[Gb-ccb] Caltech Backend Telecon Monday 07 July 4pm EDT

[John Ford]

Martin Shepherd writes:
 > 

  <snip>
 > Rich Lacasse writes:
 > > Seems like a lot of this worry could be alleviated by using opto-isolators.
 > 
 > This is a suggestion that I made for the digital control signals in
 > the first meeting that we had at GB, but it was shot down. I would be
 > more than happy to consider it again, although for the analog detector
 > signals I would first need to convince myself that opto-isolators had
 > sufficient dynamic range, SNR, and linearity for our purposes.

I don't know if it's necessary or desirable to add optos into the
analog signals.  Seems quite attractive for noise abatement on the
digital signals.  If I recall the earlier meeting, we decided that the
optos were an unnecessary feature.  Maybe we should do it, since it is
pretty cheap and a definite win.

 > There is another possibility, one that eliminates the worry about the
 > time-domain response, provided that the filters settle within 25us. If
 > a step-function convolved with a smooth filtering function is a good
 > approximation to how phase-switch transitions appear at the output of
 > the anti-aliasing filter, then one can correct for the settling time,
 > simply by taking the difference between the sample preceding the
 > transition and the following sample, scaling this by a factor that
 > depends on the filtering function, then adding the result to the
 > following sample. Given that the two arms of the radiometer normally
 > will have very similar signals, this factor wouldn't need to be known
 > to great accuracy. In theory, we then wouldn't have to loose any time
 > to blanking around phase-switch transitions, which would be a great
 > advance compared to the analog integration approach.

In another mail, off the mailing list, I suggested that we could build
the detector preamp electronics with a frequency response matching
your digitizing system, i.e. build in the anti-aliasing upstream of
your digitizing system.  Dunno if this is practical in this case The
circuits we've used in the past would lend themselves to this.  I
think we could do it.

  <snip>

John