VLBA New Digital Architecture (VNDA)

Current status: VNDA capabilities are not available to observers. Prototype systems are installed at two VLBA sites

The VLBA New Digital Architecture (VNDA) will replace the ROACH Digital Backend (RDBE). VNDA units will be installed at all VLBA sites throughout the 2027 calendar year. VNDA will retain all of the capabilities of the RDBEs, and include some significant improvements. During 2027 and early 2028, only those capabilities that replicate RDBE performance will be offered for General Observing.

For clarity, two items of VLBA terminology are defined here (the definitions are the same as for the RDBEs):

An "IF" refers to one of a maximum of four intermediate-frequency analog signals transmitted from the receiver(s) to VNDA. Most receivers provide two IFs, in opposite circular polarizations. However, four IFs are available to support specialized observing modes at some wavelengths: two dual-polarization pairs, at arbitrary frequencies within the full range of the 6-cm receiver; or from different receivers in 13/4-cm or 90/50-cm dual-receiver operation. A planned upgrade will provide support for six IFs.

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. This approach is essential for the VLBA, where capabilities are fundamentally limited by the overall data-transmission bandwidth.

The VNDA system is comprised of two components: a “producer” which samples the analog data from the IFs, and a “consumer” which reads the sampled data stream and creates the channelized complex-valued VDIF output. It is a collaboration between NRAO, Fourier Space, and Digital Radiometer Systems LLC. An effort has been made to use as many commercial off-the-shelf components as possible, both to ensure resiliency and to enable upgrades in the future.

One major advantage of VNDA over the RDBEs is that it will provide the capabilities of both the polyphase filterbank (PFB) and digital downconverter (DDC) “personalities”:

  • Generate up to 16 baseband channels per IF
  • Channel bandwidths between 1 and 512 MHz (in powers of two)

The improved capabilities of VNDA are:

  • 14-bit native sampling (vs 8-bit for RDBE)
  • 1024 MHz sampled bandwidth per IF (vs 512 MHz for RDBE)
  • 1, 2, 4, or 8 bits per sample VDIF output (vs 2 bit only for RDBE)
  • No restrictions on tuning of channels within a sampled band

Note that both the larger sampled bandwidth and the capability to produce 4 or 8 bit VDIF data will lead to increased data rates. The Mark 6 data transmission system has a maximum specified recording rate of 16 Gbps. However, NRAO currently only supports recording rates up to 4 Gbps due to disk pack shortages and other limitations.Therefore, users will need to carefully consider their scientific needs when deciding which VNDA capabilities to utilize.

The current VNDA capabilities available to observers are specified in the Call for Proposals. Observers who wish to utilize VNDA capabilities beyond what are offered in the Call for Proposals are encouraged to submit Resident Shared Risk Observing (RSRO) proposals.

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