8/3-Bit Attenuator and Requantizer Gain Setup Scans

Overview

This page covers the strategy of designing scheduling blocks using 8-bit and 3-bit observing scans. Included are scan organization for hypothetical scheduling blocks for both 8-bit and 3-bit observations, special S-band as well as P-band observation guidelines, low frequency observations heavily impacted by RFI, and guidelines for data processing.

Slew Dummy Resources

We have created five optional resources specifically for the dummy slew scan at the start of a scheduling block (SB). These new resources are located in the Resource Catalog Tool (aka Instrument Configurations) under the NRAO Defaults → Slew dummies sub-catalog.

P-slew
L-slew
S-slew
C-slew
X-slew
Ku-slew

We made these resources available if the observer chooses to use a dummy slew scan at the start of their SB which is not the same frequency tuning as the science observing band, but is the same observing band. The Special Case S-band section and the 3-bit Examples for C/X-band section below make use of the dummy slew resources. Note, the dummy slew scans do not replace the required attenuator setup scans for the science observing band.

If a dummy slew scan is desired, we recommend using the dummy slew resource that is the same observing band as the science resource. For example, an L-band observation should use L-slew as the dummy slew resource. If one of the other dummy slew resources were used, then this could cause an unintended scheduling limitation, such as: at the start of an array configuration when baselines are being updated or if we are avoiding certain observing bands due to known issues with some of the antenna receivers or avoiding RFI at the VLA from scheduled activities.

It's important to note, observers are not required to use these resources and they should not be used as a science observing band.

8-Bit Scheduling Blocks

8-Bit General Information

This section describes the importance of setup scans for the 8-bit sampler system, which delivers two pairs of 1 GHz basebands from each antenna to the correlator, and provides examples of how to setup observations at low and high frequencies.

The 8-bit system has two different types of setup scans:

A) Setting the overall attenuator levels: These scans are required to set the overall attenuator levels the first time an 8-bit setup is observed. These settings are remembered for the rest of the observing session for a particular frequency tuning. There should be one such setup scan for each 8-bit frequency tuning used in a scheduling block, with each scan being 1 minute in duration regardless of being on source or slewing to a source.
B) Setting the requantizer gain levels: This type of 8-bit setup scan is used to set the requantizer gain levels for the signals going into the correlator. Setting the requantizer gain levels ensures optimal sensitivity for cross-correlations on a per spectral window basis. If requantizers are not set, the levels will be adjusted to a pre-determined value. This pre-determined value is the same for all spectral windows and does not set requantizer gain levels to account for changes in sensitivity across the baseband. A consequence of this could be a slight loss in overall sensitivity. The characterization and quantification of the difference between pre-defined requantizer levels and setting them for each observation is under investigation.

The use of setting requantizer gain levels in scheduling blocks is strongly recommended for 4 (54-86 MHz) and P-band (230–470 MHz) observations and can optionally be used when observing with any of the other frequency bands. In 8-bit observations, a requantizer gain setting scan is triggered when all the following conditions are satisfied:

the scan with a given instrument configuration is explicitly marked with a setup intent;
it is not the first time this instrument configuration (a specific IF/LO setup and a WIDAR correlator setup) has been used in the scheduling block; and
the scan noted in (1) is either the 2nd scan with its particular LO/IF setup, or has a different instrument configuration (either in LO/IF setting, in WIDAR correlator setting, or in both) than the preceding scan.
Setting the requantizers are triggered immediately if the preceding scan utilizes the same receiver, requiring a minimum scan length of 10 seconds. If the preceding scan utilizes a different band, in that case requantizers will be triggered only after 20 seconds into the scan to account for the time the sub-reflector needs to rotate, requiring a minimum scan length of 30 seconds.

If any of those four conditions are violated, then the requantizers are not explicitly set. We note that this triggered behavior is unique to 8-bit observations/scans only, contrary to the 3-bit observations (explained below) which require the setting of the requantization level after every change of instrument configuration. Therefore, 3-bit observations/scans will use any scan that follows an instrument configuration change to set the requantizer gain levels, even if the scan is not marked with the setup intent. See the section Special Case II (below) for different approaches to utilize this type of setup scan. Examples that include requantizer gain setup scans in 8-bit can be seen in the Special Case II section below.

