Up: Details of Setup File
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
- 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
- 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).
- Argument: A character string of up to 8 characters
- Options: One of ' ', 'RDBE_PFB', 'RDBE_DDC', 'DBBC_PFB' or
'DBBC_DDC'. It is not case sensitive
- Default: 'RDBE_PFB' for a station with the RDBE DAR, 'DBBC_PFB'
for a station with the DBBC DAR, and blank for anything else.
- Usage: Defaults to previous segment. See note above about setting blank
- Example: DBE='RDBE_DDC'
Up: Details of Setup File