94 the reference signal 191 is applied to a numerically controlled oscillator u201c nco u201d 120 the signals output from both the vco 94 and the nco 120 are applied to a fourth frequency translator 130 the output signal from the fourth frequency translator 130 is band pass filtered u201c bpf u201d 140 and applied to a second input terminal of the third frequency translator 150 the output signal from the third frequency translator 150 is band pass filtered by a second bpf 170 and is divided by a second programmable frequency divider 98 before being applied to the phase detector 90 in operation the frequency of the nco 110 is controlled by the reference frequency 190 the reference frequency may be locally generated or may be received from an external circuit or device not shown the reference frequency may be applied to other similar circuits so that multiple digitally generated signals may be frequency locked to the same reference the nco 120 should be capable of providing frequency resolution to the extent required by the if signal 31 the output of the nco 120 is subtracted from the output of the vco 94 in the fourth frequency translator 130 the bpf 140 is used to filter out any undesired output from the fourth frequency translator 130 in operation the third frequency translator 150 subtracts the output signal of the programmable divider 180 from the output of the first bpf 140 the second bpf 170 removes any undesired output from the third.