SNAA411 September   2024 CDCLVC1102 , CDCLVC1103 , CDCLVC1104 , CDCLVC1110 , CDCLVD1204 , LMK00301 , LMK00304 , LMK00306 , LMK00308 , LMK01801 , LMK04832 , LMK1C1102 , LMK1C1103 , LMK1C1104 , LMK1C1106 , LMK1C1108 , LMK1D1204 , LMK1D1208 , LMX2485 , LMX2491 , LMX2572 , LMX2592 , LMX2594 , LMX2595 , LMX2820

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Generic Clock Tree
  5. 2Sine Wave Slew Rate Requirement
  6. 3Current Approach vs Clock Buffer
  7. 4Clock Buffer Implementation
    1. 4.1 Clock Buffer Common Input Stages
    2. 4.2 Choosing Between Internal or External DC Bias
    3. 4.3 Single Ended or LVCMOS Signal
    4. 4.4 Differential Inputs
  8. 5Performance Improvements, Results With Clock Buffer
    1. 5.1 FSWP Phase Noise Analyzer Measurements Case
    2. 5.2 TI LMX2820 Noise Improvements With Sine to Square Wave Clock Buffer
      1. 5.2.1 LMX2820 Phase Noise and RMS Jitter Results Summary
  9. 6Sine to Square Wave Clock Buffer Comparison
    1. 6.1 LMK1C110x Additive Noise vs Others
  10. 7Summary
  11. 8References

5.2.1 LMX2820 Phase Noise and RMS Jitter Results Summary

Table 5-1, Table 5-2 and Table 5-4 shows a quantitative summary of improvements when using sine to square wave buffer. Few key observations to note.

