From: Martin, Donald J. 
Sent: Friday, April 01, 2016 3:27 PM

The ACMs measure the charge per pulse in the Linac (and BTS).  The charge is 
averaged over 10 pulses and converted to average current by multiplying by 
the (measured) repetition rate of the external trigger.  The ACMs also include 
a housekeeping circuit that checks system integrity by injecting a simulated 
beam pulse into the current toroid when beam is not present.

The analog beam pulse is digitized at an 80 MHz rate, 400 samples per pulse, 
equivalent to 5 microseconds of integration duration.  The digitization begins 
with the rising edge of the Gate Width pulse.  The beam pulse must be within 
1.5 microseconds +/- 1.0 microsecond of the start of the Gate Width, or 
equivalently, a 0.5 microsecond delay time before beginning the acquisition, 
and a significantly larger buffer time for the acquisition to end.  The Gate 
Width extends to 40 microseconds.  The beam pulse is about 280 nanoseconds 
FWHM.  This requirement is specified in SAPE-078, Para. 1.7.2.  A similar gate 
occurs for the housekeeping signal.

This morning Steve Gierman and I measured the GATE WIDTH to X1 GAIN DELAY 
(beam) delay time for the Linac ACMs.  The measured values were 
ACM1 = 1.84 microseconds, 
ACM2 = 1.92 microseconds, 
ACM3 = 1.87 microseconds.  
The measurements are reasonably close to the norm of 1.5 microseconds and 
well within the allowed tolerance, +/- 1.0 microseconds.  Cable length 
changes of about 8-16 nanoseconds to the Linac Modulators trigger would not 
be a concern for the ACM accuracy.