vided in the "Further Reading" section at the end of this chapter). 12.3.1 Reference Signals and Calibration Previously, Figure 12-8 showed the tester connected to a PRC/PRS with a 10-MHz cable. This is used as a frequency reference for the oscillator of the device to ensure that the measurements are accurate and that the synthesized frequency signals are within specification. For example, it is im- possible to accurately measure the wander on a SyncE signal without an accurate reference to com- pare it to. As a minimum, a tester needs to have an input reference, which is normally a 10-MHz reference signal from a PRC/PRS or PRTC or alternatively can be a BITS input (where BITS in a building is taking input reference from a PRC/PRS or PRTC). For back to back phase testing, the Calnex generates its own time signals to send to the DUT and compares the return values to what it sent, SO for that mode of testing, it does not need a 1PPS phase reference (see left-hand side of Figure 12-11). But if the Calnex is being used to test the phase at the final hop of a timing distribution, then it needs to have a 1PPS phase reference to compare the phase of the DUT with (see right-hand side of Figure 12-11). Figure 12-11 shows a tester with the relevant input reference signals for the different cases. PRC/PRS 10 MHz 10 MHz (Ref) PRTC / T-GM (Ref) (Ref) Timing network Tester Tester 10 MHz PTP + SyncE (Ref) (measure) (Ref) (measure) Figure 12-11 Timing Tester with Reference Signals In the case shown on the right side of Figure 12-11, the tester is measuring the phase 1PPS or PTP signal coming from the router and comparing that with the reference phase from a PRTC. This is a test of whether the last clock is putting out a signal within the range of the budgeted phase error re- quired by the application. For example, if the application requirement is that the phase is within +1.1 us of Coordinated Universal Time (UTC), then the tester should be able to measure the value coming out of the last clock in the chain to confirm it. In th