is constant the radio frequency signals are multiplexed between rf1 to rfn where rf indicates radio frequency and n is the number of different radio frequency signals the dividend of the divider element 7 is a signal f div which is approximately equal to the comparison frequency f comp and is compared therewith the use of a relatively high comparison frequency f comp results after multiplexing in a nett individual loop comparison frequency similar to that of existing high performance frequency synthesisers the present frequency synthesiser is controlled through a data input port 11 which feeds respective divider words into data buffers 1 n 15 and stimulates the timing generator 3 the buffers 15 are coupled in parallel to a divider multiplexer 17 which in turn is coupled to the divider element 7 the timing generator 3 is coupled to the divider multiplexer 15 and the radio frequency multiplexer 5 to provide time sequencing such that a radio frequency rfn and its corresponding divider word are incident upon the program divider 7 at the correct time to give a dividend frequency substantially equal to f div when in operation the synthesiser has a master control cycle comprising several sub cycles of a possibly unequal nature i e time span this is because the frequency control of the in picture picture does not need to be as stable as that of the main picture the timing generator 3 ensures that during each subcycle a buffered divider word and corresponding radio frequency rfn are multiplexed into the divider element 7 thus the respective radio frequency generator not shown can be controlled by its control signal 19 the control.