WAPP Dual Board Mode

The WAPP dual board mode for single pixel receivers

The capability to use the WAPPs in dual board mode for single pixel receivers was added in CIMA version 2.3 with first observations performed on 1 April 2007. This is the mode when the two boards of each WAPP are used as independent spectrometers allowing each WAPP to cover up to 200 MHz bandwidth. With this mode it is thus possible to double the bandwidth covered with the WAPP during single-pixel receiver observations from 400 MHz to 800 MHz, and the mode has thus often been referred to as the 800 MHz mode. This is certainly what it is mostly going to be used for but there is nothing preventing an observer for using the dual board mode with narrower bandwidths all the way down to 0.1953125 MHz.

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

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.

You can return to the main CIMA page by clicking here.

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