The importance of this local timing source cannot be understated. It is one of three sources of timing in a 5G system that also include the network itself and the backup GNSS source that provides a pulse per second when the network goes down (see Figure 1). When this happens, the local timing source must act as a holdover clock and keep going until the primary source(s) of timing returns. It behaves like a flywheel that keeps spinning at a constant speed even when it’s not being actively driven. There can be no drift or temperature induced frequency changes, and no “activity dips” or sudden frequency jumps. The holdover clock source must be extremely stable so that the network synchronizer that selects between the three sources can perform “hidden” switching with no disruption in the signal phase of the outgoing clock.
The problem with quartz-based OCXOs in this critical 5G holdover role is that they are extremely sensitive to environmental stressors including shock, vibration, heat and rapid temperature shifts. Each of these stressors can disrupt the ability of a quartz-based OCXO to deliver a stable timing source. The lack of a stable timing source degrades network performance, reduces uptime and impacts mission-critical services such as advanced driver assistance systems (ADAS).