Boston Electronics Corporation
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Frequently Asked Questions about Signal Processing
Optical Choppers and Shutters
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Can the Scitec choppers be stopped in the open or closed position?
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Does the 350 40-slot blade come in any other material than brass?
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Why doesn’t the 300Synch work at higher frequencies (above 1000Hz)?
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Why doesn’t the 300Synch work at lower frequencies (below 10Hz)?
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Isn’t there anything you can do to make the 300Synch work in more situations?
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Is the 300Synch necessary to chop two beams with a desired phase difference?
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Can the LED display of frequencies that comes with the 300CD be included in a 300 OEM system?
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When are the Scitec 370 and 380 systems going to be available?
Lock-in Amplifiers
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Can a voltmeter with a digital readout be used with the Scitec 410 lockin?
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How does the 410 lock-in amplifier compare to EG&G’s 5101 lock-in amplifier?
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What power requirements are necessary for the 441 lock-in amplifier?
Q:
Can tuning fork choppers be coated with gold vanes?
A: Yes. The additional
cost is about $75. Please note that the "bright vanes"
option of the standard package uses shiny metal vanes which are sufficient
for some reflection purposes, and incur no additional cost.
Q:
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 spot.
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.
Q:
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.
Q:
Does the 350 40-slot blade come in any other material than brass?
A: Yes. There are stainless steel blades available.
Q:
How accurately can the phase be set on the 300Synch?
A: The
300Synch has no indication of its setting; an oscilloscope is necessary to
determine the phase difference.
Q:
Over what range of frequencies does the 300Synch function?
A: The synchronizer works best in the 10 to 1000Hz range.
Q:
Why doesn’t the 300Synch work at higher frequencies (above 1000Hz)?
A: Because the phase jitter of each chopper affects the
synchronization attempts, the effective phase jitter that the 300Synch deals
with in aligning two 30-slot blades is about +/- 30 degrees – high but
still tolerable in many applications. With
the 200-slot blade, you get about +/- 200 degrees phase jitter, which makes
it fairly useless.
Q:
Why doesn’t the 300Synch work at lower frequencies (below 10Hz)?
A: To get frequencies below 10Hz, you must use reduction gearing.
Reduction gearing results in greater jitter (by about 10 times), and
the infrequency of the signal means that the synchronizer is not getting
much information to use. Combined,
the two effects make it unfeasible for the 300Synch to align low-frequency
choppers.
Q:
Isn’t there anything you can do to make the 300Synch work in more
situations?
A: Yes, there is the possibility of switching to a digital solution,
but that isn’t happening right now. If your operation permits it,
try a shutter under TTL control for low frequencies.
Q:
Is the 300Synch necessary to chop two beams with a desired phase difference?
A: No. The best
solution is to mount both beams on the same chopper, shining the beams
through different parts of the disk. Of
course, this is not always possible.
Q:
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:
When are the 370 and 380 systems going to be available?
A: Unknown. The manufacturer, Scitec Instruments, has hit a few problems with the systems.
Namely,
1) Cost. The 380 disks are very expensive -- $500 to $1000 each.
2) Spacing. To avoid diffraction effects, the stationary and the spinning disks have to be very close together. No way has been discovered of accomplishing this yet.
3) Reference. It is not currently possible to produce a reference signal, although this could probably be overcome.
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
gear reduction
Q:
On the 300C/CD 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.
Q:
Will gear reduction affect the life of the motor?
A: Not that we know of.
Q:
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
revolution. Resultant
instability is a phase jitter about 10 times worse than otherwise.
Q: Can
the 410 or 420 be give a higher-voltage output?
A: Yes. See this application
note (pdf format).
Q:
Can a voltmeter with a digital readout be used with the Scitec 410
lockin?
A: Yes. The 410 has a BNC
out with a +/- 1V signal, as well as an analog display.
Q:
How does the 410 lock-in amplifier compare to EG&G’s 5101 lock-in
amplifier?
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.
Q:
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:
“We regret 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 only 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).”
Q:
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.
Q:
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 78L05 regulator.
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.
Q: My 500MC is
not working. What is wrong?
A: The 500MC IEEE interface is a rather old design and only supports
version 1 of the IEEE spec. If
you are using a GPIB card, please ensure that it is set to use version 1
protocols only. Alternatively, there may be a compatibility problem with
other devices on the bus. Try
disconnecting everything else from the IEEE bus while you troubleshoot.
Q: How do I know if
my 500MC is working?
A: Try writing to the 500MC. Try sending a “T1,3” command to change the time constant
setting. Does the interface LED
light? Does the error LED
light?
Q:
What is the typical operating current of a LIM-100?
A: For 5V operation, -25mA on the negative pole and +31mA on the
positive; for 12V operation, -25mA on the negative and +44mA on the
positive. The currents are
measured without input signal and without load at the output. Normally, the LIM-100 should not be damaged by reversing the
operating voltages if the current from the supply is below 1A.
Damage can occur by more than –15V on the “minus” or or more
than +14V at the "plus" power supply line.
Damage due to input overload is very unlikely.