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LO setup possibly incorrect in 7mm band

PostPosted: Thu Nov 28, 2013 2:17 pm
by ste616
To all observers who have observed in the 7mm ATCA band (frequencies between 30000 and 50000 MHz), the frequency you actually observed may have been offset slightly from where it should have been.

The problem arose because of the way the frequencies in the 7mm band are mixed down into the digitizer frequency range. The central site synthesiser can generate frequencies between 12.1 and 15.2 GHz, and this frequency is multiplied by 3 to get the tone used for the first down-mixer. The algorithm that we used (until recently) to determine the frequency of the central site synthesiser simply divided one of the simultaneously observed frequencies (plus or minus some offset frequency to account for further mixing stages) by 3. This sometimes led to the synthesiser being set to frequencies that ended in .667 MHz and .333 MHz, and thus led to an offset of +/- 1 kHz in the sky frequency.

For continuum observations, unless the observations you are making are incredibly precise, and you are worried about assigning a flux density to the wrong frequency, this offset should not pose a problem. However, if you are using the CABB 1 MHz zoom bands, where the frequency spacing of the channels is 488 Hz, this offset will be the equivalent of 2.05 channels, which may well be measurable and important. In the CABB 64 MHz zoom bands, where the frequency spacing of the channels is 31.25 kHz, the offset will be the equivalent of 0.03 channels, which may or may not be important to your results.

This LO algorithm bug has been present since the commissioning of the 7mm system in 2006. With the pre-CABB correlator, the 4 MHz/1024 channels mode is the most affected, with a channel spacing of 3.9 kHz. The offset then would be 0.26 channels.

We are in the process of compiling a list of observations that would have been affected by this bug, and we will get in touch with the PIs of the affected projects. In the meantime, we have created a web page that you can use to see if your observations would have been affected:

Currently, this calculator only works for the CABB frequency chain, but will work for the pre-CABB frequency chain within a few days.

We do not currently have a recommended method for how to correct for this offset with Miriad, but this too should be available in short order.

The LO algorithm is now fixed so that this will not occur for observations made after 2013 December 2.

Any questions about this issue can be directed to me, or you can leave a message in this forum.

Re: LO setup possibly incorrect in 7mm band

PostPosted: Mon Dec 09, 2013 9:47 am
by ste616
Good news! We have found that this bug crept in to the system on 2011 April 7, and was not present before this time.

To be clear, before 2011 April 7, there was still likely a rounding error, but it would have been at most 1 Hz, which is insignificant for all currently and previously available resolutions.

The calculator will now only be valid for observations on or after this date, and will be updated to reflect this.