High Resolution Flicker-Noise-Free Frequency Measurements of Weak Microwave Signals

TL;DR

This paper presents a novel flicker noise-free frequency measurement system for weak microwave signals that reduces amplifier count and improves measurement precision, enabling detection of ultra-stable cryogenic sapphire oscillators.

Contribution

The authors develop a new measurement system that cancels correlated flicker phase noise, reducing amplifier requirements and enhancing measurement accuracy for weak microwave signals.

Findings

System performs as well as standard with half the amplifiers.

Measurement limited by thermal noise below -80 dBm.

Next-gen cryogenic oscillators could reach parts in 10^16 stability.

Abstract

Amplification is usually necessary when measuring the frequency instability of microwave signals. In this work, we develop a flicker noise free frequency measurement system based on a common or shared amplifier. First, we show that correlated flicker phase noise can be cancelled in such a system. Then we compare the new system with the conventional by simultaneously measuring the beat frequency from two cryogenic sapphire oscillators with parts in 10^15 fractional frequency instability. We determine for low power, below -80 dBm, the measurements were not limited by correlated noise processes but by thermal noise of the readout amplifier. In this regime, we show that the new readout system performs as expected and at the same level as the standard system but with only half the number of amplifiers. We also show that, using a standard readout system, the next generation of cryogenic sapphire oscillators could be flicker phase noise limited when instability reaches parts in 10^16 or better