Conversion of Legacy Schedules to RDBE/DDC
The Digital Downconverter (DDC) is one of two digital observing systems currently supported in the VLBA. The DDC is implemented in FPGA firmware, operating within the Roach Digital Backend (RDBE). The DDC and RDBE are described in context in Section 5.4 of the VLBA Observational Status Summary.
A four-channel DDC implementation has been available since February 2013, and the full eight-channel system has been commissioned and is now in use. (A “channel” refers to a single contiguous frequency range, of any bandwidth, observed in a single polarization, that is sampled, filtered, and recorded as a separate entity.) The original “legacy” data system was maintained long as possible to facilitate a transition to the new VLBA Sensitivity Upgrade instrumentation, but was decommissioned as of the end of CY 2013.
This page describes the relatively straightforward conversion of SCHED “keyin” files as specified for the legacy system, to use the DDC observing system instead. It is designed primarily for users with some VLBA experience who wish to adapt previously observed schedules to new observations.
Comparison of Legacy and DDC Capabilities
While the implementations are completely different, and the DDC’s signal processing is inherently more stable and reproducible, the control parameters that must be specified for both data systems are substantially similar. The principal differences in capabilities can be summarized conveniently in terms of those parameters:
- The DDC supports 1, 2, 4 or 8 channels. The legacy system’s 16-channel upper-lower-sideband scheme is not available, nor are 3/5/6/7-channel cases.
- Channel bandwidths extend upwards in the DDC, beyond the legacy system’s maximum of 16 MHz, to 32, 64, and 128 MHz. The extremely narrow legacy bandwidths below 1 MHz are not available. All channel bandwidths must be the same within any setup. Selection of upper or lower sideband frequency conversion is supported in the DDC as in the legacy system.
- DDC channels are independently tunable. However, the tuning quantum differs from that of the legacy system: instead of 10 kHz, DDC channel frequencies must be specified as multiples of 15.625 kHz. This quantum is 218 times the DDC's internal tuning step of 128-MHz/231 (~59.605 milli-Hz), and is both the smallest integral-Hertz multiple, and the closest to 10 kHz. Observations requiring compatibility with legacy or similar instrumentation at non-VLBA observatories should use the smallest common multiple of both quanta, 250 kHz. The standard DDC default in SCHED sets the 15.625-kHz tuning.
Conversion of Legacy Schedules for DDC Observations
Specific SCHED parameters which may require changes are set in bold-face type below. Detailed parameter descriptions are available in the SCHED User Manual.
- A special RDBE version of the Frequency Catalog must be used:
FREQFILE = $SCHED/catalogs/freq_RDBE.dat
- If a standard DDC setup cannot be found for a desired configuration, a legacy-system version can be modified as above, and inserted inline.
- Inline setup files must specify this parameter: DBE = rdbe_ddc .
- Channel frequencies (specified by BBSYN, FREQREF, FREQOFF) must be multiples of 15.625 kHz. This applies to frequency offsets used to shift phase-cal tones away from integral-MHz frequencies within the channel, as well as to Doppler-based offsets.
Modes with 8/4/2/1 channels, equal bandwidths of 16/8/4/2/1 MHz
- A substantial majority of all legacy modes scheduled in recent years match these specifications.
- These modes require no further changes beyond those in the preceding sub-section.
Modes with bandwidths narrower than 1 MHz, or differing among channels
- The DDC cannot match the narrowest legacy channel bandwidths due to formatting restrictions, nor does it support channels of differing bandwidth. However, equivalent results can be achieved using the DiFX correlator's high spectral resolution and spectral zoom capabilities, in multiple passes if necessary.
- Set SCHED parameter BBFILTER=1, or to the widest among different bandwidths, and adjust CORCHAN as necessary to achieve the required spectral resolution.
Modes with 16 channels
- These modes all use adjacent, opposite sidebands of the eight legacy BBCs. All such cases can be replaced with equivalents by specifying NCHAN=8 and twice the channel bandwidth (BBFILTER). The double bandwidth is always possible because of the DDC’s wider-bandwidth capabilities.
- Channel frequencies (specified by BBSYN, FREQREF, FREQOFF) should be shifted to the appropriate edge of the combined band.
- To achieve equivalent spectral resolution, the requested number of correlator spectral points per channel (CORCHAN) should be doubled; the total data volume does not change, however.
Modes with 3/5/6/7 channels
- Such cases are extremely rare on the VLBA; none have been seen in the past two years. They are not currently supported in the DDC.
- Additional dummy channels must be added to bring the total up to 4 or 8. To avoid problems caused by duplicate frequencies, these extra channels should be specified with offsets from the desired subset.
- Please bring any cases not covered above to our attention via the NRAO Helpdesk.