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BBSYN is used to set the frequency for the BBC or digital filter assigned to each baseband channel (subband channel in EVLA terminology). This number will be overridden if FREQ or DOPPLER is specified in the SCHED keyin file. Even if DOPPLER is specified, this value serves as the default for continuum sources for which no velocity or DOPSRC is specified. All baseband channels assigned to the same physical BBC should be assigned the same frequency. This only applies to hardware for which the BBCs can provide upper and lower sidebands. For the digital backends where the ``BBC'' is just part of the FPGA firmware, there will only be one baseband channel per BBC. When a BBC can provide more than one channel and different frequencies are given for such paired channels, the last specified frequency will probably be used and the monitor system will get confused about what signals are present. Actually, first SCHED will complain. Please see FREQREF for a discussion of required parameters.
For MkIV all BBCs at a given station should fall in the range 100 - 220 (the ``Low patch'') or 220 - 500 the ``High patch'').
For the VLBA DAR and MkIV systems, the baseband frequency must be set on even 10 kHz frequencies.
The future is with the digital backends - the RDBE for the system being developed in the U.S. and the DBBC system from Noto being deployed in Europe. There are also systems in other parts of the world but more information is needed before those can be described.
The RDBE DDC personality (DBE = RDBE_DDC) will support frequencies set to any even Hertz. But the infrastructure in many places is not set up to handle frequencies with that many significant digits. Four byte numbers are used and print formats, including in control files such as the vex file, do not have enough digits. Thus for now, RDBE frequencies must be set to even 10 kHz as with the older systems. Finer tuning may be provided in the future, but will require and end-to-end search through SCHED and various other places for places where the precision can be lost.
With the RDBE DDC personality, there are some IF frequencies to avoid. After sampling at 1024 GHz (the usable IF frequencies are between 512 and 1024 MHz), the firmware does a polyphase filter to narrow the bandwidth to match what the FPGA clock rate can handle (256 MHz in this case. A complex filter is used which allows output bandpasses of 256 MHz width. But the edge bands are centered at the sample rate and at half that so only half of each of those bands can be used. Thus the accessible IF is divided into 3 bands: 512-640 MHz, 640-896 MHz, and 896-1024 MHz. Baseband channels can be placed anywhere in any one of those 3 filter bands, but one should not attempt to cross the boundary between two. That won't work and SCHED will issue a warning.
The DDC personality can provide 8 basebands per RDBE. Those basebands can have bandwidths of the factors of 2 between 1 and 64.0 MHz with possible extension down to 125 kHz eventually. The total output bit rate has a maximum of 2048 Mbps. The values of BBSYN can set to any frequencies that are 1024 MHz (the sample rate) divided by powers of 2. The finest setting is 0.0596046 Hz. But any frequency that does not have an integer number of cycles in one second can cause big problems with carrying phase over various events like frequency switches. So the settings must be limited to multiples of 15.625 kHz -- the smallest setting that has an integer number of cycles in a second. This can be looked at as N*125 kHz + 0, 15.625, 31.250, 46.875, 62.500, 78.125, 93.750, or 109.375 kHz. SCHED will not accept settings that are not multiples of 15.625 kHz. But until we have a chance to make sure that the frequencies are kept to sufficient precision for finer settings, it is advisable to use multiples of 10 kHz, which is only actually possible with multiples of 250 kHz. SCHED will warn of attempts to use finer settings. Note that any crd files to control the old system while the DDC is in use will have BBC frequencies rounded to the nearest 10 kHz because of hardware limitations. That should be ok as data from the BBCs are most likely not going to be recorded.
The PFB personality of the RDBE, (DBE = RDBE_PFB) has rather rigid baseband frequency options. A polyphase filter divides the 512-1024 band into 17 baseband channels, 15 of which have 32 MHz bandwidth. The other 2 are half bands on the ends (actually full bands, but centered on the sample rate and half that). Those edge channels cannot be used. In the initial implementation, only every other channel could be used, starting with the one topping at 1008 MHz. Each RDBE has 2 IF inputs and puts out 2048 Mbps (512 MHz total bandwidth with 2 bit samples and Nyquist sampling) in 16 total baseband channels. There is not yet any ability to vary the bit rate. With the original PFB personality, there are 8 basebands from each input IF in the pattern mentioned above. An early modification (almost available at this writing) is to allow any 16 total channels to be chosen from the 30 usable channels generated.
The allowed BBSYN frequencies for the PFB personality are 1008.0, 976.0, 944.0, 912.0, 880.0, 848.0, 816.0, 784.0, 752.0, 720.0, 688.0, 656.0, 624.0, 592.0, and 560.0 and the bands must be lower sideband. The freqeuncies listed are at the top of the 32 MHz channel.
The DBBC system is more of a plug replacement for the MarkIV. The DBBC has a range of possible IF frequencies, but currently only 2 are supported (10-512 MHz and 512-1024 MHz). Up to 16 BBSYN frequencies can lie anywhere in the selected IF at a multiple of 10 kHz.