A report on the current status of this issue
and the future steps which will be necessary
to perform for achieving routine operation by 2001.
The IF processor of the correlator entirely carries out the task of extracting the 16 x 16 MHz subbands (8L + 8U) from the IF band, anywhere from 100 to 1100 MHz.
Phase Cal 1 MHz signal
The comb generator mostly used is the MPI-produced Alan Rogers design,
which involves a 5 Mhz-excited step recovery diode followed by a microwave
switch that only transmits every 5th pulse. This design, although aging
now, is field-proven and has demonstrated good stability. At some point
we will have to choose either to purchase one to MPI or design a new one.
This item is essentially useful at 1mm where the sources are not strong.
Some dilution of the comb energy will take place in the addition with
the other antennas for which the comb has not been injected. This will
have to be compensated by injecting more comb power at the reference antenna.
Maser + GPS monitoring
The H-maser (provided it is permanently assigned to the PdB site) has
to be permanently taken care of by a technician. A quiet and isolated place
(a corner of the electronics room) has to be equipped to host it. A practical
sytem has to be installed to monitor its drift against the GPS over years,
and the monitor data has to be archived in a computer in the same fashion
as for example the meteorological data is archived. The surveillance of
this data is essential for maintaining the maser in good health.
A new GPS receiver has to be purchased for the PdB, which keeps in
lock not by adding/skipping 100ns pulses, but by disciplining a good quality
crystal oscillator. The model used in Pico Veleta ( GPS
truetime ) has shown good behaviour, and smooth backups in case of
temporary loss of the GPS antenna. It also can be used conveniently as
a long-term maser drift monitor.
Synthesizer
The PdBI currently uses hp 8644B synthesizers to set the sky frequency. They operate in the 1850-1900 MHz range. It is generally assumed that their phase stability (versus the external 5 MHz) is not good enough for VLBI. This will be checked soon locally. If so, the tradition in other observatories is to use Racal 3101's which are proven. The problem is that a) their frequency is limited to 1300 MHz and b) this 15-year-old design is not manufactured any longer. Although some second-handed might be available, the question of their succession is open. Manufacturer VertiCom has been cited. Their design looks excellent but since they use DDS (direct digital synthesis) technique, it may happen that the frequency they deliver would not be exactly decimal. Remember that one millihertz offset at 2GHz leads to 36 turns over 6 minutes at 230 GHz ... This has to be checked.
Adder
The adder will be a part of the correlator and will deliver an analog signal to the sampler. The level will need to be adjusted (manually, once) to the optimal level of the samplers. Since the analog signal has been reconstructed from digital data its value will not vary with receiver(s) output. The AGC circuit sometimes used elsewhere will not apply. A 10 kHz BP filter will be built in for viewing and setting the amplitude of the PCal.
Formatter
The Mark IV formatter (already ordered) will come with the samplers built-in. So the interface with the adder will be limited to a piece of coax cable. It will need 5MHz and 1pps from the observatory system. It will generate the organized streams of data together with the 32MHz writing clock. Is is bought as a turnkey device.
Tape recorder
Several question marks here. It seems that a (single-man ?) company named Signatron can customize Honeywell drives to Mark IV.
Software
A few months before we receive the formatter we need to order a special (?) control PC to SWT, Texas. This PC will need to be loaded with the FS-9 (VLBI session manager) software at GSFC (Goddard,Ca). Only when this equipment comes available in PdB can the integration start. A few pieces of software will need to be written (in Linux ?) in order to traduce the observing log file into local commands, mainly pointing data and settings originally sent to the BBC's, which now need to be applied to the correlator.
Useful links: