L M K 0 3 0 0 0 C
E V A L U A T I O N B O A R D O P E R A T I N G I N S T R U C T I O N S
20
Appendix B: Impact of Reference on Phase Noise
Inside the loop bandwidth of a PLL the phase noise is set by the quality of the
reference oscillator used. For this reason it is important to select a reference
oscillator suitable for the application.
Test Setup
Using the same loop filter as described in the General Description and by driving
the OSCin frequency with an ultra low jitter 100 MHz Wetzel Crystal (501-
04517D) and setting R = 10 to achieve a phase detector frequency of 10 MHz. A
very low integrated RMS jitter of 247 fs is measured vs. the 2.1 ps measured in
the Phase Noise section with 19.44 MHz crystal in the bandwidth of 10 Hz to 20
MHz.
10 Hz – 20 MHz integrated RMS jitter = 247 fs (shown)
100 Hz – 20 MHz integrated RMS jitter = 243 fs
12 kHz – 20 MHz integrated RMS jitter = 222 fs
Conclusion
This diagram illustrates how the phase noise inside the loop bandwidth is set by
the quality of the reference oscillator used. Phase noise outside the loop
bandwidth is set by the VCO noise level.
