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4
PLL Loop Filters and Loop Parameters
In jitter cleaning applications that use a cascaded or dual PLL architecture, the first PLL's purpose is to
substitute the phase noise of a low-noise oscillator (VCXO or crystal resonator) for the phase noise of a
"dirty" reference clock. The first PLL is typically configured with a narrow loop bandwidth in order to
minimize the impact of the reference clock phase noise. The reference clock consequently serves only as
a frequency reference.
The loop filters on the LMK04208 evaluation board are setup using the approach above. The loop filter for
PLL1 has been configured for a narrow loop bandwidth (< 100 Hz).
parameters for PLL1 and PLL2 for using the default VCXO.
TI's Clock Design Tool can be used to optimize PLL phase noise/jitter for given specifications. See:
http://www.ti.com/tool/clockdesigntool.
4.1
PLL1 Loop Filter
Table 2. PLL1 Loop Filter Parameters for VCXO with 8 kHz/V Tuning Sensitivity
Loop Bandwidth
Kφ (Charge Pump)
Reference Clock Frequency
PLL Total N
Loop Filter Components
(1)
Loop Bandwidth and Phase Margin is a function of Kφ, Kvco, N as well as loop components. Changing Kφ and N by device
programming will change the loop bandwidth.
4.2
PLL2 Loop Filter
Loop Bandwidth
Kφ (Charge Pump)
Reference Clock Frequency
PLL Total N
Loop Filter Components
(1)
Loop Bandwidth is a function of Kφ, Kvco, N as well as loop components. Changing Kφ and N will change the loop bandwidth.
SNAU200 – September 2016
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PLL1 using 122.88 MHz VCXO (Epson VG-4513CA-122.8800M-GFCT3)
20 Hz
100 µA
122.88 MHz
120
C1_VCXO = 150 nF
Table 3. PLL2 Loop Filter Parameters using Integrated VCO
PLL2 using internal VCO
320 kHz
3.2 mA
122.88 MHz
24
C1_VCO = 0.022 nF
Copyright © 2016, Texas Instruments Incorporated
PLL Loop Filters and Loop Parameters
Table 2
and
Table 3
Phase Margin
50˚
Tuning Sensitivity (K
)
8 kHz/V
VCO
Phase Detector Freq
1.024 MHz
VCXO Frequency
122.88 MHz
(PLL1 feedback and PLL2
reference)
C2_VCXO_0603 = 1 µF
R2_VCXO = 22 kΩ
Phase Margin
84˚
Tuning Sensitivity (K
)
27 MHz/V
VCO
Phase Detector Freq
122.88 MHz
VCO Frequency
2949.12 MHz
C2_VCO = 18 nF
Integrated C3 and C4 = 0.01 nF
R2_VCO = 0.56 kΩ
Integrated R3 and R4 = 200 Ω
LMK04208 User's Guide
contain the
(1)
(1)
7
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