External References

The internal references, since they need to be adjustable in a wide frequency range regardless of whether it is in standard or LN mode, have significant influence on phase noise measurement sensitivity. Measuring DUTs with extremely low phase noise would then require a lot of cross-correlation and thus be very time-consuming.

Using external references can reduce the number of cross-correlations, and therefore, shorten the measurement time. Choice of external references include:

• Frequency-tunable (voltage control input)
• Frequency tuning ranges need to overlap with DUT frequency
• Phase noise of refs can be 10…15 dB worse than DUTs
• Both single and dual ref channels possible