Filterbank Systems

This document briefly describes the main configurations available for pulsar observations using the filterbanks and multichannel digitiser at Parkes. All of these systems were constructed at the University of Manchester, Jodrell Bank Observatory.

The Multibeam Filterbank

The Parkes HI Multibeam receiver has 13 beams, each with two polarizations. See the Multibeam Receiver page for more details. Signals from these 26 channels are down-converted and sent to the pulsar Multibeam filterbank system in the control room. This provides 2 x 96 x 3 MHz channels for each beam. Signals from the centre beam are tapped off and can be sent to other filterbank systems or the Caltech correlator.

For pulsar observations, the 1st LO is normally set to 1582 MHz. There is a doubler in the focus cabin, so the reference frequency (normally set on the Rohde & Schwarz signal generator in Rack 5) is 791 MHz; the level is +11 dBm.

IF signals for the 13 beams and polarizations A and B come from the focus cabin to the Multibeam Cable Equalizer in Rack G. This corrects for the frequency-dependent cable loss and provides level setting in 1 db steps separately for each beam and polarization. It also splits off the signals for the Multibeam correlator. See Multibeam pulsar system for a block diagram of the system.

The Multibeam filterbank system accepts IFs in the range 1230 - 1518 MHz (total bandwidth 288 MHz for each polarization). Each beam and polarization is split into three subbands, each of which is downconverted to 64 - 160 MHz for input to the filterbank modules by the Multibeam up-down converter and band-splitter. This requires three LOs from the Multibeam LO module. Details of the frequency relations are given Frequency Relations. Input connections at the rear of the band-splitter are shown in Band splitters - back, and output plus LO connnections in the front are shown in Band splitters - front.

The 78 cables from the band-splitters go to the filterbanks in Racks E and F. Connections at the rear of each filterbank are shown in Filterbank Assignments. Ribbon cables take the filterbank outputs to the digitizer (see below). Sockets on the rear of the filterbanks are in parallel pairs, that is, sockets 1 and 2 of Crate A both have the polarization-summed outputs from the lowest subband (centred at 112 MHz) of beam 1. Each ribbon cable carries 32 signals to the digitizer.

Other Filterbanks

Several standard configurations are possible with the currently available 128 x 1.0 MHz, 384 x 0.5 MHz, 256 x 0.125 MHz and 128 x 0.25 MHz filterbanks. The narrow-band systems use the "640-32 MHz Downconverter" unit to pre-condition the signals. They can be used independently or in combination with the 0.5 MHz system. Summing circuits are provided to produce a 512 x 0.5 MHz by combining all of these filterbanks. The 512 x 0.5 MHz and 256 x 0.125 MHz systems are permanently connected to the digitiser at start addresses 40 and 56, respectively. Standard config files for PMCTRL are available: PMtmhr.cfg for 20cm 512 x 0.5 MHz, 50cm.cfg for 50cm 256 x 0.125 MHz, and 70cm.cfg for 70cm 256 x 0.125 MHz.

The 640-32 downconverter is particularly flexible and will convert suitably filtered 32 MHz-wide bands to a centre frequency of 32 MHz. The table below gives the general relation between the frequencies, together with those used for the 436 MHz and 660 MHz systems:

Input Band (MHz)(f-100)-(f-68)420-452 644-676
Output Band (MHz)16-4816-4816-48
LO1 (MHz)f520744
LO2 (MHz)116116116

The 32/0.25 MHz system

2*128*0.25 MHz, covering a total of 32 MHz. As well as the 128 output channels for each input polarisation, the outputs of adjacent frequency channels can be summed in polarisation and in pairs or fours to provide 0.5 MHz or 1.0 MHz channels. This filterbank is in a single unit which accepts 4 IF bands of 4-12 MHz, provided by a downconverter which accepts a single IF band of 16-48 MHz. Note that the frequency order of the output channels can be reversed by simply inverting the order of the input IF cables from the downconverter. All outputs are available from 37-way D-type connectors at the rear on pins 1-32 in order, with earth return provided on pins 33-37.

The 32/0.125 MHz system

2*256*0.125 MHz, covering a total of 32 MHz. As well as the 256 output channels for each input polarisation, there are 128 and 64 outputs obtained by summing adjacent frequency channels in polarisation and in pairs and fours to provide channels of 0.25 MHz and 0.5 MHz. This filterbank is in two units which together accept 4 IF bands of 4-12 MHz, provided by a downconverter which accepts a single IF band of 16-48 MHz. Note that the frequency order of the output channels can be reversed by simply inverting the order of the IF cables from the downconverter, as well as changing the conversions between upper- and lower- sidebands on the 4 switches on the front of the downconverter. All outputs are available from 37-way D-type connectors at the rear on pins 1-32 in order, with earth return provided on pins 33-37.

The 384/0.5 MHz and 128/1.0 MHz system

Filterbank modules F and G provide polarisation-summed outputs covering 192 MHz and 128 MHz respectively. The 0.5 MHz filters are normally used in conjuction with the two 32-MHz filterbanks and summing junctions to provide a polarisation-summed 512 x 0.5 MHz system. Currently these systems are connected to the Multibeam system, but they can be used with other receivers. Diagrams of the downconversion scheme and cabling can be found here and here, respectively.

Down-Converter Setup

We are gradually moving to integrate the Parkes Down-Converter into observing systems. Until the filter-bank systems are brought under TCS control, it is necessary to run scripts to set up the down-converter for different receiver/back-end configurations. These scripts live in /psr1/losetup/. Presently available scripts are: On any workstation: 
%perseus lorun /psr1/losetup/name.cmd

The Digitizer

A one-bit digitizer has been built (at Jodrell Bank) to provide the capacity necessary to use the Multibeam and other filterbanks. It is located at the bottom of Racks E and F and is interfaced to the Pulsar Multibeam Data Acquisition (PMDAQ) system running on the Dec Alpha puma. (See Pulsar Filterbank Data Acquisition System for further details of the operation of this system.) The digitizer can take up to 2560 frequency channels in 80 sockets. Layout of the input sockets is shown in Digitiser Connections. In channel order, the input sockets alternate between the top and bottom rows. Beam and subband inputs for the Multibeam filterbanks are shown in the Digitiser Connections figure. The start address for the digitiser is under program control, allowing remote selection of different filterbank systems.

The digitizer requires several control lines. These are normally left connected. The External 1 MHz light should be green. When observing the Enable light comes on.

Lines at the rear of the digitizer are:

The cal control line is a TTL signal, switched in synchronism with the sampling, which can be used to control a pulsed cal. The 5-sec synch pulse is currently generated by the `Front-End' computer (Line EVDS8 on Rack B).

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14 November, 2003 - Dick Manchester