Phase-lock loop test-signal generator

An instrument for testing the performance of an antenna utilises a microwave signal source 1 of the type having coarse and fine frequency-control inputs, a sample of the output frequency being mixed 3 with a digitally-controlled synthesised local oscillator frequency 2 and the phase of the mixer output compared 5 with that of a reference oscillator 4 to provide an input 6 to the fine control of the signal source. A phase-locked loop is thus produced which locks the signal source 1 to a frequency set by the synthesised local oscillator 2. In an initial setting- up procedure the coarse control is swept through its operating range until phase locking occurs for each of a number of different frequencies. The value of the coarse control when locking occurs is recorded and the recorded values are used during a subsequent test procedure to enable almost immediate locking of the signal source onto the correct frequency. In a preferred arrangement the loop strain of the phased locked loop is monitored 12 and the coarse control adjusted so as to minimise loop strain. It is this adjusted value which is then memorised.