Texas Instruments LMK04208, Руководство пользователя

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PLL Loop Filters and Loop Parameters
7 SNAU200 – September 2016
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Copyright © 2016, Texas Instruments Incorporated
LMK04208 User’s Guide
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).
Table 2 and Table 3 contain the
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
(1)
Loop Bandwidth and Phase Margin is a function of K φ , Kvco, N as well as loop components. Changing K φ and N by device
programming will change the loop bandwidth.
Table 2. PLL1 Loop Filter Parameters for VCXO with 8 kHz/V Tuning Sensitivity
(1)
PLL1 using 122.88 MHz VCXO (Epson VG-4513CA-122.8800M-GFCT3)
Loop Bandwidth 20 Hz Phase Margin 50 ˚
K
φ
(Charge Pump) 100 µA Tuning Sensitivity (K
VCO
) 8 kHz/V
Reference Clock Frequency 122.88 MHz Phase Detector Freq 1.024 MHz
PLL Total N 120 VCXO Frequency 122.88 MHz
(PLL1 feedback and PLL2
reference)
Loop Filter Components C1_VCXO = 150 nF C2_VCXO_0603 = 1 µF R2_VCXO = 22 k
Ω
4.2 PLL2 Loop Filter
(1)
Loop Bandwidth is a function of K φ , Kvco, N as well as loop components. Changing K φ and N will change the loop bandwidth.
Table 3. PLL2 Loop Filter Parameters using Integrated VCO
(1)
PLL2 using internal VCO
Loop Bandwidth 320 kHz Phase Margin 84 ˚
K φ (Charge Pump) 3.2 mA Tuning Sensitivity (K
VCO
) 27 MHz/V
Reference Clock Frequency 122.88 MHz Phase Detector Freq 122.88 MHz
PLL Total N 24 VCO Frequency 2949.12 MHz
Loop Filter Components C1_VCO = 0.022 nF C2_VCO = 18 nF Integrated C3 and C4 = 0.01 nF
R2_VCO = 0.56 k Ω Integrated R3 and R4 = 200 Ω