When B+ and B- are set to 0V, the
voltages we see at V+ and V- are just the JFET offset voltages, which
should be well matched (within 10-20 mV of each other).
During normal operation, B+ and B- are set to equal and opposite
values. If Rb is nonzero, equal and opposite voltage appear its
two ends. The JFETs, acting as following, communicate this signal
to their outputs, so V+ and V- move up and down by equal but opposite
amounts, respectively. The signal we read is the difference
between the two.
When the bolometers are warm, Rb ~ 0Ω and no change will be seen in V+
and V- if B+ and B- are varied in a symmetric fashion. However,
by tying B+ or B- to zero and varying the other to some voltage Vb, the
bolometer and the JFET gats move up by Vb/2. We expect both JFET
output voltages V+ and V- to increase (or decrease) by equal and
Thus, using asymmetric biasing, we can test whether the JFETs are
working and the bolometer is connected even when Rb = 0. We
cannot distinguish short-circuited bolometers from working bolometers,
but all other failure modes -- open-circuit bolometers, non-functional
JFETs, or broken JFET output lines -- are visible.
Do the test as follows. It is easiest to do all three bias
settings for each hextant and then cycle through hextants.
- You will need to monitor the bias being sent into the dewar. To
do so, connect clip leads from pins 1 (B-) and 2 (B+) of the AD624 chip
on the bias board corresponding to the hextant you want to check and
connect to a DMM. There are six such bias monitor circuits on the
board; they should be labelled "1", "2", and so on. The AD624s should
be obvious from their gold-plated
packages. The chips of interest are in the top half of
- You will need to alternately ground the + and - sides of the
bolometer bias in order to send an asymmetric bias into the
dewar. This is done by attaching clip leads from the board ground
to either the B+ or B- bolometer bias lines. It is easiest to do this, and least likely to cause
damage, by attaching a chip clip to the AD624 for the hextant bias
monitor and grounding either pin 2 (B+) or pin 1 (B-).
- You will need to
measure the voltages of all the JFET source lines relative to ground
(V+ and V- in the above picture). We normally do this by connecting the
cable G DB50 connector of the hextant of interest to a breakout box
that converts from DB50 to 25 isolated BNCs (you may want to use a
DB50M-DB50F extension cable so you put the breakout box in a convenient
place). Each source line pair goes to one BNC (center conductor
and shield) and pins 1 and 34 go to the 25th BNC.
- Put the bias board
into adjustable DC bias mode; see the Electronics page for
- To do the bias = 0 V measurement, set the bolometer bias to 0
using the front panel knobs and ground B-. It may not seem necessary to
ground B- for the 0V measurement, but it is necessary because voltage
offsets appear when one side of the bias is grounded.
- Measure the voltages at all the source lines with respect to
ground. These voltages are called the "JFET offsets" -- these are the
source line output voltages when the gates are at 0V. Source lines that read
out the same bolometer should have offset voltages within 10 mV of each
other because the JFETs are on the same die. If the voltages are much
larger than 10 mV apart, then either the JFET is problematic or you
have a measurement problem. Record all the source line voltages. There
is a checkout sheet available for recording these voltages: ps, pdf. (Print out 6
copies of the second page). Record these values in the bias = 0 mV column of the checkout
NOTE: The offsets are temperature dependent. It takes about 20 minutes to do the full set of
measurements on a single module. So you have to make sure your JFET
stage temperature has stabilized sufficiently that the offsets are not
drifting while you are trying to
make your measurement.
- Vary the bias so
that you see 40 mV at your bias voltage monitor. You have grounded the
- side of the bias, so you are applying 0V at the B- line and
+40mV at the B+ line. As explained above, this should raise the
voltage of the bolometer by +20 mV, which should cause both the V+ and
V- JFET outputs of each pair to go up by 20 mV. Record
these values in the bias = +40 mV
column of the checkout sheet.
- Unground B- and ground B+. Do not change the bias
setting. You should now be sending 0V to B+ line and -40 mV to
B-, so the JFET gates are held at -20 mV and the JFET source lines
should all be 20 mV below their 0V values. Record all the source
- Disconnect the
ground clip and the bias monitor clip. Set the bias to 0V.
- Repeat for each hextant.
- Once you are finished, tape the sheets into the Bolocam
logbook. If you will be doing both warm (JFETs at 300K) and cold
(JFETs at 140K) tests, leave space on facing pages for both the warm
and cold sheets for each hextant so it is easy to compare whether
things change with temperature.
Any channels that did not
respond to bias as expected should be noted. You can refer to the
previous time the test was done (just look back in the logbook) for the
known bad channels. If you have time, try to get in touch with
the Bolocam support person
to go over
any unexpected bad channels.