[TASS] Coming Up for Air #2

[Tom Droege <droege@WWA.COM>]

I feel much better.  Things are now working like they were six months or so
ago.  Sigh!  It has been a tough fight.  There were several problems at
once, and some were caused by me trying to "improve" things.

There is a pretty complicated signal chain.  When I changed the gain in the
camera head to try to improve the signal to noise, this caused the
differential receiver on the electronics board to see a larger signal from
the camera.  I finally put my oscilloscope probe on this amplifier's
summing junction, and saw something.  Summing junctions are supposed to be
pretty booring.  They should just sit there.  So seeing a spike showed that
the signal was overloading the range of the amplifier.  This was a subtile
problem, since everything still worked, but there was increased noise from
the loss of common mode rejection.  OK, I did check this when I first
turned things on, but not after the gain change.  Sigh!

I could not do a really proper fix.  I just reduced the gain of the
differential amplifier.  This sort of defeats the whole idea of increasing
the gain in the camera head.  It depends on where the noise is being
generated.

A gain and noise check now gives (at -20 C):
V   A=2.02   Noise = 15 e-
I    A=1.98   Noise = 20 e-

Computing what the gain should be assuming the manufactures gain of 3 uv
per e- is correct I would expect a gain of 2.4.  This is close enough
considering that there is an integrator in the path with a now well known
capacitor, and I am running with 20k instead of the manufacturer's
recommended 2k load resister.  (This give the right direction for a gain
change, I think.)

OK, I owe a lot of people stuff.  Hopefully I will get a package off to
Richard soon.  For Aaron I will send some Basic code, for others, I hope to
get some good data so that I can send out another round of CD ROMs with
requested stuff (and the drawings).  Give me another day or two, then I
will take requests again.

There is still a problem with ice crystals on the CCD.  This means I have
to run above zero where it is noisier.  But the noise here is still
dominated by the sky, so this will only be a problem for Arne, if for him.
I will work on a better seal of a camera head, and try to get the desiccant
working really well.

Meanwhile, day time I am back to assembling Arne.  My goal is sill to get
it running by the 21st of September so I can run it a month before I take
it apart and carry it out there.

Tom Droege