Note on scan intents: Although we do not require observers to use the VLA CASA calibration pipeline, we request observers to select setup intent as the intent for all setup scans throughout the scheduling block even when the observed source is a calibrator. The setup intent will automatically flag the data as bad for data reduction and prevent the pipeline from choosing a setup scan as a calibrator scan. If any other intent is selected for these setup scans, it will cause calibration problems in the output data from the pipeline, but not for hand-edited calibrated data. One should never trust the data from either the initial attenuator/gain slope equalizer setup or the requantizer-setting scans.

Generally, and due to dynamical scheduling, we recommend 10–12 minutes for the start-up sequence. During this time, all of the required setup scans that fall in type (A) above (to set the attenuator levels) can be observed even when the antennas are slewing. This will help to ensure that the first observing or reference pointing scan has plenty of on-source time (note, reference pointing requires a scan with an on-source time ≥2m30s).

8-Bit Examples

Below we give 8-bit low and high frequency examples that do not include requantizer gain setup scans. Examples that include requantizer gain setup scans in 8-bit can be seen in the Special Case II section below. Note that the requantizer gain setup scans can be used with any 8-bit resource, however, they are optional for L- through Q-band observations.

Low Frequency

Single band observing


01m00s L-band setup scan [intent = setup intent]
10m00s L-band calibrator observing scan [intent = calibrator intent(s), scan mode = standard observing]
L-band observing
etc.

Multiple band observing


01m00s C-band setup scan [intent = setup intent]
01m00s L-band setup scan [intent = setup intent]
09m00s L-band calibrator observing scan [intent = calibrator intent(s), scan mode = standard observing]
L or C-band observing
etc.

High Frequency

Single band with reference pointing


01m00s K-band setup scan [intent = setup intent, scan mode = standard observing]
01m00s X-band setup scan [intent = setup intent, scan mode = standard observing]
10m00s X-band reference pointing scan [scan mode = interferometric pointing]
04m00s K-band calibrator observing scan [intent = calibrator intent(s), scan mode= standard observing]
K-band observing
etc.

Multiple bands with reference pointing


01m00s K-band setup scan [intent = setup intent, scan mode = standard observing]
01m00s Q-band setup scan [intent = setup intent, scan mode = standard observing]
01m00s X-band setup scan [intent = setup intent, scan mode = standard observing]
09m00s X-band reference pointing scan [scan mode = interferometric pointing]
04m00s Q or K-band calibrator observing scan [intent = calibrator intent(s), scan mode = standard observing]
etc.

Special Case I: S-band

In general, S-band observations can be set up following the guidelines presented above (also see the Low Frequency examples). S-band, however, is subject to very strong RFI from a number of satellites, in particular those providing satellite radio service. A satellite passing through the VLA beam during the initial slew will play havoc with the attenuators. While the probability of this happening may be fairly low, observers wishing to follow a very conservative approach may adhere to the following guidelines to set up their S-band observations:

The setup scan needs to be done while pointing at a section of sky guaranteed to be free of geostationary satellites at all times and most emission from roaming constellations providing satellite radio service.
There are two such zones:
- 250° < Azimuth < 330° and Elevation > 30°
- 45° < Azimuth < 90° and Elevation < 60°
Since the OPT does not allow you to specify Azimuth and Elevation directly, you will have to find an object (any gain calibrator will do) in your possible LST start time range that lies in one of the two satellite-free zones. We refer to the NRAO Helpdesk for any questions.
Slewing to the satellite-free position needs to be done in a different band (or at a different frequency) from any that is going to be used (the NRAO default "S-slew" can be used, as seen in the example below). Attenuators are set during the first scan at a newly specified tuning, which can include the initial slew. The possibility that a satellite may pass through the VLA beam during the initial slew can never be excluded. To prevent unwanted, erroneous attenuator settings, it is necessary during this slew to tune to a frequency not used for actual observing. Any ill effect will then be on this attenuator setting rather than any used in the remainder of the Scheduling Block.

Here follows an example of the first few scans of an S-band Scheduling Block:


09m00s  S-slew (or a different dummy slew resource) scan slewing to and reaching a position
        in the satellite-free zone. This scan should have a frequency tuning that will not be
        used again in the scheduling block.
        [intent = setup intent, scan mode = standard observing]
01m00s  S-band attenuator setup scan in the satellite-free zone
        [intent = setup intent, scan mode = standard observing]
05m00s  S-band actual calibrator scan 
        [intent = calibrator intent(s), scan mode = standard observing]
etc.

