[Gb-ccb] Minutes from 07jul03 videocon

[Brian Mason]

All- please have a look at this and let me know if you have any
additions or changes.

 cheers,
  Brian

Caltech Continuum Backend Videocon
Monday 07jul03

present: Tim Pearson, Tony Readhead, Martin Shepherd, Phil Jewell,
Brian Mason, John Ford, Galen Watts, Rich Lacasse, Roger Norrod


Background: We held the videocon in order to provide feedback on a
number of specific issues that Caltech had raised now that they are
getting on with the hardware design.  The main issues were: the
overall architecture change (to a faster, lower-resolution ADC further
up in the signal chain, followed by low pass filters and integrating
in subsequent FPGAs); cabling, wiring, and connectorization strategy.

Roger points out that if we're sampling at 100ns (new scheme) then the
time constant of the post-detector video amps must be fast ("fast"
meaning the step response of 1 usec, or a few MHz bandwidth).  However
the slew times of the post detector op-amps certainly limited WMAP's
switching rate-- perhaps we can use better components than them.
Clarifying our earlier definition of "fast" we said the step response
should be 0.5 usec, settling to -84dB at 1 usec (-84 dB is a part in
4000 that is, 12 bits).  Generally the properties of this circuit may
be key and the circuit is as yet undesigned; GB should have someone
look at a design and Spice simulations soon, but indicated that it is
ok for Caltech to plan on us meeting this spec.  GB also indicated
that it was acceptable and sensible to plan on putting video filters
on our side of the interface, probably in the Rx before the
wiring/connectors running between Rx and Backend.  Brian indicated
that 1 usec blanking out of 25 usec is acceptable and we don't need to
push too hard to reduce this (although if feasily feasible that's
obviously good).  Rich stated a rule of thumb that we have for our
backends which I didn't quite get down clearly (the signal is down by
15 dB at the nyquist frequency, which results in aliased signal being
30 dB down?)-- and wonders whether the aliased stuff is signal or
noise (worst case: some sort of systematic noise which doesn't average
down).

The full-scale voltage of the ADC being considered is 2.5 Volts.  We
indicated to Caltech that from our point of view, the signal levels
are open to specification by them. The ADC is clocked at 10 MHz
(5xNyquist).

In response to Martin's query about the detector noise budget, we
decided that it was fair to assume that the noise budget is dominated
by the receiver.

We decided that differentially driven, individually shielded, twisted
pairs were good for the analog signals.  The Skewclear cables Martin
has found look sensible.  These are D-connector-izable. Since the
signals are differential crosstalk will be lower; video filters in the
receiver will also reduce the crosstalk.

Discussion of the control signals was somewhat less conclusive.  Will
we use LVDS or TTL?  Not clear, although we think that either could be
used.  We currently use mostly TTL although this needn't be a
constraint as there will simply be a multiplexer/lockout card that
both (CCB and switching signal generator) signals feed into for the
receiver.  Are both TTL and LVDS compatible with using the opto
isolators? Not clear.  How fast can the opto-isolators be driven?
Martin will look into the last question. We also need to clearly
determine the bandwidth required for them.  Brian claims that
according to our initial design & specifications no individual
component in the receiver is toggled faster than every 50 usec.  The
backend may allow driving all the switches every 25 usec but there is
no intent to do this (intent is to drive them alternately). The
specific bandwidth requirement then depends on how the control signals
are combined and sent.  A sketch of the control signals would be
helpful and GB will produce this.  It should be kept in mind that the
3mm Rx has half as many phase switches (2 not 4), and half as many cal
diodes (1 not 2), as the 1cm Rx.

As to interconnection: the general feeling was that more connectors
rather than fewer was good: this permits empirically optimizing the
grounding and bundling scheme, and presents more options for
troubleshooting should they be needed.  The specifics of whether we
have four bundles of four cables each, or 16 individual cables, is
open, and probably not critical.  It should be kept in mind that the
3mm Rx has 12 not 16 detectors (3 groups of 4, not 4 groups of 4).

Galen asked if the phase switches and cal diodes will be driven
directly by the backend, and they will.

Tony readhead inquired as to the front end schedule.  Galen Watts is
hard at work on the receiver and we plan to have it ready in the lab
by 01jan04.  Tony also requested that if possible we get back to them
within a month or two on the software which has been delivered; and
indicated that it would be sensible to have someone from JPL
participate in the CCB CDR since our current efforts have some overlap
with the 91 element MMIC array under development.

Martin asked if we have some video filters lying around which we could
have a quick look at.  We do, and we might be able to have a look at
them; failing that we could probably give some pointers.

We will touch base in a week or two to see when is a good time for the
next videocon.

Summary of Actions
==================

*Martin: check opto isolator bandwidth.

*???: look at post detector circuit design.

*Galen: sketch control signals for 1cm Rx.  Run this by Mike for obvious
deltas to 3mm.

*Rich: see if we have a representative video filter lying around and
do a test or send Caltech the part number (or the part).

*Brian: follow up on software with GB software development division.


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Brian Mason                           | office: +1(304)456-2338
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