Introduction

The Paella card is a compact and flexible pulse height analyzer developed and used for several nuclear and elementary particle physics projects at UCL.

The input to the Paella card is the pulse shaping amplifier output. The Paella card digitizes the input, applies real-time digital processing to the signal, detects the peak amplitude (digitally), and bins this value in its histogramming memory, generating an energy spectrum. The spectrum is then transmitted over the USB interface to the user's computer.

The main blocks of the Paella card are:

• Digitazing block, based on the 20 MSPS, 12-bit ADC, AD9228;
• Signal processing block, implemented in a Cyclone III EP3C25 FPGA from Altera;
• USB interface, based on the EZ-USB FX2LP controller from Cypress.

The ADC output is processed continuously using a pipeline architecture. There are two parallel signal processing paths: the "fast" and "slow" channels, optimized to obtain different data about the incoming pulse train. The "slow" channel is optimized to obtain accurate pulse heights. The "fast" channel is optimized to obtain timing information: detecting pulses which overlap in the slow channel, measuring the incoming count rate, measuring pulse risetimes, etc.

The pulse selection logic rejects pulses for which an accurate measurement cannot be made. It includes pile-up rejection, risetime discrimination logic, etc.

The histogramming memory operates as in a traditional MCA. When a pulse occurs with a particular peak value, a counter in a corresponding memory location is incremented. The result is a histogram, an array containing, in each cell, the number of events with the corresponding peak value. This is the energy spectrum and is the primary output of the Paella card.

The unit also includes several counters, counting the total number of selected pulses but also counting input pulses, rejected events, etc.

The Paella card includes hardware and software to interface between these various functions and the user's computer. A primary function of the interface is to transmit the spectrum to the user. The interface also controls data acquisition, by starting and stopping the processing and by clearing the histogram memory.