Conversion of Legacy Schedules to RDBE/DDC
The Digital Downconverter (DDC) subsystem is the final stage of the VLBA Sensitivity Upgrade Project. 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 for observing 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.)
Dual operation of the VLBA's original “legacy” data system and the new Sensitivity Upgrade instrumentation was maintained as long as possible, to facilitate transition. However, ongoing VLBA operations were increasingly impacted by difficulties in maintaining the legacy data system, and by inefficiencies arising from dual recording media pools. The last proposals for which the legacy system could be requested were submitted at the August 2012 deadline, and the legacy data system, including the Mark 5A recorders, 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 data system, to use the DDC 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.
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.
- Special RDBE versions of the Station and Frequency Catalogs must be used:
STAFILE = $SCHED/catalogs/stations_RDBE.dat
FREQFILE = $SCHED/catalogs/freq_RDBE.dat
- (These catalogs will revert to the usual file names (without the 'RDBE' suffix) once the legacy system is decommissioned. That change will be announced with the SCHED release in which it is implemented.)
- Currently, there are no standard setups for the DDC, but if desired a legacy-system version can be copied from the SCHED setups directory, modified as above, and inserted inline. A DDC inline setup is included in example egddc2.key in the User Manual.
- Inline setup files must specify these parameters:
DBE = rdbe_ddc … [The rarely specified default DBE = ‘ ’ should be replaced if present.]
FORMAT = vdif
- 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.
- Schedules specifying DOPPLER must add DOPINCR=15.625 in the main section of the schedule file (not in an inline setup definition) in order to quantize the Doppler-based frequency offsets correctly.
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.