 7500 watts/cm2,
or 300 watts in a 2.25mm diameter spot.
Knowing that the fixed 200-slot blade in the 300HF-200 is of identical
construction to all our standard blackened surface rotating blades, we
can therefore estimate that the rotating blade should be unharmed by laser
power up to perhaps 7500 watts/cm2 in a 2.25mm spot. Gold coated
blades should withstand considerably high power densities; the user of
gold coated blades is cautioned to account for where the reflected energy
is going for safety’s sake.
Can the Scitec choppers be stopped in the open or closed position?
A: No. This feature has never been supported. At one
time, the blades were made of spring steel and it was possible to rig
an unorthodox setup whereby an electromagnet held the blade at the desired
position, but the blades are now made of brass.
Does the 350 40-slot blade come in any other material than brass?
A: Yes. There are stainless steel blades available.
Can the LED display of frequencies that comes with the 300CD be included
in a 300 OEM system?
A: Yes. The LED display,
part 300F, will plug straight into the 300 OEM version B, and can be added
to the 300 OEM version A if extra wires and a DPDT switch are added.
Q: Is a
one-slot disk available?
A: No. A one-slot disk is
not possible due to balance problems. For very low frequencies, use
On the 310CD systems, do the 200mm and 250mm disks use the same heads
and control units as the regular system?
A: The control units on the standard system are suitable for driving
the large chopper systems, but are normally tweaked to give approximately
10% faster output. The heads
(mounting) is modified to accommodate the larger disks.
Will gear reduction affect the life of the motor?
A: Not that we know of.
Will gear reduction affect the stability of the chopper?
A: Yes. The gearbox
head causes the disk to speed up and then slow down slightly once per
instability is a phase jitter about 10 times worse than otherwise.
the 410 or 420 be give a higher-voltage output?
A: Yes. See this application
note (pdf format).
Can a voltmeter with a digital readout be used with the Scitec 410
A: Yes. The 410 has a BNC
out with a +/- 1V signal, as well as an analog display.
How does the 410 lock-in amplifier compare to EG&G’s 5101 lock-in
A: The 410 was originally designed for a customer who was using
the 5101 but wanted a replacement.
The only specification difference we are aware of where the 5101
is better than the 410 is that the 5101 has a 1µV sensitivity,whereas
the 410 only goes down to 3µV. In
all other aspects, the 410 is the same or better.
Is a Scitec 441 lockin available for low temperature use?
A: The 441 has not been tested at low temperatures.
However, according to the manufacturers:
that we haven’t been able to test the 441 at low temperatures as the costs
for accessing an oven are prohibitive.
However, we have had experience of operating boards down to –50
deg C with a previous company. We
don’t believe there would be any problems with operating at –30 deg C.
All the resistors fitted to the board are 1% 50ppm/degC or better
with the input gain stage resistors being 0.1% 15ppm/deg.
The IC’s are a mixture of industrial temp parts and commercial
temp parts. However, after
speaking to one of the manufacturers of a commercial temp range part,
the only difference between the industrial and commercial temp range parts
was the testing they went through.
“We can arrange to have boards made with only
industrial temp range parts and with an additional conformal coat applied.
This should help with moisture problems at 0 deg C.
However, we will not be able to guarantee operation down to –30 deg
C without a large hike in price to pay for the necessary testing.
“We have been through all
the IC’s that are used on the 441 and checked their specified temperature
range. Fortunately, the
majority were already specified to –40 deg C.
A number do need to be changed but we can get hold of –40 deg
C parts within a week and they only cost a nominal amount more.
The y chip we are having problems with is an OPA602AP. It has a specification of –25 deg C which you may or may not
consider acceptable. There
is a part OPA602SM which is specified to –55 deg C but we can’t get
hold of it (nobody seems to have heard of it here).”
Can the 441 lock-in amplifier board be vertically mounted?
A: Yes. It is possible to purchase card mounts that allow a board to
be mounted on its edge – they are basically plastic pillars with a slot
cut into them which allows the board to slide into place.
For a greater cost, it is also possible to buy cases that will accept
a single eurocard board; again, the board will just slide into place.
What power requirements are necessary for the 441 lock-in amplifier?
A: +/-15V and ground
are required. The +5V input
can be simply powered off of the +15V line using a 78L0regulator.
The +5V line uses very little power, and so places little load
on the +15V line; the overall figure is still below 50mA. Typical actual
loads on the +/-15V lines in this configuration are 30 to 40mA.
Any power adapter with good ripple performance will do.
The 441 will also operate satisfactorily from +/-12V, but the dynamic
reserve will be reduced slightly due to the reduced headroom.
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