The Wideband Arecibo Pulsar Processor Home Page

WAPP - Wideband Arecibo Pulsar Processor

• Introduction Modes
• Mode Table and Limits
  • IF Modes
  • Bandwidth

Introduction

The WAPP is a new facility pulsar backend for Arecibo Observatory. In its initial implementation, it will handle wideband pulsar observations with particular utility as a search machine or for single pulse work. This instrument represents the first operational stage of an on-going development program. It is our intention to expand and improve the initial design, however this document will only consider the system that has been (or soon will be) released for general use.

The WAPP does incoherent pulsar detection. It uses a Linux computer to rapidly unload a hardware digital correlator. The resulting correlation functions are written to disk as 16 bit unsigned integers with minimal processing. The initial system will have two IF channels of up to 100 MHz each. Nominally these channels will be configured for dual polarization observations. It is possible to measure autocorrelations from a single band or from both bands simultaneously. Also, it is possible to measure cross-correlations to produce stokes parameters. Figure 1 gives a block diagram of the WAPP

Modes

The WAPP has several configurations to adjust it's operating parameters to match the desired scientific goals

IF Modes

WAPP has two IF channels which are normally configured to measured othogonal polarizations from the same receiver. By changing the configuration of the WAPP, its possible to measure the power spectrum of a single IF, both IF simultaneously or all auto and cross productes to create a full stokes measurement. In addition, it is possible to combine the IFs after correlation (post-detection). This mode ("Summed IFs") is very useful for search, however because the signals are combined as raw correlations, there is a small residual non-linearity because the quantization correction is not applied before summing.

Bandwidth

Both IFs are bandpass filtered to avoid aliasing during sampling. By reconfiguring the correlator, it is possible to directly process IFs of 100 MHz or 50 MHz. Both modes are supported by RF filters. Thus, WAPP has two bandwidth modes: 100 MHz and 50 MHz. There are a number of tradeoffs that occur when selecting the bandwidth. For example, becuase the 100 MHz mode is effective sampled at twice the rate, it will tend to overflow it's buffers twice as fast. Also the 100 MHz mode requires four times as many correlator chips. This means some combinates of modes can not be implemented such as 9 level, 2 IF, 100 MHz. This requires more than the available 16 correlator chips.

Quantization Level

The raw voltage signal from the IFs are sampled with an 8 bit A/D converter (Analog Device AD9054). However the implementation of the correlator multipliation table is limited to 1.5 bit (3 level) or by combining several chips it is possible to create a 3.125 bit (9 level) sample of the input voltage.

16/32 Bit Data Files

Raw correlation values receive only minimal processing after they are read from the correlator chips. In general, they are simply re-order and combined to create "lag-continuous" correlation functions; in other words, sequenced in a nature order from lag 0 to N. This data is then written to disk in two possible forms: 16 bit or 32 bit modes. The 16 bit correlations data uses 2 bytes, 16 bits to represent the raw correlations. It follows that 32 bit mode uses 4 bytes to represent the correlation values

Data Acqusition Limits

Modes Table and Limits

This document maintained by wapp@naic.edu.
Material Copyright © 2000 Arecibo Observatory - Cornell University