The Link Hut
The Link Hut contains the Hydrogen Maser Frequency Standard and the equipment required to transfer this standard frequency to the MERLIN telescopes and to receive, over microwave links, the signals collected by them.
The Sigma Tau Maser provides our Standard Frequency. In the Lovell Control Room we use this frequency to keep Jodrell Atomic Time. The frequency is also transferred to our outstations over an L-Band Link system where it is used to keep all the local oscillators used within the MERLIN array coherent.
The reference frequency received at each telescope site is re-transmitted back to Jodrell where its phase is compared to that originally transmitted. As a consequence of changes to the atmospheric conditions along the link path to the telescope the relative phase of the transmitted and received signal will change. A computer, seen at the right of the rack, continuously measures the relative phase and sends this information to the MERLIN correlator enabling an appropriate correction to be made to the data.
This computer also receives control commands from the MERLIN computer to set the synthesizers used to define the frequency in use at the MKII telescope and the attenuators to set the correct level for the signals received by it. The two hands of polarization are then modulated onto a 70 MHz carrier for transfer to the MERLIN correlator.
The link hut also contains the Ferranti Microwave Link receivers. Signals from microwave dishes mounted on the adjacent link tower are brought to them using very low loss heliax cables. They carry the two hands of polarization as independent sidebands on an amplitude modulated signal. In the link hut these are brought down to an IF of 70 MHz and transferred to the MERLIN correlator along with those from the MKII telescope.