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DBE is used to specify the personality to use in a digital backend that is based on an FPGA. Such personalities can be changed quickly, so this parameter is not appropriate for the station catalog. However personality changes have not been integrated with the observing system yet so they will not be allowed within one key file. Use multiple key files if you must switch, and don't do it often. The personality determines much about the capabilities of the hardware. In particular, it determines what baseband frequencies can be set.

The possible options are:

RDBE_PFB is the original polyphase filter personality developed by Haystack for the RDBE digital backend (DAR=RDBE in the station catalog). It is restricted to 16 baseband channels of 32 MHz bandwidth each, lower sideband, with frequencies of 1024-16-N*32 MHz, where N is an integer between 0 and 15. The initial version is restricted to N even and there are no choices. In that version, channels come in pairs, one on each input IF, which are usually polarization pairs. Switching is being added to allow general selection.

RDBE_DDC is the digital downconverter personality being developed for the RDBE digital backend at NRAO. It has a high degree of flexibility of tuning frequency and bandwidth. The restrictions will be given here once they are known in detail. For this personality, the input IF (512-1024 MHz) is split into three signals by a complex polyphase filter into three bands (512-640, 640-896, and 896-1024 MHz). Note that the central one is 256 MHz wide while the other two are 128 MHz wide. The few MHz around the transition between these bands should be avoided as signal will be degraded, and any one baseband can only access one side of such a ``crossover'' frequency. SCHED will warn of attempts use degraded frequencies (eventually). The RDBE_DDC can provide 4 baseband channels from each of the two input IFs. Those baseband channels can be as wide as 128 MHz so the full input bandwidth can be covered. See the wideband observing section for more information about tuning restrictions and about the use of dual RDBEs.

DBBC_PFB This is for the DBBC digital backend being built in Noto for the EVN. It is meant to have the flexibility of the old MarkIV and VLBA backends. Like the RDBE, it has PFB and DDC options. The PFB option is much like the RDBE equivalent and the two will be treated the same in SCHED until that proves to be incorrect.

DBBC_DDC This is the digital down converter option for the DBBC. Unlike the RDBE equivalent, it does not have crossover issues as the frequency conversion is done on the full bandwidth data stream. Each channel, or upper and lower sideband pair of channels, is derived in a separate converter. Basically it tries to duplicate the function of the legacy BBC/VC in a digital package with sampling before the filter. There are complicated restrictions on the IFs that can be assigned to each BBC as described in DBBC. These restrictions should be alleviated with future firmware upgrades.

DBE should not be specified for DAR's other than the RDBE and DBBC. Note that it may be necessary to explicitly set DAR to ' ' (blank) if there are multiple segments to the setup file and an earlier one has it set to something else.

When the DAR is the RDBE, the output channels and all the input channel information given to SCHED are written to the VEX file. But the crd files that control the old VLBA hardware also has to be told something. SCHED does not have a separate set of variables for all those configuration parameters, so it just does something reasonable. It sets the number of channels to the maximum of the number requested and 8. It sets the frequencies and sidebands to match the RDBE requests. It sets the sample rate to the maximum of that requested and 32 Ms/s. It sets the channel bandwidth to the lesser of the request and 16 MHz. It only writes the first 4 pcal extraction requests (avoiding going into channel numbers that are too high).

next up previous contents
Next: DUALX Up: Details of Setup File Previous: BITS   Contents
Craig Walker 2014-06-17