Phase noise describes jitter in the frequency domain. From the phase noise information, you can extract the RMS Phase Jitter value which is often given in datasheets as a measure of quality and spectral purity. The frequency spectrum plot in Figure 1 is of a real sinusoidal clock with nominal frequency Fc (also called the carrier frequency) plus sideband offset noise frequencies Fo. That means, sometimes the clock will have a slightly larger frequency Fc + Fo and sometimes it will have a slightly lower frequency Fc - Fo. The small changes in frequency appear as phase shifts in the clock waveform, hence the name phase noise. The noise power in a 1Hz band at an offset frequency Fo compared to the power of the carrier frequency Fc is called the dBc Phase Noise (dBc = power in dB relative to carrier). Refer to application note AN-815 for more details. For other questions not addressed by the Knowledge Base, please submit a technical support request.




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AN-815 Understanding Jitter Units Application Note PDF 565 KB