A Method for the Precise Measurement of the Difference between Two Low Frequencies

A method is presented for measuring the difference between two low frequencies with high resolution. The method is based on multiplying the two incoming frequencies by a large factor and then using a BCD up/down counter to store and display the resulting frequency difference. To achieve a resolution of n decimal places, the multiplication factor must be 10n and the decimal point must be placed n digits to the left of the least-significant digit of the displayed results. The inherent ± 1 count error will result in a resolution of ±10-n Hz. Frequency multiplication is achieved using a phase-locked loop stage which provides a multiplication factor of 103. This method has the advantage of being simpler and less expensive compared to other methods using period-measuring techniques.