The Galileo Energetic Particles Detector

Galileo EPD Handbook

Chapter 1. Instrument Summary

Hardware Changes

Source: Stephen Jaskulek, October 2, 1987

• General TOF Sensor Hardware and Signal Flow - below; please scroll down
• Output High Voltage Reference
• Convert HV Reference Digital/Analog Conversion
• Switch High Voltage Input Power
• Switch TOF Input Power
• High Voltage Output Analog Monitor
• Polytwist Wire Allocation

General TOF Sensor Hardware and Signal Flow

The new TOF sensor and associated electronics will replace the present CMS unprime end telescope sensor and the TOF analog electronics built by NOAA.  The new TAC board will be built by APL (based on a previous MPAE Giotto design), and the rest of this new hardware is to be provided by MPAE.  APL will also build a new version of the CMS rate logic board, and modify a number of existing EPD boards.

The present plan is to take the start and stop signal from the microchannel plates (MCP1 and MCP2) and bring them directly into the new TAC (Time-to-Amplitude-Converter) board via coax cables. The board will create a bipolar pulse whose amplitude is proportional to the time between the start and stop pulses.  This waveform, which resembles that produced by the previous TOF circuitry, is run to the CMS analog electronics board via the same polytwist lines as the old TOF signal used; the signal processing is then performed as with the previous design.

The energy signal produced by the KT detector is processed by the analog chains previously used by the L and Jb detectors. Unlike the L detector, the KT detector requires a negative bias voltage to operate, due to the way it will be mounted relative to the telescope ground. Although the charge sensitive preamp previously worked with a positive input pulse, it can also process the negative pulse produced by the KT detector. The detector will be biased (via an additional divider network) with approximately -6 to -10 volts.

The shaping and amplifier circuit on the analog filter board's old L-channel is a noninverting stage, so that the result of a positive detector signal is a positive bipolar pulse.  Since we are reversing the detector signal polarity, we need to reverse the buffer amplifier's gain polarity (to avoid a negative-going output pulse).  This is done by taking the output of the old L-channel charge sensitive preamp (U5) and running it to the input of the old JB-channel shaping and amplifier stage, which is inverting.

The signal is sent from the preamp to the amplifier stages via the analog FET switch U6.  The prime side signals are multiplexed as before.

As with the TOF signal, the output of the KT analog chain is sent down the polytwist to the analog electronics board.  The signal is input to the analog electronics previously used by the Jc detectors, which are being eliminated in the new design.  The amplified log signal is sent to various discriminators, most of which were previously used with Jc signals  (thus reducing rewiring).

The discriminator signals will be combined with those from the TOF discriminators to form new TOF x E rate channels.  The channels will be defined by combinations of logic gates on the rate logic board.  This task turned out to be one of the more difficult in the new design, because the channel definitions are hardwired on the rate logic.  The changes were so extensive that we did not feel they could be safely implemented on the present flight CMS rate logic board (see discussion of rate channel below).

The Jc' and Kc' detectors are being replaced, respectively, by ones called Jb' and Kb'.  The new detectors will be the same thickness and size as the Ja' and Ka' detectors.  The only difference is the amount of active area exposed to space; the Jb'/Kb' detector pair will have a lower geometry factor.  The output of these detectors will be processed by the analog filter electronics previously used by Jc'/Kc'. The Jb' signal will then be input to the previous Jb circuitry on the analog electronics board downstairs; the Kb' signal will be summed with Ka' in the analog filter, as it was with the previous design.

The signals from the prime and TOF sensors were previously multiplexed through the same electronics, so only one sensor could be active at a time.  In the new configuration, each sensor has its own dedicated analog processing circuits, so no multiplexing is needed prior to the polytwist.  Although the downstairs analog and digital processing of the two sensors' data is still somewhat coupled, it appears possible to run both simultaneously, as well as separately. Accordingly, the new configuration supports operation in the single sensor mode, alternating sensor mode, and simultaneous mode.

The KT detector will be 10-15 mm thick, and will measure the total energy, less that lost from the front foil and detector dead layer, of incoming ions on the TOF x E sensor.  In the old CMS design, the TOF x E sensor is mounted on the 180° end of the CMS telescope, and is not shielded from radiation when the motor is in the calibration position (sector 0).  In the new sensor, this detector will be much more susceptible to radiation damage than the Ja' or Jb' detectors, since its lowest energy threshold (KTO @ 100 keV) is significantly lower than theirs (JA0 @ 660 keV).

A large radiation dose will therefore affect the TOF x E measurements more than the delta E x E measurements.  For this reason, we have decided to reverse the direction of the CMS telescope:  the delta E x E sensor (Ja'/Jb') will now be on the 180° end, and the TOF x E sensor will be on the 0° end (above the low-energy LEMMS sensor aperture).  The delta E x E sensor will still be referred to as the "prime side," and the TOF x E sensor the "unprime side."  This mechanical change will obviously affect the calibration shield design and radiation sources.

Continue: Output High Voltage Reference 

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