The
BUSCA CCD Controller CCU2
Overview
The CCU2 is a 4-channel system – one channel
per BUSCA-CCD. It consists of 2 units, the 19" rack at the BUSCA
instrument (with interface, sequencer and ADC modules) and the USB2.0 interface
box near the instrument control PC. A
single fiber pair connects the two.
Main characteristics of the CCU2 are:
o
The control of the detector operation is based
on a combination of microcontrollers and FPGAs (Field Programmable Gate
Arrays).
o
Read out rates of up to 0.66 Megasample per
second are technically possible. Rates between 30Ksamples and 250Ksamples are
available with predefines read out modes (so-called sequences).
o
Read out noise between 3 and 4 electrons (for
the new CCDS).
o
16 bits digitization is standard 18 bits is
possible
o
Static and dynamic voltages are fully
programmable with a time resolution of 20ns. Dozens of sequences can be stored
simultaneously. They are uploaded from the instrument control PC.
o
A set of standard sequences can be stored in
non-volatile memory as ``factory
default''. They are immediately available after power on.
o
Easy adaption of the analog signal chain to
various CCDs (i.e. CCD gains) and read out modes is possible with a
programmable pre-amplifier stage.
The
basic design.
The schematic below shows how the main
components of the CCU2 are related to each other:
o
For each CCD channel exists of a pair modules –
a sequencer and an ADC module.
-
The sequencer module provides all bias and
clock voltages for the CCD operation. All clock and bias voltages are
programmable (see command list in the appendix).
-
The ADC module does all the signal conditioning
necessary (DC levelling, filtering, gain) and the A/D conversion (18 Bits).
Gains can be set by command. No CDS takes place on this board. Instead multiple
samples of the Reset and the pixel levels are taken and sent to the interface
board FPGA where these samples are used to perform a digital CDS.
o
The interface module (left) performs i) digital
CDS for all channels, ii) multiplexing of data and control commands onto a glas
fiber, iii) exposure shutter control.
An extra RS232 connector (Dsub9, not shown in the scheamtics) is only for
engineering purposes. Usually control commands (ASCII) are multiplexed onto the
fiber together with the data. Important: As long as a command terminal is
connected to the extra connector, the command communication over fiber is
blocked.
o
Data and control commands are demultiplexed by
the "Interface Box" near the instrument control PC (busca1 or busca2)
. From there two connections exist: USB2.0 for the data and RS232 (9600,N,8,1) for commands.
The read out clocking for the 4 CCDs is
perfectly synchronised as all signals are derived from one master clock on the
interface board.
Hardware
overview.
The 19" rack contains these three board
types (left to right, see image below):
o
The interface board with a piggy back board
with FPGA and mC.
Connectors are for the fiber and control commands (RS232, 9 pin DSub) and for
the shutter signal (TTL).
o
The sequencer board. The analog part (e.g.
clock drivers) is on the main board. The piggy back board is identical to the
one on the interface module, but the programming is different of course.
Pattern generation occurs in the FPGA on the piggy-back.
o The ADC board.
The sequencer/ADC moduel pairs for each channel
are all identical by hardware. But they contain channel specific programming on
the piggy back boards. The base boards are exchangable. Although the ADC
modules are identical, small differences of the component properties may lead
to less than optimum behavior if they are exchanged among the channels: There
may be the need for a new tuning of the ADC board parameters (done by
programming).
Connections.
Connectors (see images below):
CCU2 19" rack frontside:
o
glas fiber for data and command communication
(100MHz Bit rate).
o
9 pin Dsub for command communication (only for
enineering purposes, if connected command communication via fiber is blocked).
CCU2 19" rack backside:
o
4 Connectors for CCD cables.
o
Connector for Shutter cable
USB2.0 interface box:
o
glas fiber for data and command communication
(connects to CCU2 19" rack).
o
9 pin Dsub for command communication (connects
to PC).
o
USB for data (connects to PC).
These images show the connectors at the front
and back sides of the 19" rack.