erence clock SO that it can do an easy comparison to the output signal returned from the clock under test. Note that if the tester is a different piece of equipment from the reference input clock, the same clock signal needs to be also passed to the tester to allow it to compare the measured clock to the reference clock. You cannot use two separate signals, one as reference and one in the tester. Synthesized reference Clock capture & clock (Clock) measurement equipment generator Timing testing equipment Figure 5-23 Time Error Measurement Setup The testing equipment that is generating the reference clock is referred to as a synthesized refer- ence clock in Figure 5-23 because of the following: For noise transfer tests, the reference clock needs to be synthesized such that jitter and wander is introduced (under software control). The tester generates the correct amount of jitter and wander, according to ITU-T recommendations, for the types of clock under test. The clock un- der test is required to filter out the introduced input noise and produce an output signal that is under the permissible limits from the standards. This limit is defined as a set of MTIE and TDEV masks. For noise tolerance tests, the reference clock needs to emulate the input noise that the clock un- der test could experience in real-world deployments. And for this purpose, the testing equip- ment simulates a range of predefined noise or time errors in the reference signal to the clock un- der test. To test the maximum noise that can be tolerated by the clock as input, the engineer tests that the clock does not generate any alarms; does not switch to another reference; or does not go into holdover mode. For noise generation tests, the reference clock that is fed to the clock under test should be ideal, such as one sourced from a PRTC. So, for this measurement, there is no need for artificially synthesizing the reference clock, unlike the noise transfer and noise tolerance case, where the clock is synthesized. The testing equipment then simply co