Control of the HP3325 frequency synthesizer by a PDP 11 computer for gravitational experiments

The digital interface for the control of the frequency synthesizer is described. The system is composed of a room temperature 2500 kg gravitational antenna, a transducer with a low noise amplifier coupled to two phase sensitive detectors controlled by reference signals from a high stability oscillator (frequency synthesizer) working at the exact resonant frequency of the antenna. The temperature variations change the resonant frequency of the resonant modes of the antenna, requiring an automatic control of the reference signals. The electrical schema of the synthesizer command module which interfaces the PDP11 computer to a maximum of 8 HP3325A synthesizers is given. The program for the management of the interface is described.