Daisy-chaining PPS via a USRP to another device

Application Note Number

AN-345

Abstract

This application note describes daisy-chaining the PPS GPSDO signal between USRPs that support PPS/TRIG IN and OUT.

Overview

Some USRPs such as the X300/X310, N300/N310, and N320/N321 provide ports for PPS/TRIG IN and OUT, and the hardware is designed to replicate the PPS GPSDO signal from IN to OUT as well as use the signal on the USRP itself. In theory these PPS ports could be used to create a daisy-chained signal distribution system that could be used by all USRPs -- a ring topology. The typical approach for GPSDO signal distribution is using a common source and then each USRP receives the signals from that common source -- a star topology.

While the hardware itself supports signal distribution via either topology, the USRPs themselves do not support the ring topology because their programming does not compensate for the time latency caused by each link in the ring. Ettus Research does not support a ring topology for GPSDO signal distribution; customers are free to try this topology and it may work, but it is an unsupported use-case.

The only supported GPSDO signal distribution systems are star topology, for example using an OctoClock-G to provide the common source for all GPSDO signals and then connecting up to 8 USRPs with those signals, each one requiring 2 cables between the OctoClock and USRP; this use-case works for all USRPs but is limited to a few meters distance between the OctoClock and each USRP, and all cables between these devices must be the same length in order to minimize timing jitter between USRPs.

For those USRPs that support White Rabbit -- the N3xx -- the distribution is still a star-like topology, but in this case the signals can be distributed across many kilometers rather than a few meters. This use-case is fairly niche as White Rabbit devices can be comparably priced to a single USRP, adding possibly significant cost to any USRP-based RF system.