This assumes that the S-slew resource is not being used in the remainder of the SB, and that a 5 minute calibrator scan is sufficient for slewing from the calibrator in the satellite-free zone to the true calibrator while leaving enough time on the latter.

Multi-frequency S-Band Scheduling Block

If you are creating an S-band SB that will be using other frequencies, you can use these other frequencies for the slew to the satellite-free region where you can then set up S-band.

Here follows an example of the first few scans of a multi-frequency Scheduling Block that includes S-band (observing at S, L, and C-bands):


05m00s  L-band attenuator scan slewing to and reaching a position in the satellite-free zone.
        [intent = setup intent, scan mode = standard observing]
05m00s  C-band attenuator scan slewing to and reaching a position in the satellite-free zone.
        [intent = setup intent, scan mode = standard observing]
01m00s  S-band attenuator setup scan in the satellite-free zone
        [intent = setup intent, scan mode = standard observing]
05m00s  S-band actual calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
        Continue with science observing
etc.

Special Case II: The use of requantization setup scans in 8-bit observations

As noted above, it is possible to utilize requantizer gain level setup scans in 8-bit observations. While these can be used in any observation that uses the 8-bit samplers, we strongly recommend their use in P-band (230–470 MHz) and 4-band (54-86 MHz) observations, which utilize 16 spectral windows each 16 MHz wide in the former case, for the following reasons:

Setting the requantization levels optimizes the digital power (i.e., maximizes the signal-to-noise ratio) within each spectral window.
There is a significant variation of power within the 240 MHz span of the P-band. Therefore, without spectral window dependent requantization level adjustments, a single value is used for all spectral windows. Spectral windows at the edge of the passband have lower than average power, while some in the middle will be higher.
Higher power in spectral windows can also result from strong RFI, from observing very strong isolated sources (e.g., Cas A and Cygnus A at 4 or P-band), or from observing part of the sky with high background power (e.g., the Galactic center at P-band).
Such high digital power can lead to large correlation errors and reduce the quality of the data. Therefore, the optimization of the digital power through the use of requantization setup scans can become important, if not critical, to avoid these errors.

The utilization of the 8-bit requantizer setup scans can vary depending on the project and the observed sources or the regions of the sky. Therefore, the following scenarios are supported in the OPT:

A scheduling block with a single instrument configuration with a single requantization setup (most P-band observations). For this scenario, first slew to the source of interest using a band different than P-band (e.g., 8-bit X-band, L-band, or C-band). Once on source, observe a scan at P-band for 1 minute with setup intent to set the attenuator levels, this scan is then followed by a 10 second scan at P-band with setup intent to set the requantizer gain levels. See the following example:

09m00s ?-band slew to source of interest [intent = setup intent]
01m00s P-band attenuator/gain slope equalizer setup scan [intent = setup intent]
00m10s P-band requantizer gain setup scan [intent = setup intent]
03m00s P-band calibrator scan [intent = calibrator intent(s)]
etc.

A scheduling block with multiple instrument configurations with one or more utilizing 8-bit requantization setup scans. For this scenario, insert the appropriate setup scans to set requantization levels when needed, similar to a standard 3-bit scheduling block. Requantizer levels are not remembered, thus they will have to be reset every time the instrument configuration changes. In the situation where the observing receiver changes, the length of the requantizer scan has to be 30s to account for the time it takes to rotate the sub-reflector. See the following example:

05m00s L-band attenuator/gain slope equalizer setup scan [intent = setup intent]
04m00s X-band attenuator/gain slope equalizer setup scan [intent = setup intent]
01m00s P-band attenuator/gain slope equalizer setup scan [intent = setup intent]
00m10s P-band requantizer gain setup scan [intent = setup intent]
03m00s P-band calibrator scan [intent = calibrator intent(s)]
03m00s X-band calibrator scan [intent = calibrator intent(s)]
03m00s L-band calibrator scan [intent = calibrator intent(s)]
00m30s P-band requantizer gain setup scan [intent = setup intent]
01m00s P-band calibrator scan [intent = calibrator intent(s)]
06m00s P-band science target scan [intent = observe target]
01m00s P-band calibrator scan [intent = calibrator intent(s)]
etc.

