Fig. 6 Frequency and Time Supply
Translation of signals from the radio frequency range to the intermediate frequency range occurs with one or more oscillators. During observation, with a few exceptions where free running oscillators are used, the phase of these oscillators is held fixed to the output of our Hydrogen maser.
For the tunable oscillator we use the "Universal Local Oscillators" (ULO). This is for ULO1 a frequency synthesizer (2 synthesizers actually to allow frequency switching) connected to the frequency standard followed by a frequency doubler and has an output frequency in the range 1.0 to 2.0 GHz, for ULO2 a frequency synthesizer connected to the frequency standard with an output frequency in the range 0.01 to 1.0 GHz. ULO1 and ULO2 can be set either by computer or by hand in steps of 2 Hz (ULO1) or 1 Hz (ULO2). These signals are amplified and transmitted to the focus cabins and, if necessary, translated up to the desired frequency range in the receivers either by frequency multipliers or by means of phase locked oscillators. It is recommended to check for interference caused by mixing of harmonics of both ULO's, if untested frequency combinations are used.
The station clock is driven by the Hydrogen maser or a Rubidium standard (in case of a maser problem), regular comparisons are made with UTC signals obtained from a GPS receivers. (See Fig. 6)
During frequency switching the ULO1 frequency is changed periodically by switching between two synthesizers set to different frequencies. Both frequencies can be set by the computer. Using frequency switching it is necessary to take into account the switchover times of all the phase locked oscillators involved in establishing the end frequency.