AN279 Silicon_Laboratories, AN279 Datasheet - Page 3

• Manufacturer Part Number: AN279
• Description: Estimating Period Jitter FROM Phase Noise
• Manufacturer: Silicon_Laboratories

4. Example
Consider the example in Figure 2, which represents the SSB phase noise in dBc/Hz measured for a 160 MHz
oscillator.
For this particular plot, there are three curves illustrated as follows:
The particular phase noise instrument that took the original phase noise data had a default maximum offset
frequency of 20 MHz when measuring clocks less than 250 MHz. For the purpose of estimating the period jitter, the
"ultimate noise floor" of the original data set has been extended from the last data point at 20 MHz out to the carrier
frequency of 160 MHz.
In this example, there are no discrete components or spurs. However, if there had been any significant spurs in the
vicinity of the half-carrier frequency, they would need to be weighted similarly as described in "Appendix B—
Calculating with Spurs Included" on page 7.
By numerical integration, we can determine that the integrated phase noise under the entire SSB phase noise
curve from 10 Hz to 160 MHz yields a total phase noise power = –54.46 dBc. This "brick wall" integration is
equivalent to wideband RMS phase jitter of 2.663 ps or 0.00268 radians.
Phase Noise—This is the upper solid curve going from –78.32 dBc/Hz @10 Hz on the left down to
–142 dBc @80 MHz on the right.
Period Jitter Weighting Function—This is the dashed line depicting the 4sin
predominately a +20 dB/dec sloped line until reaching a peak at the half-carrier frequency where it turns over to
a null at the carrier frequency.
Resulting Phase Noise—This is the lower, thicker solid curve representing the summation of the phase noise in
dBc/Hz with the Period Jitter Weighting Function in dB.
Figure 2. 160 MHz Oscillator SSB Phase Noise
Rev. 0.1
2
(πfτ) weighting factor in dB. It is
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