SHARC II Electronics

Electronics Topics:

amplifiers
battery
analog control
multiplexer
A/D (analog-to-digital)
acquisition computer and DSP
thermometry
miscellaneous notes

Amplifier Electronics

SHARC II has 12 amplifier modules, each handling the signals from 32 bolometers.  There is a 13th enclosure which has the same shape as the amplifiers (but no multiplexer unit); this is used for thermometry.  The thermometry unit should never be swapped with the amplifiers.
It is labeled "Thermometry" and marked with green marker to distinguish it.

Each amplifier unit contains 8 amplifier cards, 0-1 bias cards, and 0-2 PID/thermometry cards.  The amplifier units have limited interchangeability due to differences in the configuration of those cards from unit to unit.  At this time, amplifiers should not be swapped without first consulting with Darren Dowell.  There is at least one spare amplifier unit in the lounge.

The amplifier attaches to the cryostat via an RFI box, so called because it has a 78-pin RFI-filtered D connector on its output.  The wiring internal to each RFI box connects in turn to the cryostat via a circular 79-pin connector which mates with the hermetic connector on the cryostat.  An RFI box can be removed by removing the screws holding it to the hermetic connector plate, lifting the box gently, and reaching under to de-mate the circular connector.  RFI boxes also have limited interchangeability.

List of amplifier channels disabled on amplifier cards due to lack of response or high noise (updated 2003 Aug. 26):
Row 1:  cols. 1, 2, 4, 32
Row 2:  col. 27
Row 3:
Row 4:  cols. 3, 4, 7, 14, 31
Row 5:  col. 1
Row 6:  col. 20
Row 7:  col. 32
Row 8:
Row 9:  col. 20
Row 10:
Row 11:
Row 12:  cols. 3, 8, 21, 24, 26

Amplifier Battery

The amplifier battery is located downstairs.  It is charged by a National Railway Supply unit.  The "charge" mode or "operate" mode can be selected with the downstairs control panel, or, more conveniently, the alidade control panel.

Analog Control Electronics

The analog electronics control chassis distributes power from the amplifier battery and routes programming signals from the sharcii computer and user to the amplifiers.  There are several switches on the front panel that control the operation of the unit.  The switch on the bias master should be set to divide by "2N+2" for normal operation.  All of the "high/low" switches on the slave cards should be set to "low".  The "on/off" switches on odd slaves should be set to "off"; the switches on even slaves should be set to "on".

Multiplexer Electronics

The outputs of each amplifier are routed through a 2-to-1 multiplexer.  The multiplexer control originates in the analog electronics control chassis and passes through a buffer before connecting to the multiplexers in a daisy-chain fashion.  The sharcii acquisition computer is made aware of the multiplexer state via a long cable from the second multiplexer buffer output.  The switch on the buffer box should be in the "normal" position.

A/D Electronics

The analog-to-digital converter collects analog signals from the amplifiers/multiplexers and sends digital signals via fiber optic cables to the sharcii computer.  In unusual circumstances (e.g., following power-up), the A/D converter malfunctions and does not provide correct signals to the DSP.  In such cases, pressing the red reset button on the front of the A/D should return the A/D to the proper state.  After pressing the reset button, it is essential to reset the DSP also.

The A/D power supply is downstairs and should always be powered on at 6.0 V and -7.0 V. The typical current draw is +2.7 A and -2.3 A.

Acquisition Computer and DSP

The sharcii computer resides in the Lounge.  From left to right, the boards in the computer are:
The IP address of sharcii is 128.171.86.12.

Thermometry Electronics

Except for JFET diodes and heaters, cryogenic thermometry and control is routed through the special thermometry enclosure, which is labeled and identified by green markings.  We currently read only two thermometers (detector GRT and a diode on one of two charcoal pumps in the 3He refrigerator), which is accomplished with the Neocera LTC-21 on the top of the rack.  The Neocera also heats the charcoal pumps in order to recycle the 3He refrigerator. Here is the manual for the LTC-21.

A motherboard diode interface is under test.  It routes diodes located in and read in selected amplifiers to the A/D converter.

Miscllaneous Notes

Here are some photos of the rack configuration in August 2003:
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