Delays Exhibit 8ns Jumps

This usually indicates an Early/Normal problem (see Appendix K for more details of Early/Normal). You can expect to see these delay errors when the Local Oscillator (LO) to an antenna has been disrupted. This will occur when the antennas are moved during an array reconfiguration, however, this may occur at other times as well due to LO maintenance or equipment failure.

It is important to check that the delays are jumping by exactly 8ns, (anything else is not an Early/Normal problem). Inspection of the cross-products on VIS (eg VIS$>$select ab,cd) will show a figure similar to that for an Early/Normal problem.

Figure 7.2: An example of a VIS display with an Early/Normal problem (i.e., delay jumps of 8ns).

To correct for this problem observing must be stopped. You should close your current data file (CAOBS$>$corr closefile). The EARLY program (see Appendix K) will then have to be run. However, to do this the correlator configuration must be FULL_128_2. To perform corrections with the EARLY program do the following steps:

	CHECK the correlator configuration		Should be FULL_128_2.
	VIS$>$select ab,cd		To display cross-product polarisations
	VIS$>$onsource off		Display visibilities when the antennas are off source. Can toggle this mode on and off by repeating the onsource command.
	CAOBS$>$ early		This sets up the required scan pattern.

Then to run the EARLY program, use a XBONES window to type:
XBONES$ early

After starting the EARLY program ASSISTANCE will report that all the antennas are disabled. At the XBONES terminal a title message will be edited, as well as the correlator products to be used in the program. The program will then wait for data. To start type:

CAOBS$>$track n

Where n any source with 128MHz bandwidth and the FULL_128_2 correlator config.

The EARLY program will then display the auto-correlations being calibrated. If the recommended correlator file is being used this will be for both observing frequencies, ie ##AB and ##CD for antenna #. The record number will increment from 1 to 32 during the program's operation and can be used to check that the program is working. The final output of the program for each antenna is in tabular form showing the sampler phase offset in degrees and the adjusted delays. These adjustments should be near-zero or near $\pm$7.8125; those that are not should be in reverse-video on the screen indicating an error.

For each antenna you will be asked to either:

• (A)ccept the results,
• (R)evise them if you know a better solution, or
• go to the (N)ext antenna and perform no correction.

Note: After running the EARLY program, the delays will need to be recalibrated (with CACAL)