P
Phil Hobbs
Hi, all,
I don't have much to contribute to the LCR MOSFET argument except
possibly some ethological insight:
<http://electrooptical.net/papers/TerritorialityInTheWhiteRhinoceros.pdf> .
But I digress.
I'm on a trip to SoCal to debug the pre-production models of my
spectrometer. The SNR is pretty good, way over 60 dB (which is good for
a SWIR spectrometer).
Looks like the last remaining problem is that the spectrum has a way of
sort of swimming around a bit, i.e. the gross shape stays roughly the
same, and the small scale noise is low, but there are small systematic
variations on scales of 1/10 to 1/2 of the scan range, where multiple
spectra don't quite line up with each other.
One thing I noticed is that the Hall effect shaft encoders are quite
unstable with temperature, something like 3 arc min per kelvin. Since
the grating only has to rotate about 8 degrees for the whole
measurement, that winds up being, like, 3 nm/K, which is _horrible_.
(They're US Digital type MAE3-P12-125-500-7-1: 12-bit PWM.)
The RC airplane servo that I used in the proto appeared much more stable
on the medium scale motion, although it didn't do small steps very
repeatably. It used a pot for the encoder, of course.
Right now we're working on putting a temperature controller on the
encoder, mostly by thermally grounding the leads to a controlled plate,
since the case is plastic and there's no contact between the encoder
body and the shaft.
Is the tempco usually this bad?
Any additional wisdom on these things? We can rip out the encoder and
bodge in a pot if we have to, but it'll limit the instrument lifetime
fairly severely.
Thanks
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
I don't have much to contribute to the LCR MOSFET argument except
possibly some ethological insight:
<http://electrooptical.net/papers/TerritorialityInTheWhiteRhinoceros.pdf> .
But I digress.
I'm on a trip to SoCal to debug the pre-production models of my
spectrometer. The SNR is pretty good, way over 60 dB (which is good for
a SWIR spectrometer).
Looks like the last remaining problem is that the spectrum has a way of
sort of swimming around a bit, i.e. the gross shape stays roughly the
same, and the small scale noise is low, but there are small systematic
variations on scales of 1/10 to 1/2 of the scan range, where multiple
spectra don't quite line up with each other.
One thing I noticed is that the Hall effect shaft encoders are quite
unstable with temperature, something like 3 arc min per kelvin. Since
the grating only has to rotate about 8 degrees for the whole
measurement, that winds up being, like, 3 nm/K, which is _horrible_.
(They're US Digital type MAE3-P12-125-500-7-1: 12-bit PWM.)
The RC airplane servo that I used in the proto appeared much more stable
on the medium scale motion, although it didn't do small steps very
repeatably. It used a pot for the encoder, of course.
Right now we're working on putting a temperature controller on the
encoder, mostly by thermally grounding the leads to a controlled plate,
since the case is plastic and there's no contact between the encoder
body and the shaft.
Is the tempco usually this bad?
Any additional wisdom on these things? We can rip out the encoder and
bodge in a pot if we have to, but it'll limit the instrument lifetime
fairly severely.
Thanks
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net