Revised
-
December 2013
LMK04906 Family: Low-Noise Clock Jitter with Dual Loop PLLs
SNAU126A
35
Copyright © 2013, Texas Instruments Incorporated
Appendix B: Typical Phase Noise Performance Plots
PLL1
The LMK04906B’s dual PLL architecture achieves ultra low jitter and phase noise by
allowing the external VCXO or Crystal’s phase noise to dominate the final output phase
noise at low offset frequencies and the internal VCO’s phase noise to dominate the final
output phase noise at high offset frequencies. This results in the best overall noise and
jitter performance.
Table 10 lists the test conditions used for output clock phase noise measurements with
the Epson 25 MHz VCXO.
Table 10: LMK04906B Test Conditions
Parameter
Value
PLL1 Reference clock input
CLKin1 single-ended input, CLKin1* AC-coupled to
GND
PLL1 Reference Clock
frequency
125 MHz
PLL1 Phase detector frequency 2083.33 MHz
PLL1 Charge Pump Gain
400 uA
VCXO frequency
25 MHz
PLL2 phase detector frequency 50 MHz
PLL2 Charge Pump Gain
3200 uA
PLL2 REF2X mode
Disabled
25 MHz VCXO Phase Noise
The phase noise of the reference is masked by the phase noise of this VCXO by using
a narrow loop bandwidth for PLL1 while retaining the frequency accuracy of the
reference clock input. This VCXO sets the reference noise to PLL2. Figure 15 shows
the open loop typical phase noise performance of the Epson VG-4231CA 25.0000M-
FGRC3 VCXO.