P-band observations that include a mix of very strong and weak sources, or observing sources across the whole sky, i.e., having significant variation in sky background power depending on the pointing direction. For such observations, requantization gain levels can be set for every scan (i.e., calibrators and science targets).

After the initial slew and the setting of the attenuator levels, and for each scan intended to benefit from the requantization setting, a sequence of three scans will be needed (example given below):

A very short scan of 5 seconds: for this, one must use a different P-band resource—which can be a duplicate of the original resource—that has a different name in the RCT and will need to differ in LO/IF frequencies and/or the correlator configuration to trigger the requantizers.
A 10 second P-band requantizer scan with a setup intent using the science P-band resource, if the dummy setup uses the same receiver. Note: If the dummy scan uses a different receiver, a 30 second scan is needed to set requantizers.
The original intended/desired scan using the science P-band resource.

09m00s ?-band slew to source of interest 
              [intent = setup intent, source = calibrator-1]
01m00s P-band attenuator/gain slope equalizer setup scan 
              [intent = setup intent, source = calibrator-1]
00m10s P-band requantizer gain setup scan 
              [intent = setup intent, source = calibrator-1]
03m00s P-band calibrator scan 
              [intent = calibrator intents, source = calibrator-1]
00m05s P-band-different-name dummy setup scan 
              [intent = setup intent, source = gain cal]
00m10s P-band requantizer gain setup scan 
              [intent = setup intent, source = gain cal]
02m30s P-band calibrator scan 
              [intent = calibrator intent, source = gain cal]
00m05s P-band-different-name dummy setup scan 
              [intent = setup intent, source = target-1]
00m10s P-band requantizer gain setup scan 
              [intent = setup intent, source = target-1]
08m00s P-band science target-1 scan 
              [intent = observe target, source = target-1]
00m05s P-band-different-name dummy setup scan 
              [intent = setup intent, source = gain cal]
00m10s P-band requantizer gain setup scan 
              [intent = setup intent, source = gain cal]
01m30s P-band calibrator scan 
              [intent = calibrator intent, source = gain cal] 
00m05s P-band-different-name dummy setup scan 
              [intent = setup intent, source = target-2]
00m10s P-band requantizer gain setup scan 
              [intent = setup intent, source = target-2]
08m00s P-band science target-2 scan 
              [intent = observe target, source = target-2]
00m05s P-band-different-name dummy setup scan 
              [intent = setup intent, source = gain cal]
00m10s P-band requantizer gain setup scan 
              [intent = setup intent, source = gain cal]
01m30s P-band calibrator scan 
              [intent = calibrator intent, source = gain cal]
etc.

3-Bit Scheduling Blocks

3-Bit General Information

This section describes the various setup scans required for observing with the 3-bit sampler system, which delivers four pairs of 2 GHz basebands from each antenna to the correlator. It also demonstrates how to set up observations at both low (C and X-bands) and high (Ku, K, Ka, and Q-bands) frequency bands using the 3-bit system.

The 3-bit system requires two different kinds of setup scans which need to be done separately for each frequency tuning:

A) Setting the overall attenuator and gain slope equalizer levels: The first type of 3-bit setup scan is used to set the overall attenuator and gain slope equalizer levels the first time a 3-bit setup is observed in a scheduling block. These settings are remembered for the rest of the observation for a particular frequency setting. There should be one such setup scan per 3-bit frequency tuning used in a scheduling block, with each scan being 1 minute long and preferably observed at about the same elevation as the target source.
B) Setting the requantizer gain levels: The second type of 3-bit setup scan is used to set the requantizer gain levels for the signals going into the correlator, and are set every time a 3-bit correlator setup with a different LO/IF tuning is requested. The requantizer gains are therefore reset when returning to the 3-bit system after every reference pointing scan in a high frequency observation, and at every band or frequency change using the 3-bit system. The requantizer gain setup scans must be 30 seconds long in duration (on source time is not required), and a new setup scan is needed every time the system changes back to using a 3-bit LO/IF tuning or another 3-bit tuning. For scheduling blocks observing more than one 3-bit tuning it is therefore more efficient to group all scans using the same 3-bit tuning together in order to avoid the overhead of multiple requantizer gain setups scans required every time there is a band or tuning change.

