Dual Channel Data and 1.8 GHz Bandwidths

Stack manipulation for dual-channel data

When you ask SPECX to read a spectrum (with the 'r-g-d' command), if you are (were) tuned in Dual Polarity (i.e dual-channel) mode the channel A data will be read into the stack first, followed by the channel B data. Thus the channel A data will be pushed into the y-stack position and the channel B data will be placed in the x-stack position. If you wish to average together a number of scans you should be aware of this and remember that the 'average-spectra' command combines the x-stack with the y-stack, and places the result in the x-stack position; if you are averaging both channels this will be what you want. For example, to average both channels for three scans numbered 34, 35 and 36, you would enter the following sequence of SPECX commands:

r-g-d 34;ave
r-g-d 35;ave;ave
r-g-d 36;ave;ave
d-m

Useful SPECX commands to see the contents of the stack and to manipulate the stack are

show-stack	- displays stack 

xy		- interchanges the contents of the x and y 
		  stack positions

roll-stack	- rolls the contents of the stack without losing 
		  anything -  i.e. x,y,z,t ==> t,x,y,z

Wideband (1.8 GHz) Mode

Although the receiver must be tuned in single polarity for the maximum 1.8 GHz bandwidth, the maximum DAS bandwidth per IF channel is 920 MHz, and hence both DAS IF channels are used to accommodate the full bandwidth. It is worth noting that the appearance of such spectra on the on-line display at the telescope can be quite confusing: the two halves of the spectra are flipped end-for-end when observing in the upper sideband, so that the ends of the spectrum are in the middle as displayed, often giving the appearance of large noise spikes at the center of the spectrum. To reduce such wideband spectra SPECX loads half of the raw spectral scan (the "B" portion) into the x-register, which pushes the other half (the "A" portion), having been loaded first, into the y-register.

The "das-merge" (or d-m) command can be used to create a single spectrum from an observation made in this mode; a third optional parameter added in the past few years makes the procedure quite straightforward. Having loaded a wideband spectrum with the usual r-g-d command, next type d-m and note the following exchange:

 >> d-m
There are  110 overlapping channels
 Number of overlap channels to use? [  55] 
 Adjust any DC offset quadrants? (Y/N) [N] 
 Wideband mode (also merge Y spectrum)? (Y/N) [N] Y

You will then have a merged and concatenated spectrum containing the complete band covered by both halves of the DAS. To avoid the Q&A sequence, just type (the hashes take the offered defaults, and can often be omitted altogether):

 >> d-m\#\#\y

It may be helpful to understand the sequence of steps involved in this procedure, since there are circumstances where one may wish to have full control over the process, and in particular keeping the two halves of the spectrum separate. The following procedure is required:

1. (a) issue the "das-merge" command for both the x & y stacks,
2. (b) to bring the two halves two a common frequency scale use the "concat" command, and finally,
3. (c) use "merge-quad" on the concatenated spectrum.

In that case the following command sequence could be used to reduce a 1.8 GHz wide scan; after the second d-m command one has the option to inspect the two parts of spectrum.

r-g-d nn	- read the scan "nn"
d-m\#\#\#\      - DAS-merge the first half of the scan (i.e. the x stack)
xy		- interchange the x and y stacks
d-m\#\#\#\	- DAS-merge the other half
xy              - reorder stack positions
concat		- combine the two halves onto a single scale
merge-quad      - make a single spectrum out of them
n		- plot the scan

See the SPECX cookbook section on "Making very long spectra" for further obfuscation, including how to average wideband spectra together (hint: average the "A" and "B" parts and das-merge them separately, then concatenate and merge the two).

Please send comments and questions to the undersigned.

(Original by Lorne Avery 12 July 1997)