The WAPP dual board mode for single pixel receivers

The following freedoms and restrictions apply in the current implementation of the dual board mode:

• Any WAPP or any combination of WAPPs can be used. You don't need to run all WAPPs and You don't need to select WAPPs in sequential order. If You, for some reason, want to use only WAPP-2 and WAPP-4, You can do that.
• It is either dual board mode or single board mode for all selected WAPPs. If You select to use dual board mode, all enabled WAPPs will be running in dual board mode. You can not mix and have one WAPP running in dual board mode and another one running in single board mode.
• You can only have one bandwidth selection per WAPP. When using dual board mode, both boards on the same WAPP will use the same bandwidth. You will thus only see one bandwidth menu for each WAPP in the user interface when You are running dual board mode. You can, however, select different bandwidths for different WAPPs. If You want to cover 600 MHz of band and at the same time get some high resolution spectra of two known lines in Your band, You could set up three of the WAPPs to use 100 MHz and set the fourth one to use a narrow bandwidth.
• You can use any bandwidth selection normally available for the WAPPs from 0.1953125 to 100 MHz.
• You can use any WAPP configuration normally available for the WAPPs. As with the bandwidths, the same configuration will be applied to both the boards on each WAPP, but You can have different configurations on different WAPPs. NOTE though that '2 chan, 9-level' and '9-level, polarization' can NOT be used together with 100 MHz bandwidth.

The example above shows how to set up an observation for dual board mode with the new 'Receiver IF/LO selection menu' introduced in CIMA version 2.3. In this example, the C-band receiver has been chosen and the WAPP mode has been selected to be 'Dual board' to allow us to cover 800 MHz between 5000 and 5800 MHz with the center frequency set to 5400 MHz. The rest frequencies for the eight boards have then been put in with a separation of 100 MHz. The first line of rest frequencies refers to the first board of each WAPP while the second line refers to the second board. Note the order of the frequencies; an observer is, of course, free to put the selected bands in any order, but this is the order that will make the consecutive bands to be numbered sequentially in the CIMAFITS file.

In the example given, we have set the boards with 100 MHz separation. This may not be the optimal set-up for projects like searches for molecular lines, since the bandpass falls off at the edges of each band. For such projects it may be better to have a bit of overlap (10-15 MHz) at the edges to ensure that any interesting line doesn't fall in the gap with reduced sensitivity between the boards.

The data is currently written to FITS-files in a similar way as ALFA data is written. The INPUT_ID keyword is given a value between 0 and 7 to indicate which board the data refers to, with 0 and 1 being board A and B on WAPP-1. The two arrays SYNFRQ and MIXER are 4-value arrays and are set up to contain the synthesizer frequencies and mixer settings for the four A-boards in records with INPUT_ID = 0, 2, 4 and 6 and for the four B-boards in records with INPUT_ID = 1, 3, 5 and 7. Other variables have currently not been modified.

For pulsar data, the files look very much like ALFA data. However, there is a change of the pulsar file headers to include information on the eight channels as well as a new flag. The 'wapp_split' program can extract the data from the two boards into separate files and this program is replacing the old 'alfasplit' program.

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This page is administered by Prakash Atreya ( patreya (a) naic . edu ) and was last updated on 19 September 2008.