| |
MR. SQUID® FAQ
Download
PDF Version
(115 KB)
|
MR. SQUID® Operation
The output of my Mr. SQUID® unit is a straight line or a
slight curve with no flat region in the center of the trace
as shown in the manual. What can I do?
If you do not observe any zero-voltage
supercurrent (flat, horizontal region in the V-I characteristic
at zero volts), it is possible that the critical current of
the junctions has been suppressed by external noise sources
or the junctions have trapped magnetic flux. To check this possibility,
adjust the oscilloscope to display the V-I characteristic while
the probe is cooled in liquid nitrogen. Remove the probe and
note what happens to the characteristic as the probe warms.
At some point, the curve should suddenly rotate and become nearly
vertical. When this happens, turn the bias amplitude to zero
and adjust the bias offset to roughly center the oscilloscope
display. Now cool the probe again and wait until the probe is
sufficiently cold (the display should jump to zero when the
SQUID returns to the superconducting state). Now increase the
amplitude to display the full V-I characteristic.
If there still is no supercurrent, it is possible that the critical
current is being suppresses by ambient Electromagnetic Interference
(EMI). A nearby microwave oven, cordless telephone, or other
electronic equipment may cause EMI and prevent the SQUID from
operating normally. If this is the case, turn off this equipment
and try re-cooling the SQUID again.
If the above two steps do not improve operation, another possibility
is that the magnetic shield has become magnetized. Demagnetize
the shield using a demagnetizing coil (available from Radio
Shack, for example), reinstall the shield on the probe and re-cool.
If a supercurrent is still not visible, complete the resistance
tests below and contact Customer Service at STARCryo for further
support.
How should I cool Mr. SQUID® to avoid trapping flux in the
junctions?
Connect the X (current) output
of the Mr. SQUID® electronics box to the horizontal input
on the oscilloscope, the Y (voltage) output to the vertical
input on the oscilloscope. Set the FLUX BIAS and CURRENT BIAS
offset pots to the 12 O’clock position and turn the AMPLITUDE
pot to its fully counter-clockwise position. Select the V-I
mode and connect the Mr. SQUID cable to the back of the electronics
box, but do not connect the cable to the Mr. SQUID® probe.
Turn on the power to the Mr. SQUID® electronics box, then
plug the 9-pin cable into the connector on the Mr. SQUID®
probe. Adjust the CURRENT BIAS offset to move the “dot”
on the oscilloscope display to roughly the center of the display.
It may be helpful to adjust the vertical gain to around 1 V/div.
Cool the Mr. SQUID® probe in liquid nitrogen. Once cold,
the “dot” on the oscilloscope display should jump
to zero voltage. Now increase the AMPLITUDE to see the SQUID
V-I characteristic.
The voltage output of my Mr. SQUID® electronics appears
to have a small offset – how can I fix this?
The flat portion of the SQUID
V-I characteristic should be at zero volts. If it is not, you
can adjust the amplifier offset pot on the amplifier board inside
the Mr. SQUID® electronics box. This may be done with the
Mr. SQUID® probe cold while monitoring the V-I characteristic.
Check the zero setting of the oscilloscope by grounding the
input for the voltage channel. Open the cover of the Mr. SQUID®
electronics box and locate the trim pot on the amplifier board
at the rear panel (there is only one trim pot inside the electronics
box). Using a small screwdriver, adjust the trim pot to shift
the V-I characteristic until the flat portion of the curve is
at zero volts. Replace the cover to the electronics box.
The output of my Mr. SQUID®
unit is just 9 Volts all the time. What can I do?
This symptom can have a number of different causes:
1. Inspect the cable connections and make sure they are tight.
2. Try replacing the batteries in the Mr. SQUID® electronics
box. Sometimes one of the batteries can be too low yet the battery
check light may still indicate that the batteries are good.
This is
because the battery check circuit looks at the total voltage
from the two batteries, but not the balance between the two
batteries.
3. If neither of the above helps, then you will need to contact
customer support. But before doing so, check the Mr. SQUID®
probe itself using a digital voltmeter. Using a digital voltmeter,
measure the resistance, at room temperature and at 77K, of the
following pin combinations on the DB-9 connector at the top
of the probe;
|
Terminals
|
Room
Temp
|
Liquid
Nitrogen Temp
|
| Pins 1 and 2 |
|
|
| Pins 3 and 4 |
|
|
| Pins 5 and 6 |
|
|
| Pins 7 and 8 |
|
|
| Pins 1 and 3 |
|
|
| Pins 5 and 3 |
|
|
| Pins 7 and 3 |
|
|
If the resistance between pins 3 and 4 is higher than 500 Ohms
at room temperature, substitute pin 4 for pin 3 for the last
three measurements. Have these measurements available when you
contact Customer Service.
Repairing Mr. SQUID®
Will STAR Cryoelectronics refurbish Mr. SQUIDs that are out
of warranty?
Yes. Contact info@starcryo.com
for pricing and information.
|
|
