An apparatus comprises: a first oscillator; a second oscillator; a first frequency synthesizer configured to receive a first input from the first oscillator and to provide a scaled first output frequency based on the first input; a second frequency synthesizer configured to receive a second input from the second oscillator and to provide a scaled second output frequency based on the second input; a mixer configured to receive the scaled first output frequency and the scaled second output frequency and to output a difference between the scaled first output frequency and the scaled second output frequency; and a processor configured to determine a change in an output frequency of the first oscillator based on the difference. The change in the output frequency provides an indication of a change in a measurement parameter.