Notes:

High frequency observations require reference pointing at X-band. Such scans use an 8-bit correlator setup, and therefore a 3-bit high frequency scheduling block should also include the required 8-bit setup for the reference pointing scan at the beginning of the scheduling block. Such 8-bit setup scans should be at least 1 minute long, but are often scheduled for longer in order to ensure that the following 3-bit setup scan (see (A) above) takes place close to the elevation of the target source.
3-bit and 8-bit mixed observing setups: For observations that would use a mix of 3-bit and 8-bit samplers in a single setting, e.g., for simultaneous continuum and high resolution spectral line observations, the guidelines to set up the 3-bit samplers should be followed as described on this page.
For low frequency (C- and X-band) observing, while the NRAO strongly advises the use of an initial 8-bit dummy scan, this is not necessary for the creation of a valid 3-bit low frequency SB. Please see the example below on how to create such an SB. For high frequency (Ku-band and higher), we still require the initial 8-bit scan in order to get the antennas near the first source of interest in the SB to better set up the 3-bit attenuator and the gain slope equalizer levels. (See section (A) above for further explanation.) Please also note that removing the 8-bit dummy scan from the start of a low frequency 3-bit SB does not save on overhead as the antennas will, most likely, still need to slew some distance to the first source of interest. This amount of slew will depend upon where the antennas were pointing at the end of the previous observing session.

3-Bit Examples

High frequency with single 3-bit setting
In this example there is a single high frequency 3-bit setting. Because the observations need a reference pointing scan at X-band, an 8-bit setup scan should also be included. Considering that initial slew can take up to 9 minutes, and the reference pointing scan needs to be at least 2m30s long on-source, this example will allow up to 12 minutes at the start of the scheduling block for the 3-bit attenuator and gain slope equalizer scan (at least 1 minute), the 8-bit setup scan (at least 1 minute), and the reference pointing scan. Note that the attenuator values for the 3-bit and 8-bit tunings will be remembered for future scans. In this example, the 8-bit setup scan has 6 minutes duration to allow for more slewing to the appropriate elevation for the 3-bit setup scan that follows it. Note the inclusion of the 30 second requantizer gain setup scan after every X-band reference pointing scan.


06m00s X-band (8-bit) attenuator setup scan [intent = setup intent, scan mode = standard observing]
01m00s K-band (3-bit) attenuator/gain slope equalizer setup scan
                      [intent = setup intent, scan mode = standard observing]
05m00s X-band (8-bit) reference pointing scan [scan mode = interferometric pointing]
00m30s K-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
01m00s K-band (3-bit) calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
.
.
.
03m00s X-band (8-bit) reference pointing scan [scan mode = interferometric pointing]
00m30s K-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
01m00s K-band (3-bit) calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
08m00s K-band (3-bit) target source scan [intent = setup intent, scan mode = standard observing]
etc.

High frequency with multiple 3-bit settings
In this example there are two high frequency 3-bit settings, at K and Q-bands. These observations also need reference pointing scan(s) at X-band, and therefore an 8-bit setup scan should be included. Considering that the initial slew can take up to 9 minutes, and the reference pointing scan needs to be at least 2m30s long on-source, this example will allow up to 12 minutes at the start of the scheduling block for the two 3-bit attenuator and gain slope equalizer scans (each at least 1 minute), the 8-bit setup scan (at least 1 minute), and the reference pointing scan. Note that the attenuator values for the 3-bit and 8-bit tunings will be remembered for future scans. In this example, the 8-bit setup scan has 3 minutes duration to allow for more slewing to the appropriate elevation for the 3-bit setup scans that follow it. Note the inclusion of the 30 second requantizer gain setup scan after every X-band reference pointing scan and after every change in band (from Q to K-band for instance).


03m00s X-band (8-bit) attenuator setup scan [intent = setup intent, scan mode = standard observing]
01m00s Q-band (3-bit) attenuator/gain slope equalizer setup scan
                      [intent = setup intent, scan mode = standard observing]
01m00s K-band (3-bit) attenuator/gain slope equalizer setup scan
                      [intent = setup intent, scan mode = standard observing]
07m00s X-band (8-bit) reference pointing scan [scan mode = interferometric pointing]
00m30s Q-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
01m00s Q-band (3-bit) calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
04m00s Q-band (3-bit) target source scan [intent = setup intent, scan mode = standard observing]
.
.
.
01m00s Q-band (3-bit) calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
00m30s K-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
01m00s K-band (3-bit) calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
etc.

