Frequency Biases in Pulsed Atomic Fountain Frequency Standards Due to Spurious Components in the Microwave Spectrum

A mechanism by which spurs in the microwave spectrum may cause biases in pulsed atomic fountain frequency standards was identified by Levi et al. Here we present models for the cases of phase modulating (PM) and amplitude modulating (AM) spurs in the spectrum. In the case of a PM spur, the bias is first-order in the spur amplitude. Under common experimental conditions, this effect may be much larger than the second-order bias presented by Audoin et al. (1978), We present measurements made using NIST-F1 whereby PM spurs of known amplitude were added onto the microwave signal used to interrogate the cesium (Cs) clock transition. The measured biases are compared to the predictions of the model. Finally, we discuss the difficulties of evaluating the frequency bias in fountain frequency standards using traditional techniques such as operating at elevated microwave powers