Using software defined radios for baseband phase measurement and frequency standard calibration

We devise a novel phase and time error measurement technique for 10 MHz and 1 PPS reference signals, based on Software Defined Radios (SDR) and Digital Signal Processing (DSP). The proposed measurement setup allows us to employ two synchronized type N210 Universal Software Radio Peripheral (USRP) devices with LFRX daughterboards to realize an economically priced and easy-to-use quad-channel measurement setup. Our real-time C++ DSP software is based on the USRP Hardware Driver and enables flexible long-term measurements with verified sub-nanosecond precision. We demonstrate our intended use case, the calibration of two off-the-shelf rubidium standards (SRS FS725) against each other, using their RS 232 interface. By means of two 120 hour long-term measurements with a power cycle and cold start in between, we verify it is possible to reproducibly maintain a relative drift of ±5 parts-per-trillion, which is an order of magnitude better than specified by the FS725 data sheet. Additionally, we investigate the influence of the ambient temperature on short-term stability.