Oscillator metrology with software defined radio

TL;DR

This paper demonstrates that commercially available software defined radios can perform high-precision time and frequency metrology, surpassing traditional devices in accuracy and enabling novel comparisons across widely different frequencies, including optical clocks.

Contribution

It introduces a method using SDRs for high-precision time and frequency measurements, outperforming purpose-built devices and enabling comparisons at optical frequencies.

Findings

SDRs achieve 20 fs and 1 fs time deviation noise floors at 10 MHz and 6 GHz.

Relative amplitude measurement instability of 3e-7 at 5 MHz.

Femtosecond-level comparisons of ultra-stable lasers across terahertz frequency differences.

Abstract

Analog electrical elements such as mixers, filters, transfer oscillators, isolating buffers, dividers, and even transmission lines contribute technical noise and unwanted environmental coupling in time and frequency measurements. Software defined radio (SDR) techniques replace many of these analog components with digital signal processing (DSP) on rapidly sampled signals. We demonstrate that, generically, commercially available multi-channel SDRs are capable of time and frequency metrology, outperforming purpose-built devices by as much as an order-of-magnitude. For example, for signals at 10 MHz and 6 GHz, we observe SDR time deviation noise floors of about 20 fs and 1 fs, respectively, in under 10 ms of averaging. Examining the other complex signal component, we find a relative amplitude measurement instability of 3e-7 at 5 MHz. We discuss the scalability of a SDR-based system for simultaneous measurement of many clocks. SDR's frequency agility allows for comparison of oscillators at widely different frequencies. We demonstrate a novel and extreme example with optical clock frequencies differing by many terahertz: using a femtosecond-laser frequency comb and SDR, we show femtosecond-level time comparisons of ultra-stable lasers with zero measurement dead-time.