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Long Spectra (WADDS)

General

The signals of the Wideband ADDing System (WADDS) can be used to make long spectra (i.e. with a very large number of frequency channels) for a single fan-beam on the sky using the spectral line correlators (DLB or DXB). This has been already done, although it is not yet (21 August 1996) a fully supported operational mode at Westerbork. Fan beams of different arrays of telescopes at the same sky frequency can be correlated with one another to give a compound interferometer mode of observing.

The receiver setup used is basically the same as for a DCB observation, except that the DCB correlator is not used. For this observing mode all DCB bandwidths need to be the same (either 5MHz or 10MHz), and an arbitrary array of telescopes can be used per DCB band. Pairs or larger groups (up to the maximum of 8) of DCB bands must be set to the same frequency to get correlations between subarrays. Subarrays should be chosen while specifying the observation in the program STFMA, by answering 'YES' to the question about use of the Wide Band Adding System. It is of course possible for a subarray to have only one telescope. The observation is in fact a DLB (or DXB) observation and needs a full specification for these correlators.

The sub-arrays should be phased-up correctly for the observing frequency(ies) to be used, and any non-operational telescopes should excluded in the adder box. If any have no total powers then their places in the equalizer unit should be connected to the noise source. Great care should be taken that the correct telescopes are added into the correct subarrays in the adder box, as the online system can not read what telescopes were actually used. It is of course very unusual to have any telescope in more than 1 subarray at any frequency, but possible if autocorrelations are wanted.

Hardware Setups

The WADDS uses the tied array output (from power splitters at back of ASTI just before mixing).

The outputs from the WADDS are 16 (psuedo)analogue signals (i.e. 8 DCB bands x 2 linear polarizations). The frequency ranges are:

This setup is done because the bands are always centred at 6.25MHz, where a residual LO peak is seen.

These WADDS outputs can be fed into the IF input of the DXB correlator, after upconversion to the DLB IF frequency, and spectra made in the usual manner. This is done by putting the video signals into the back of the equalizer units (where the DLB input would normally be linked through) and mixed with a signal from an external mixer. This is at ~25MHz, to give the central frequency for the DLB input at LO2 (35.625MHz); for 10MHz bandwidth use 24.375MHz which is derived as follows: 

35.625MHz - 6.25MHz - 10/2MHz = 24.375MHz (LO2 - band centre - half bandwidth)

and for 5MHz use 26.875MHz 

35.625MHz - 6.25MHz - 5/2MHz = 26.875MHz

Points to note are:

See the See section WADDS checklist

Hardware devices used

The majority of settings are as for a normal DCB observation, except that the DCB correlator is not used. The following devices are used (and relevant settings are given):

PTS synthesizer
Control as a normal DCB observation.
DLB LO2 synthesizer
This synthesizer is needed to mix the video output (taken from the input to the ASTI) up to the 35.625MHz needed by the DLB IF system, so that the frequency setting in the 20-40MHz range can be covered by the power distribution unit.
DCB devices
Primary fringe
This is done in LO2 of the DCB. The LO3 fringe of the DXB needs to be suppressed. This is done by manually switching it to `VAST' at present (21 August 1996) although it will be under software control in future.
DCB IF
All bandwidths must be the same.
A/D converters
Normal 3x3 mode with noise tube switching. Total power must be read out every 10 seconds.
Delay
Behaves as normal DCB delay. Values must be sent every 10 seconds
RCU
Receiver control is with a specified pattern (e.g. Walsh pattern).
AS
The wide band adder is used to produce the phased arrays. The attenuators immediately before the ASTI are used to adjust the total power level of this before sending it to the equalizer units.
RFDS
The RFDS is needed to demodulate any modulation pattern and correct for the differential fringe before adding. Differential fringe information is sent to the RFDS every 10seconds. Any Walsh phase switch pattern used must be demodulated here. Each band has a Walsh pattern length of 8, so the pattern repeats after 80 seconds. Observations should therefore not be shorter than 80 seconds.
DCB processor
The processor must be idle. This is made necessary by a software limitation which makes it impossible to read both the DCB and DXB correlator outputs.
DLB or DXB
Only specific configurations are suitable (see below). It is important

Correlator Configurations

All the special configurations have numbers between 192 and 255. Bearing in mind limitations in the correlator (no recirculation for nonstandard baselines etc) some suggestions are below. 

       Cabling table     
DCB band         DXB input
(software)      X pol.  Y pol
1               6X      6Y
2               7X      7Y
3               8X      8Y
4               9X      9Y

5               AX      AY
6               BX      BY
7               CX      CY
8               DX      DY

If 10MHz bands are selected then only the DLB can be used. For smaller bandwidths the DXB can be used with recirculation.

The use of this restricted set means that the telescope word will normally only includes telescopes 6,7,8,9,A,B,C and D. However the total powers should be measured for all telescopes in all subarrays.

Users who need a different configuration should contact the head of the system group at Westerbork well in advance of observing.

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