Sideband Separation Using Walsh Function Switching
Question from Clint and Bill...
A question came up at the BE CDR on use of Walsh functions for calibrating out noise:
BW of the function, 180 deg enough or 90 deg wanted?, where best to insert the function:
always 1st LO or 2nd LO, too?
I believe that this issue is being resolved by a recent CRE, which has recently passed the CCB. The short answer is 90 degrees phase switching at the first LO. See CRE Number : ALMA-56.00.00.00-001-A-CRE.
- 21 Jul 2004
-- Comment by BillBrundage
- 23 Jul 2004
My understanding is that only the 1st LO requires both 180d and 90d phase switching, which is accomplished by the DDS in the FLOOG, each per a 128? bit Walsh function that maintains orthogonality among all antennas.
D'Addario says 180d phase switching in the 2ndLO is desirable, but not required (80.04.00.00-002-D, System Design Description, L. D'Addario, 2004-02-20). I presume that the 2ndLO 180d phase switching would be used only when 180d switching fails in a 1stLO.
Both 180d and 90d switching are "removed, or de-switched" in the correlator per the appropriate Walsh function.
- 10 Sep 2004 :
This requires a little clarification:
- 180 degree phase switching is applied at the 1st LO, to suppress unwanted slow varying signals; it is demodulated at the antenna, not in the correlator. It has nothing to do with side band separation.
- 90 degree phase switching (recent CRE) is also applied at the 1st LO, to perform 1st LO sideband separation for DSB receivers (high frequency receiver bands). The demodulation is after correlation (in the correlator computer).
-- Comment by AlWootten
- 13 Feb 2007
A question arose: How does ALMA produce singledish single sideband spectral line data from a double sideband receiver?
I thought that the Science Software Requirements
cover that. See http://almasw.hq.eso.org/almasw/bin/view/SSR/SsrDocuments
The Use Cases state:
"Measuring the sideband gain ratios could be done in the preamble if both sidebands can be measured."
and what is I think the same thing:
"Measuring the sideband gain ratios could be done in an initialization step if both sidebands can be measured."
"User specifies whether to use software sideband suppression or sideband suppression by LO offset."
"The system will allow the user to examine which transitions
could be expected from image sidebands and as a result tune the receiver
The actual requirements are:
- 2.3-R12: When a band is equipped with a double sideband receiver, it shall be possible to process and store data from both sidebands using software sideband separation or to store data from a single sideband using sideband suppression by local oscillator offset. Priority: 0
- 188.8.131.52 Astronomical Calibration: Single Dish Data
- 6.3.3-R1: The Pipeline shall reduce the temperature scale calibration, using sideband ratios determined from the most recent interferometric calibration. In on-line mode, the results must be made accessible to the dynamic scheduling system; they must also be made available to later convert the raw data into temperature scale whenever required. Priority: 1
What does this mean?
I think that software sideband suppression or sideband suppression by LO offset must both be available. What is 'software sideband suppression'? I think that this is not the sort of deconvolution done by the CSO folks when they produce a spectral survey. Herschel plans to implement this; ALMA does not. Sideband suppression is available to both single dish and interferometric observers. Single dish observers benefit in that during initialization, the sideband gain ratio of the receiver using the observing setup may be determined. See C) below.
What is LO offset suppression?
(A) Suppression of the Other sideband in DSB receivers
This is exactly equivalent to the "sideband smear"
operation, where we stepped the LOs at the 12 M as a function of time, so that the image sideband power was smeared relative to the targeted sideband. There are two flavors of sideband smearing. In one, all antennas in the array are offset by an amount, then all are summed for the total power result. As proposed by Napier, this compromises spectral resolution; suppression is a function of number of antennas used and thus does not really work for, i.e. four antennas. General opinion as expressed by Darrel: I don't think the "N antenna" version on its own its going to be adequate. I think you need more different frequency offsets even than the number ~50 of antennas.
(A1) A version of LO offset smearing was contemplated with the NRAO 12m antenna once. This works with n=1. We could think of using this in combination with A) above.
Darrel explained the NRAO 12m concept: With that, you step 1st LO and 2nd LO in synchronism, which means that the unwanted sideband smears out. For example, you might step the LOs once a second, which means that after 60 seconds you've smeared the unwanted sideband into 60 spots over the spectrum. That would be essentially identical to Peter's concept of averaging 60 antennas each with a different but constant-in-time LO offset. Question
: Can the computing and electronics work with this? We think the answer is yes (LO changes at ~1Hz rate).
- The optimum will be to use a version of the 12 M frequency-with-time stepping, or sweeping. That would enable the stepping-with-time to get through a complete set of steps N times more quickly, which is probably a good thing.
- Simulations are needed! Remijan to work on this.
Lucas notes that we will not be able to determine the sideband gain ratio with the two antennas at the ATF. This should be tried with a DSB receiver. This can only be sky-tested on an ALMA system when antennas are installed at the AOS (when there are plenty of other things to keep us busy). It may be possible to test the scheme at the ATF using the evaluation receiver at 3mm which can be configured to be DSB at its upper end. We do not plan to use the DSB 1.3mm receiver for any tests.
(B) Improving the suppression in 2SB receivers
Darrel noted: One good thing is that for most bands we do at least have 10 dB of suppression between the sidebands, which is a starting point. The better thing is that I think most of the sideband separation mixers have a good deal better than 10 dB suppression, with 10 dB being the worst case spec that has to be met over most of the band. One sideband separation option has been pointed out by Tony Kerr in a draft memo that he has at: http://www.cv.nrao.edu/~akerr/EnhancedImageRejection01.pdf. Tony's scheme can only be used with sideband separating mixers, not with true double sideband mixers, and relies on very careful calibration measurements that enable you to separate the 2 sidebands by a process of linear combinations in the post processing. Tony's scheme might work quite well, but so far as I know hasn't actually been tried on a mm-wave telescope.
This could be tried with a 2SB ALMA receiver. Basically, it just calibrates the sideband gains, which one can do interferometrically with the three elements at the AOS. There will be no ALMA receiver which can be used at the ATF under the present plan. It may be possible to test the scheme at the SMT, probably at large expense.
(C) What is software suppression?
This would be a version of the 'sideband clean' algorithm which Schilke and collaborators used on the CSO spectral surveys. ALMA has the advantage that the sideband gain ratio is measured. However, in all implementations of this algorithm I am aware of there is some residual 'ghosting'. This is not an ALMA requirement and, therefore, would be up to individual users to implement.
- 20 Feb 2007