Position Encoding Module

(LPSD position decoding electronics)



  • Two nuclear-type pulse shaping amplifiers with computer controlled gains.
  • Computer controlled hardware LLD.
  • Two 16-bit ADCs with 2.5 usec pulse processing time.

  • Trimpot adjustable pole-zero input compensation.

  • Four front-panel output connectors for monitoring the Amp_A, Amp_B, Sum (Amp_A+Amp_B) and LLD outputs.

  • Two rear-panel amplifier input BNC connectors .

  • Computer controlled software LLD for LPSD position calculations.

  • LPSD data maintained in four internal 12-bit wide and 32-bit deep histograms containing the ADC_A, ADC_B, ADC_SUM and Position histograms.

  • Computer controlled histogram width: 7-bits < Histogram width < 12 bits.

  • Amplifier gains and LLD settings maintained by non-volatile memory

  • Connection to host via USB.

  • 1-wide NIM module


The Position Encoding Module (PEM) has been designed for processing the signals from charge division LPSDs. These units are packaged in 1-wide NIM enclosures and derive their power from a NIM bin and NIM power supply. Each module is equipped with two linear pulse-shaping amplifiers, a summing amplifier, an integral discriminator and two 16-bit successive approximation ADCs. A block diagram of the PEM is shown in the figure below which displays the typical arrangement of a single charge-division LPSD, connected to two preamplifiers, one at each end.

The detector preamplifier output signals are connected to each PEM through BNCs mounted on the rear panel of the NIM module. On the front face of the module are four output connectors: one each for the two amplifiers (Amp_A and Amp_B), one for the output of the Sum amplifier ( = Amp_A + Amp_B) and one for the output of the lower level integral discriminator (LLD). On the front face of the PEM are pole-zero adjustments (20-turn trimpots) for Amp_A and for Amp_B and two LED indicators: one that shows “events” (signals in the Sum amplifier that exceed the LLD) and another that indicates USB-Bus activity. The gains of Amp_A and Amp_B, the threshold of the LLD and the zero-offset controls for ADC_A and ADC_B are adjustable via command of the control computer with 10-bit resolution.

The PEM amplifiers have been designed to match the pulse shaping and output amplitude range of the hybrid circuit preamplifiers used for LPSDs, which produce a 10-30 nsec rise time pulse with a exponential fall time (~50 micro sec). The signals from the preamplifiers for neutron events in the detector (at ~2000 V bias) are approximately 200 mv for neutrons near the maximum in the neutron pulse height spectrum. The gain of the PEM amplifiers is adjustable over a 2:1 range under the control of the host computer. The exact range of gain settings needed for the operation of the PEM with the detector will depend on the bias voltage used, and the characteristics of the detectors. The range of gain settings available with the digital controls can be altered by the selection of internal gain setting resistors.

The PEM is normally operated with the outputs of Amp_A and Amp_B balanced (equal) and signals from the Sum amplifier that exceed the LLD cause the Amp_A and Amp_B signals to be digitized by the 16-bit ADCs, ADC_A and ADC_B. Data acquisition can be for a fixed time (with a clock maintained internally on each PEM) or it can be under control of the host computer. During data acquisition, the PEM maintains the histograms for ADC_A, ADC_B, ADC_Sum and Position (ADC_B/(ADC_Sum)). The histograms are all 4-bytes deep. In addition, during data acquisition, the PEM maintains a 4-byte counter that records the number of events that exceed the LLD.

The PEM is equipped with Base Line Restoration and Pole Zero Correction. The peaking time for the analog signal is 1.5 msec a baseline width (at 1% of the amplitude) of 3.6 msec. The PEM ADCs digitize the input pulse in 10 msec with an additional 2 msec cycle time for a total of 12 msec/pulse. Experiments with the PEM demonstrate that the system is capable of processing signals at a synchronous rate of 66 kHz.

It is possible to perform a first-order dead-time correction to the PEM data to recover the pulses lost at high input count rates. The Sum histogram records (in the top histogram channel) the number of events where either the Sum signal or ADC_A or ADC_B has exceeded the range of the ADC. These signals are rejected and their position is not calculated or histogrammed.

Let  f = (Sum(Position histogram) + the number of signals rejected)/(LLD events). This represents the fraction of the incident neutrons analyzed. The quantity (1-f) is the fraction lost by the signal processing electronics. The fraction of events lost has the same position distribution as those actually recorded so a first order dead time correction to the data is formed by multiplying the contents of each histogram cell by 1/f.

For high-resolution applications, the 16-bit ADC data is truncated to 12 bits when the data is histogrammed and the system maintains 4 12-bit wide 4-byte deep histograms internally. For lower resolution requirements, the data can be further truncated. If a different resolution were to be desired, changes in resolution can be accomplished with alterations of the PEM firmware.

Communication between the PEM and the host (control) computer is by USB-bus. Up to 127 devices can communicate with the host through one USB input connector. Each PEM has a unique serial number recorded in an on-board Serial-EEProm that can be used by the customer's control software to associate a particular PEM with a particular detector element. A driver, suitable for use with Microsoft  Windows XP through Windows 8 is supplied with the PEMs.

The driver provides the functionality required to:

Add the PEM to the list of devices on the USB bus

Load the firmware that sets the characteristics of the PEM

Control the level of the amplifiers, ADC offsets and LLD threshold.

Specify the time for data acquisition

Clear the histograms and event counter

Start and stop data acquisition.

Set the histogram resolution between 7-bits and 12-bits.

Return the selected histogram to the host computer.

The USB specification provides for a raw transmission speed of 12 Mbit/sec. In addition to the device driver, a Java test program, suitable for exercising the PEM and providing an example use of the driver is supplied.