[mmaimcal] Band switching times revisited...

[Steven T. Myers]

Here is a "heads-up" on a probable complication for ALMA observing due
to the limitations in changing frequency bands.  Bryan reminded me that
there was this mysterious spec of 15min to switch between bands that
was circulated late last year.  He pursued this with the front end gang
and came up with the response from Darrel appended at the end of this
message.

In short, due to cooling capacity limits in the dewar, there will be only
two "hot" receiver cartridges at any given time - you can switch between
freqs in the hot bands at 1.5sec or faster (10ms for df/f<0.03% in-band).
However, it can take as long as 15min (possibly shorter, though we cannot
bank on it) to switch to a new "cold" band (meaning shutting off one of
the current hot Rx and warming up a cold one).  This means that WHICH
BANDS ARE ACTIVE AND WHICH ARE NEEDED FOR THE NEXT SCHEDULE MUST BE
ACCOUNTED FOR IN THE SCHEDULING.  Furthermore, SBs for a given experiment
that needs more than 2 bands will need to be scheduled in such a way as
to allow warm-up without undue loss of time.  Bryan suggests that this
would be a good use of subarrays for multi-band snapshot experiments.

At the very least I think there are some significant complications to
the dynamic scheduling we envision.  For example, during very good weather,
we would shade toward the high bands but you will likely want to pair with
low bands for calibration etc. so one would probably have one low band "hot"
for this, meaning you wouldn't want to switch often the upper band.  This
doesn't seem ideal but I guess its just another limitation we have to deal
with at this point.

Just thought I would pass this on, as I was not aware of this limitation
(and it is not yet in the Project Book).

  -Steve

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<included message>

On 2001.04.20 12:25 Darrel Emerson wrote:
Hi Bryan,
   I've received confirmation now from everyone except Wolfgang Wild
that my interpretation of the specs is correct. (Because of the time
difference, Wolfgang won't have seen my question yet.  But as John Payne
already agrees, I'm assuming Wolfgang will too.)  Larry's response was
also in agreement, although there were some semantic issues .  [Larry
prefers to think of the spec as system-wide spec of 1.5 seconds, but
with the 2 exceptions for (1) frequency switching and (4) where
frequency cartridges have been powered down.] Larry also correctly
pointed out that my "several minutes" to bring an off-line receiver (not
one of the standy-by receivers) into action is in fact set at "15
minutes," although we don't know how much better it really will be in
practice.

   It is likely that in practice all systems will exceed these
requirements, but this is what's set at the moment.

  My revised version of the specs, which should be put into the PB in
due course:

(1) The time to change frequency by <0.03%, as in frequency switching,
is
not more than 10 ms.  This is really a function of the LO system.

(2) The time to change frequency to a different receiver band, where the
change is foreseen, changing to a preselected different receiver
cartridge, is not more than 1.5 seconds.  This change will involve
removing LO signal from the previous receiver, powering up the LO signal
to this new receiver, switching over the IF output of the new receiver
to
replace the previous one, and changing antenna pointing to select the
new
receiver.  Not more than 2 receiver bands can be ready in standby
mode for this relatively quick response.  The pointing receiver,
probably 90 GHz, is likely to be one of the stand-by receivers available
(within 1.5 secs) at all times.

(3) The time to change frequency by an arbitrary amount within the same
receiver band is less than 1.5 seconds. 

(4) The time to change frequency to a different band, where the new band
is not preselected in preparation for the change, may be as long as
15 minutes.  As well as all the changes necessary in (2), this will
require time for thermal stabilization of the IF amplifier and other
components  which, because of cryogenic loading, will have been turned
off when not needed.  The precise warm-up time won't be known until
after cartridges are built, and
will probably vary from band to band.


                 Cheers,
                        Darrel.


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|:| Steven T. Myers                      |:|  Associate Scientist      |:|
|:| National Radio Astronomy Observatory |:|                           |:|
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|:| smyers at nrao.edu                      |:|  FAX: (505) 835-7027      |:|
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