The T&F System consists of three major elements:

• Two active hydrogen masers (AHM) located in the ‘Maser Room’ (MAR).

• One main rack located in the Antenna Equipment Room (AER) driven by the 2 AHMs

• One rack located in the Main Equipment Room (MER), synchronised to AER rack via optical fibres.

• Cabling between elements in Maser Room and AER and to the interface points for cross-site connections between AER and MER.

• The M&C interface is realized with a TCP/IP LAN connection to the AER rack.

System Overview (simplified), including elements in MAR, AER, MER and the interconnections via the three sites.
Two GPS antennas are mounted outdoor.

Two redundant, highly stable active hydrogen masers (AHM) form the reference for the F&T system. They are located in a dedicated room (Maser Room) together with their dedicated uninterruptible power supplies. One of the AHM is the active frequency source, which is selected with the Maser selection. The second one is free running as a hot standby and continuously compared to the active maser.

The selected maser output is buffered and distributed to a range of 5 MHz frequency outputs – balanced and unbalanced - available at the interface panel of the system racks. 10 MHz and 100 MHz outputs are also available.

The frequency reference is complemented with date & time information using redundant GPS receivers. Using the maser-provided frequency reference, the GPS derived date & time information is used to generate timing signals according to the IRIG-B standard with 1 kHz and 5 MHz carrier frequencies.

Continuous system performances monitoring is achieved with a Phase Comparator unit, which measures the phase and frequency difference between the masers. It monitors the delay stability of the AER distribution system as well. Measurement data is sent to the external monitoring and control host and stored in an internal database on the Linux server.

Redundant power supply concept of the F & T System

All critical elements are in redundant configuration. A redundant primary power concept is implemented in the maser room and for the AER rack. The MER rack implements a rack-internal UPS feeding a redundant power distribution system.

Additional equipment is located in the Main Equipment Room (MER). It exhibits its own precision frequency reference, which is synchronized by a computer controlled PLL circuit to F&T signals received from the AER. A range of T&F signals is generated in the MER room synchronous and in time with respect to the F&T signals in the AER.

The Phase Comparator (PCO) unit performs all system-internal monitoring and control functions. It is the only interface to the external ground station M&C system, which is realized with a dedicated Ethernet external LAN port. The F&T units communicate to each other by a system-internal ‘private LAN’, which extends to the MER by an optical link. The private LAN is the backbone of the internal data flow and logically and physically separate from the external LAN.

The physical LAN access point is realized with a LINUX PC acting as manageable router (ROU).

Additional serial data lines provide M&C to the units located in the maser room. In particular, the maser performance is continuously monitored as basis for the maser selection criterion. Maser data and system status and system events are logged on the LINUX PC for the purpose of preventive system health monitoring and in particular for preventive maintenance of the AHM’s at intervals of approx. 3 months.

Dedicated sensors inside the MAR monitor the MAR room conditions (temperature, magnetic field and humidity).

The two masers are mounted in thermally controlled Heater/Cooler Boxes (HCB’s). The HCB’s are controlled by two Heater/Cooler Units (HCU).

All units in MAR are DC isolated from the AER rack, either by optical isolators (4 serial lines to AHMs and HCUs), transformers (1 pps from maser) or by capacitive decoupling (AHM output signals 5 MHz and 100 MHz).