C or X-band with single 3-bit setting
In this example there is a single low frequency (C or X-band) 3-bit setting. Such observations usually do not require reference pointing scans. The scheduling block may start with an optional 8-bit dummy slew scan, e.g., C-slew (or X-slew for 3-bit X-band observations). The dummy slew scan resource should use a frequency tuning that will not be used later. In this example, the 3-bit attenuator scan has 3 minutes duration to allow for more slewing to the appropriate elevation for the 3-bit attenuator and gain slope equalizer scan (at least 1 minute). This should be followed by the 10 second requantizer gain setup scan.


03m00s C-slew (8-bit) {OPTIONAL} dummy scan with a frequency tuning that will not be used again in the scheduling block
                      [intent = setup intent, scan mode = standard observing]
01m00s C-band (3-bit) attenuator/gain slope equalizer setup scan
                      [intent = setup intent, scan mode = standard observing]
00m10s C-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
06m30s C-band (3-bit) calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
etc.

C and X-band with multiple 3-bit settings
In this example there are two low frequency 3-bit settings, at C and X-bands. Such observations do not require reference pointing scans. The scheduling block may start with an optional 8-bit dummy scan that uses a frequency tuning that will not be used later (one of the NRAO defaults, C-slew or X-slew would be appropriate). In this example, the 3-bit attenuator scan has 3 minutes duration to allow for more slewing to the appropriate elevation for the two 3-bit attenuator and gain slope equalizer scans (each at least 1 minute). This should be followed by the 10 second requantizer gain setup scan for one of the 3-bit tunings, which precedes the regular calibrator and target scans that use that particular tuning. A 30 second requantizer gain setup scan for the other tuning will be needed before observing its regular calibrator and target scans. If more alternation between the C and X-band tunings occur in the scheduling block, the appropriate 30 second requantizer gain setup scans should be inserted as needed.

03m00s C-slew (8-bit) {OPTIONAL} dummy scan with a frequency tuning that will not be used again in the scheduling block [intent = setup intent, scan mode = standard observing]
01m00s C-band (3-bit) attenuator/gain slope equalizer setup scan [intent = setup intent, scan mode = standard observing]
01m00s X-band (3-bit) attenuator/gain slope equalizer setup scan [intent = setup intent, scan mode = standard observing]
00m10s X-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
06m30s X-band (3-bit) X calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
.
.
.
01m00s X-band (3-bit) X calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
00m30s C-band (3-bit) requantizer gain setup scan [intent = setup intent, scan mode = standard observing]
01m00s C-band (3-bit) C calibrator scan [intent = calibrator intent(s), scan mode = standard observing]
etc.

Low Frequency Observations in the Presence of Strong RFI

Below we discuss two types of strong RFI and how to best avoid it.

Strong RFI from Geosynchronous Satellites

C, X, and Ku-bands are subject to strong RFI from satellites in the Clarke Belt—the zone of geosynchronous satellites that can severely affect observations of sources in the declination range of −15° to +5°. While such strong RFI may not saturate the 3-bit samplers as reported in EVLA memo 187, here we present an approach to better ensure the success of the observations.

In the example below, there is one low frequency 3-bit setting (C-band) but the sources of interest are in the declination range noted above.

Start the scheduling block with a dummy scan that observes a source that is either below −15° declination or above +5° declination to avoid the satellites in the Clarke Belt. Set the frequency of this dummy scan to a tuning that will not be used later (hence the ? in the example below). This is important for the attenuator and gain slope equalizer setup scan that will follow, which should be on the same source selected for this scan.
For the 3-bit scans (all setup and science scans), the NRAO default instrument configurations cover the full frequency ranges of the C-band (4–8 GHz) and X-band (8–12 GHz) receivers. Observers wishing to follow a conservative approach, particularly in order to minimize the effect of the RFI from satellites in the Clarke Belt while observing in the declination range of −15° to +5°, can tune the 2 GHz-wide basebands of the 3-bit samplers to frequencies that would exclude this RFI. (With the following recommended tunings, the OPT will produce a warning that part of the sampler output lies outside the nominal range of the receiver. This warning can be safely ignored, for the purpose of avoiding this strong RFI.)

