The PdB interferometer features provision for two independent and simultaneous LO1's at each antenna.
In 2010 a new frequency plan (called LOSystemIII) was applied and all the receivers were equipped with new and up-compatible LO1Ref Boxes. They
were designed around a 15Ghz YIG oscillator. Twenty-for boxes were built or prepared, in view of the future.
In 2013, NOEMA being on its way, new LO Control units capable of driving 12 antennas were built and installed in the correlator room.
|The building section of the coaxial
cable system sends to each antenna two independent sets of frequency and phase
references. One set of
references consists of 3 signals:
- The synthesizer frequency, in the 1.6 - 2.1GHz range, to set the sky frequency
- The offset frequency, 0.5 MHz (fixed phase), also used for the cable round trip phase measurement
- The 100 MHz , which carries the sum of four components:
- The natural fringe rate (main component, for the exact LO1 frequency)
- The Walsh switching sequence current argument
- The cable phase correction, computed from the phase meter measurement.
- The zero complement for alignment on the reference antenna in phased array mode (vlbi).
The LO1 signal is generated in the receiver LO1 box (one per band ) by combining the 100 MHz with an harmonic of the LO1ref box output. Every LO1ref box can drive any number (1 to 4) of LO1boxes .
The current practice is that LO1ref#1 drives band 1 and LO1ref#2 drives bands 2+3+4. This makes full use of the two reference systems.
During the commissionning period of the new antennas in interferometric mode, the two resources will have to be shared between main array and sub array, calling for a temporary different connection, which will be normalized as soon as the new antennas join the main array.
The diagram above shows the physical layout of the complete system. All the pairs of cables from the stations will be concentrated on a pair of patch panels in the correlator room. These may represent the tracks for clarity. The selected stations are connected to their respective LO control units by wiring N/SMA jumpers of good quality cable and short length (1m and 1.40m are available).
On the antenna side, the 4 LO1 boxes are connected by semirigid wiring to either LO1ref#1 or #2 according to the development phase of each antenna.
This computer is mainly dedicated to LO1 control. It drives an EtherCAT loop that serves two phasemeters and two phase rotators.When the PolyFiX correlator will require it, the Barycentric generator will be added to this loop as a fifth element . Until then, the traditional 5MHz/1pps fixed referencing is used.
For historical reasons, this computer also hosts the "watchdog" program (on a dedicated EtherCAT loop) .
More on the Barycentric reference sytem.
32 times per second it wakes up the computer to send the argument for the next period to the gate, later to be applied by the 100 MHz DDS at the exact time. The same mechanism is reproduced in the correlator, in order to achieve synchronous Walsh encoding and decoding.
The clock value of 320 MHz allows the exact frequency of 100 MHz to be generated without synthesis residuals, which is required for the reference antenna during a vlbi session. This is because 100=(320/64)*20 and 64 is a power of two.
32 times per second, the software reads the status of the phasemeter integrator and sends a correction value to the 0.5 MHz DDS. The software averages 32 values to get the round trip phase every second. The hardware integration time of the phasemeter is independent and may or may not follow the barycentric timing. More on the phasemeter principle.
The amplitude of the 500 kHz analog input signal is proportional to the attenuation of the HiQ cable in service. This is a valuable indication on the health of the cable run and its associated connectors. It is measured in true dB and sent to the computer for monitoring. It has to be checked at each configuration change in order to prevent operation under degraded phase noise conditions, which would not be reported by the lock alarms.
wiring of the 1pps/5MHz distributions by means of two
Timetech racks. When the Barycentric generator will be installed, these
will be affected to 1pps-B
and 128MHz-B distribution to PolyFiX.