User name gilman
Log entry time 10:25:43 on October 23,1998
Entry number 3310
keyword=helicity documentation, as of 10/22/98
Updated documentation for helicity circuitry for FPP/GDH. R Gilman 10/23/98
This can be accessed also as
----- Helicity scheme -----
In original FPP runs, we used "toggle" mode, with 30 Hz of pulses,
33.3 ms + alternating with 33.3 ms minus. From the source we got 2
helicity correlated logical pulse signals, + and -, that were
complements of each other, and we sent both downstairs.
During GDH, we are running in pseudorandom mode. We get pairs of 1 s
long pulses. The second of each pair is the complement of the first,
but the phase of the first is chosen pseudorandomly. Thus the first
pulse of a pair may be the same as the second of the previou pair,
and apparent pulse widths vary from 1 to 2 s. We get two signals
from MCC, a correlated signal plus a clock signal. We use the correlated
signal to generate an h+ signal and the complement h- signal that we send
----- Cables to downstairs, hadron arm -----
The h+ and h- signals go downstairs to the equipment aisle through
patch 1H75B03, numbers 17+18.
The signals are connected with 4 foot BNC cables to the patch to the hadron
arm "nose", through numbers 9+10.
At the back of the detector stack, 10 foot BNC cables bring the signals to
patch 161B, numbers D5+D6, which takes them upstairs to the top level
of the stack.
----- Circuit in hadron arm -----
The signals sent downstairs are complements, and do not blank off the
first ~200 us, to give time for the helicity to stabilize and the electrons
to reach the hall. This blank-off period is introduced in electronics in the
hadron detector stack.
For both h+ and h- signals, the circuitry looks like the following...
(Note: diagram adjusted for going from fixed width to variable width font...)
linear fan out-----\
| | GO output
Phillips gate+delay | from TS
200 us delay | |
200 us long gate output | |
| | |
| | assorted level
| | shifting
| | |
| | |
| | |
veto input input b input c
LeCroy 465 coincidence module
linear output logical output (100 ns wide)
| | |
| | - unused -
for scalers _fan out_
_/ | | \_
_/ | | \_
_/ / | \_
_/ / | \_
_/ / | \
_/ ADC for backup ADC LeCroy level shifter&,^
| event for event channel 6+, 10- for
| helicity helicity input to scalers --
| tagging* tagging* level shifter output
| is fanned out through LeCroy ECL
to electron arm delay box to inputs to the MLU/TS
scaler and the helicity gated versions
% Since the scaler gate is an AND of run and good helicity, the helicity
gated scalers should reflect
* For the event data, the long helicity gates are:
h+ gate into channel 15 of the S1 and gas Cerenkov 4413 discriminators
h- gate into channel 16 of the S1 and gas Cerenkov 4413 discriminators
Experimentally we find that ~50% of events are +, ~50% are minus, a
small fraction are neither, and perhaps 1 in 10^6 events are both
helicities, a situation which should not happen.
& We originally used scaler channels 9 and 10, but this was shifted to
6 and 10 sometime. When GDH added the additional V-to-F scalers?
^ The scaler inputs used to be the short logical outputs of the
LeCroy 465 coincidence module; we do not know when this was changed.
I will attempt to change this back when there is time to check it out.
----- Cables to electron arm -----
To enable independent DAQ for hadron and electron arms, we need to bring
the first-200-us-blanked-off signals to the electron arm. The ~1 us
delay from hadron to electron arms should not matter.
The cabling is as follows:
From the top level of the stack, the cables go to the back of the stack
through patch panel 161B, numbers D7+D8.
Two ~10 foot cables then connect the helicity signals to the "hadron nose"
patch panel, cables 1 + 2.
Two short BNC cables connect the "hadron nose" channels 1 + 2 in the equipment
aisle to "electron nose" channels 3 + 4.
In the back of the electron spectrometer, we cable the "electron nose" to
patch panel Alfred D 13+16 to take the signals to the upper electronics
level. As of 10/22/98, we have seen that we get good complementary,
200-us-blanked-off signals in the electron arm; we have not identified
into which two ADCs to send the signals.