For C-band (4–8 GHz), the affected frequency range is 4 to 4.2 GHz, and the 2 GHz-wide basebands from the 3-bit system can be centered at 5.25 GHz and 7.2 GHz to avoid this RFI.

Note, C-band is also impacted by strong RFI caused by microwave links near 6 GHz in the A and B configurations. As a result, 3-bit data obtained with the standard setup are corrupted. We advise observers to use a mixed 3-bit and 8-bit samplers. See the next section for more details.

For X-band (8–12 GHz), the affected frequency range is 11.7 to 12 GHz, and the 2 GHz-wide basebands from the 3-bit system can be centered at 9 GHz and 10.65 GHz.

Clarke belt example


03m00s C-slew (8-bit) dummy scan using a source below −15° or above +5° declination.
         This dummy scan should have a frequency tuning that will not be used again in the scheduling block.
         [intent = setup intent, scan mode = standard observing]
01m00s C-band (3-bit) attenuator/gain slope equalizer setup scan using the same source as the previous scan
         [intent = setup intent, scan mode = standard observing]
00m10s C-band (3-bit) requantizer gain setup scan using the same source as the previous scan
         [intent = setup intent, scan mode = standard observing]
07m30s C-band (3-bit) scan on a calibrator that may be in the satellite zone
         [intent = calibrator intent(s), scan mode = standard observing]
etc.

C-band Observations in the Presence of Strong RFI Microwave Links

At C-band, communication transmitters near the VLA have been observed to produce strong interference (RFI) at 6.2 and 6.4GHz. These point-to-point signals tend to affect subsets of antennas rather than the entire array, depending on array configuration and pointing direction. These signals are sometimes strong enough to cause compression that corrupts an entire 3-bit baseband. The 8-bit samplers are more resistant to this effect due to their higher dynamic range; in 8-bit data the narrowband RFI can be flagged and removed using standard procedures.

So far this issue has only been observed in the extended array configurations (A and B). The intensity is variable and can be stronger on some days than others. To mitigate this effect, we produced an alternative default continuum resources named C32f2Amixalt and C32f3Bmixalt. This uses both 3-bit and 8-bit samplers, placing the 8-bit baseband on the strong RFI. This setup maintains the same frequency coverage as the previous NRAO default resource setups for 3-bit C-band. There are also new resources that incorporate both microwave RFI and RFI blanking called, C32f2Amixalt-blank and C32f3Bmixalt-blank (link to section on RFI blanking).

Our testing indicates that the resource with the mixed 3-bit and 8-bit samplers performs better than the standard 3-bit resources in these conditions. Therefore, we strongly recommend usage of the new resource for C-band projects at A and B configurations in place of the previous 3-bit defaults.

Since this resource is a mixed 3/8-bit setup it will require the requantizer gain setup scans.

03m00s C-slew (8-bit) {OPTIONAL} dummy scan with a frequency tuning that will not be used again in the scheduling block
         [intent = setup intent, scan mode = standard observing]
01m00s C-band (3/8-bit) attenuator/gain slope equalizer setup scan
         [intent = setup intent, scan mode = standard observing]
00m10s C-band (3/8-bit) requantizer gain setup scan 
         [intent = setup intent, scan mode = standard observing]
06m30s C-band (3/8-bit) calibrator scan 
         [intent = calibrator intent(s), scan mode = standard observing]
etc.

Data Processing

The requantizer setup, which occurs after every change in the tuning, introduces small (~5-10%) gain changes that must be corrected for before proceeding with the rest of the calibration. The values of the corrections needed are stored in the measurement set, but must be converted to a gain table to be applied to the data. It is one of the predetermined prior calibrations applied by the VLA calibration pipeline.

The following gives an example of how to make a gain table that corrects for the requantizer gains in CASA:

gencal(vis=myms, caltable='requantizergains.g', caltype='rq')

In subsequent calls to gaincal, the calibration table requantizergains.g must then be applied as a priori calibration by specifying it in the gaintable parameter. Failure to apply requantizer gain corrections to the data will result in an increased uncertainty in the flux density scale. The requantizer gain correction option in gencal is available in CASA 4.1.0 and above. Visit our Obtaining CASA web page to download the latest CASA distribution.

In AIPS, to apply the requantizer gain corrections, use the task TYAPL with optype ='PNG'.