Two Correlators for the Price of One How a VLBA Correlator C

The proposed 40-station WIDAR EVLA correlator [1] uses a novel technique for efficient wideband correlation. This technique requires the correlator to be capable of fringe stopping and as such, it is fundamentally capable of operating as a VLBI correlator.

Two Correlators for the Price of One:How a VLBA Correlator Could Fit Within theProposed 40-Station WIDAR EVLA Correlator

NRC-EVLA Memo# 006Brent Carlson, September 28, 2000

ABSTRACT

The proposed 40-station WIDAR EVLA correlator [1] uses a novel technique forefficient wideband correlation. This technique requires the correlator to becapable of fringe stopping and as such, it is fundamentally capable of operatingas a VLBI correlator. NRC has made the commitment [2] to design the BaselineBoard for VLBI compatibility and will endeavour to design the Station Board sothat the installation of a tape interface daughter board (or some equivalent) wouldallow VLBI operation. This memo explores how a VLBA correlator could fitwithin the planned 40-station correlator if the entire 40 stations are not requiredfor EVLA operation. The size of the VLBA correlator, of course, depends on thenumber of unpopulated correlator station inputs. Some gain in station capabilityis made if the additional VLBA antennas require only 4 GHz of total bandwidtheach—every unused correlator station input can handle two VLBA antennas.Additional gain is made if the required VLBA antenna bandwidth is reduced to 1GHz—in this case every unused correlator station input can handle four VLBAantennas. Decent spectral-line capability will be available, with 512 frequencypoints per sub-band in wideband modes. Increased spectral resolution for lineobservations can be obtained by reducing the number of sub-bands and/or sub-band bandwidth. Finally, this memo shows that with some small correlator chiprouting modifications, VLBA antennas can operate and correlate with EVLAantennas in real-time.

1 Introduction

The proposed 40-station WIDAR EVLA correlator [1] contains two essential elementsrelevant to this memo—the Station Board and the Baseline Board. The Station Board iscapable of processing two, 2 GHz basebands. Data enters the Station Board via a

daughter module that, for the EVLA, accepts data from two fiber-optic interfaces1—onefor each baseband. Each baseband has its own completely independent delay model andthus can be on a different phase-center on the sky. Each 2 GHz baseband is split into 16

1

The actual number of fibers coming into the board may be more, but conceptually there is one interfacefor each baseband.

The proposed 40-station WIDAR EVLA correlator [1] uses a novel technique for efficient wideband correlation. This technique requires the correlator to be capable of fringe stopping and as such, it is fundamentally capable of operating as a VLBI correlator.

sub-bands with digital filters on the Station Board. Each sub-band can be any bandwidthin decreasing powers of 2 from 128 MHz down. The minimum width of each sub-band isdetermined by the size of the digital filters and normally can be as narrow as a fewhundred kHz2. Four Station Boards are used in parallel to provide 16 GHz of totalbandwidth from each EVLA antenna (i.e. 8 basebands or 4 baseband pairs). The fourStation Boards plug into the “Sub-band Distributor Backplane”, and there are 16 cableoutputs from this backplane3. Each of these 16 cables contains data for all 8 basebands ofone sub-band. Each cable goes to a separate “sub-band correlator” and each sub-bandcorrelator correlates the data for one sub-band from all antennas. Each sub-band

correlator contains several Baseline Boards (15 for a 40-station correlator) and data onthe input cables gets distributed to Baseline Board inputs via one “Station Data FanoutBoard” per antenna (per sub-band correlator). The Baseline Board has 8 ‘X’ inputs and 8‘Y’ inputs and can correlate all baselines for an 8 x 8 parallelogram of the baseline matrix(see Figure 1). Each of the 64 correlations in the 8 x 8 matrix is performed with onecorrelator chip on the Baseline Board.

This memo will show that if an antenna only has 4 GHz total bandwidth (two 2 GHzbasebands), then only one Station Board is required for the antenna. Each input to theBaseline Board can then be arranged so that it actually contains data from two antennasand each correlator chip correlates the data for four baselines. With small routingadditions to the correlator chip, it is possible to correlate 4 GHz antennas with 16 GHzcan be correlated for every unused (16 GHz) station input in the correlator.

2 Correlator Baseline Matrix Layout

Figure 1 shows a 40-station baseline matrix with an example 6-station, integrated 4 GHz(VLBA) correlator. Each Baseline Board correlates data (for one sub-band) for one 8 x 8parallelogram (an example of which is highlighted in the figure) or an edge triangle. Thefigure shows an array of 64 correlator chips on the Baseline Board. Normally, eachcorrelator chip performs all of the correlations for one baseline and one sub-band of 8basebands (or 4 baseband pairs). However, if a station has only 4 GHz of totalbandwidth, then each correlator chip does 4 baselines with two basebands.

The VLBA-only correlations are performed in the bottom left triangle with the layout ofthe associated Baseline Board as shown. The board also performs 16 GHz correlations(VLA only) and 4 GHz x 16 GHz correlations (VLA x VLBA or VLBA x VLA). Thehighlighted parallelogram and associated Baseline Board layout shows how other VLA-only and VLA x VLBA correlations are performed along the matrix diagonal.

2

Except for the “radar-mode” filter which can be narrower.3

...for correlation—more outputs are required for going to the phasing subsystem.

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