Checking and Setting the Wobbler Frequency and Blanking Time
	Bill Peters					13 May 97

    The chopping subreflector frequency and blanking time are currently set
manually.  The frequency is adjusted with the function generator in the
leftmost rack in the Computer Room.  The frequency on the function generator
should be set to double the desired chopping rate.  (N.B., its knob is *not*
well calibrated.)  Before adjusting the frequency or blanking time, turn on the
subreflector chopping by releasing the START button in the Control Desk Rack
and choose a wobbling observing mode such as POINT or WSWITCH in OBST. 
(Chopping begins as soon as the LOAD button lights.  One must remember to
depress the START button again when ready to take observations.)  When
adjusting the frequency, one should monitor the subreflector position on an
oscilloscope and take care that a frequency is chosen which does not coincide
with one of the resonant frequencies of the telescope.  (Ones that cause the
subreflector to oscillate about the command position or cause excessive noise
and vibration during the chopping.) 

    The blanking time is set with the blanking generator unit in the rack to
the right of Drumbeat in the right receiver room (at about chest level).  It
has two sets of thumb wheels on its front panel which can select the phase
shift of Blank relative to the Sync input (top set) and the length of the
blanking pulse (bottom set).  Both thumb wheels specify times in milliseconds,
and only take effect after the Enable button is pressed. 

    For SIS receivers, the phase shift should be set to zero.  At some chopping
frequencies, the 4-color bolometer signal-to-noise ratio can be improved by a
phase shift. 

    (The narrower units to the right of the Blanking Generator are additional
sources of a fixed-width blank pulse of 50 ns which can be phase shifted 
relative to the Sync input signal.  One of these is used for the hot-sky
chopper mounted in front of the 4-color bolometer.) 

    The blanking time required is best judged by examining the subreflector
position monitor on an oscilloscope while it is chopping.  It is usually set
to 30-40 ms and is only slightly dependent upon the chopping amplitude.

    One can most accurately determine the chopping frequency by doing a 
POINTING observation and examining the Drumbeat output in the BEORGA window.
The numbers in the fourth column are the number of microseconds, Tchop, that 
Drumbeat integrated during one or more cycles of the subreflector.  The
number of cycles, Ncycle, is the product of the 3 components of the PREINTEG_C
adverb in OBST.  (Type SHOW DRUM or SHOW PREINTEG_C to see these.)  Tchop
excludes the blanking time, Tblank (ms), set above.  So the subreflector 
frequency, Wfrq (Hz), is given by

	Wfrq = 1./((Tchop/1000000)/Ncycle + 2*Tblank/1000)

    After measuring/changing Wfrq and Tblank, these should be specified to
OBST with either

	SET_DEFAULT WFRQ <Wfrq>
	SET-DEFAULT WBLANK <Tblank>
or
	WSWITCH /WFRQ <Wfrq> /WBLANK <Tblank> /DEFAULT
or
	WRASTER /WFRQ <Wfrq> /WBLANK <Tblank> /DEFAULT

These are used to calculate the AOS readout time per phase of the subreflector
chop.  Inaccurate values can result in a lower duty cycle if the time
calculated is too short or, if the time is too long, the integrations could
extend into the next blanking period or even into the next subreflector phase.