Analysis of frequency biases and noise in sampled digital frequency servos for primary frequency standards

Unlike previous frequency standards at NIST, the main frequency servo of NIST-7, the U. S. primary frequency standard, does not lock a local oscillator to the atomic resonance. Instead it compares the frequencies of the atomic resonance and a reference oscillator. This paper presents an analysis of biases and noise in this digital servo. We show that a slight modification of the servo permits sensitive measurements for the presence of frequency biases.