  • LMX2820 PLL noise is significantly improved around 10kHz offset. Far out noise is dominated by VCO.
  • RMS jitter and phase noise at higher power level is better with buffer compared to without buffer case. For worst case ((lowest power level) jitter reduced by 75.6% while the best case (highest power level) buffers reduces the jitter by 20%.
  • There is slight improvement at 100MHz input frequency as well but improvement is not significant compared to lower frequency use case as shown in Table 5-3 and Table 5-5. One reason for this is the doubler stage used on the LMX2820 to improve slew rate on the device input.
Table 5-1 Phase Noise: 10MHz Input, 100MHz Output
Parameter Data Unit
Input Power -10 -6 0 5 10 14 dBm
Trace Trace 1: Yellow Trace 2: Blue Trace 3: Green -
Case 1: Phase Noise without buffer 10Hz -108.93 -109.24 -110.16 -111.43 -111.28 -110.3 dBC
100Hz -114.68 -117.08 -119.04 -120.96 -121.32 -120.84
1kHz -116.29 -119.31 -125.16 -128.95 -131.84 -132.96
10kHz -115.2 -119.78 -124.95 -129.29 -133.58 -135.34
100kHz -128.82 -121.58 -134.10 -137.62 -140.04 -141.22
1MHz -154.72 -147.92 -158.94 -161.33 -162.04 -162.20
10MHz -166.76 -166.75 -166.87 -166.79 -166.80 -166.84
100MHz -167.49 -167.53 -167.57 -167.54 -167.46 -167.57
Trace Trace 4: Orange Trace 5: Dusty Blue Trace 6: Red -
Case 2: Phase Noise with buffer 10Hz -109.73 -111.12 -110.34 -112.72 -112.29 -111.24 dBC
100Hz -121.17 -122.06 -119.96 -123.5 -123.32 -122.79
1kHz -127.26 -129.94 -129.64 -135.14 -135.62 -134.98
10kHz -128.17 -130.12 -133.81 -137.32 -138.06 -138.07
100kHz -140.83 -142.73 -142.23 -142.97 -143.47 -143.57
1MHz -162.13 -162.50 -162.52 -162.75 -162.82 -162.77
10MHz -166.79 -166.77 -166.71 -166.82 -166.80 -166.79
100MHz -167.51 -167.51 -167.6 -167.54 -167.56 -167.44
Phase Noise Delta: Case 2 - Case 1 10Hz -0.8 -1.88 -0.18 -1.29 -1.01 -0.94 dBC
100Hz -6.49 -4.98 -0.92 -2.54 -2 -1.95
1kHz -10.97 -10.63 -4.48 -6.2 -3.78 -2.02
10kHz -12.97 -10.34 -8.86 -8.03 -4.48 -2.73
100kHz -12.01 -21.15 -8.13 -5.32 -3.43 -2.35
1MHz -7.41 -14.58 -3.58 -1.42 -0.78 -0.57
10MHz -0.03 -0.02 0.16 -0.03 0 0.05
100MHz -0.02 0.02 -0.03 0 -0.1 0.13
Table 5-2 Phase Noise: 10MHz Input, 1GHz Output
Parameter Data Unit
Input Power -6 5 10 dBm
Trace Trace 1: Yellow Trace 2: Blue Trace 3: Green -
Case 1: Phase Noise without buffer 10Hz -89.51 -90.23 -91.79 dBC
100Hz -97.59 -101.37 -101.54
1kHz -100.09 -109.46 -112.42
10kHz -99.74 -109.75 -113.45
100kHz -106.25 -116.11 -119.71
1MHz -132.78 -141.33 -143.48
10MHz -159.69 -160.05 -160.09
100MHz -161.12 -161.08 -161.21
Trace Trace 4: Orange Trace 2: Dusty Blue Trace 3: Green -
Case 1: Phase Noise without buffer 10Hz -91.55 -93.20 -92.29 dBC
100Hz -101.64 -103.60 -103.29
1kHz -109.23 -114.71 -115.47
10kHz -110.154 -117.31 -118.00
100kHz -122.66 -122.68 -123.22
1MHz -144.60 -144.63 -144.95
10MHz -159.83 -159.94 -160.01
100MHz -161.17 -161.04 -161.29
Phase Noise Delta: Case 2 - Case 1 10Hz -2.04 -2.97 -0.5 dBC
100Hz -4.05 -2.23 -1.75
1kHz -9.14 -5.25 -3.05
10kHz -10.414 -7.58 -4.55
100kHz -16.41 -6.57 -3.51
1MHz -11.81 -3.3 -1.47
10MHz -0.14 0.111 0.08
100MHz -0.05 0.04 -0.08
Table 5-3 Phase Noise: 100MHz Input, 1GHz Output
Parameter Data Unit
Input Power -6 5 10 dBm
Trace Trace 1: Yellow Trace 2: Blue Trace 3: Green -
Case 1: Phase Noise without buffer 10Hz -98.37 -99.46 -98.25 dBC
100Hz -110.61 -110.59 -110.44
1kHz -122.42 -122.73 -123.05
10kHz -127.55 -131.08 -131.60
100kHz -127.63 -133.04 -133.93
1MHz -138.10 -142.00 -142.47
10MHz -159.88 -159.90 -159.97
100MHz -160.91 -161.00 -160.94
Trace Trace 4: Orange Trace 2: Dusty Blue Trace 3: Green -
Case 1: Phase Noise without buffer 10Hz -98.28 -97.79 -99.78 dBC
100Hz -109.90 -110.39 -110.44
1kHz -122.34 -122.71 -122.99
10kHz -130.77 -131.49 -131.77
100kHz -133.43 -134.26 -134.46
1MHz -142.43 -142.67 -142.70
10MHz -160.06 -159.93 -160.01
100MHz -161.08 -161.07 -161.88
Phase Noise Delta: Case 2 - Case 1 10Hz 0.08 1.67 -1.54 dBC
100Hz 0.70 0.12 0
1kHz 0.08 0.02 0.05
10kHz -3.23 -0.40 -0.17
100kHz -5.80 -1.22 -0.52
1MHz -4.33 -0.66 -0.23
10MHz -0.18 -0.04 -0.03
100MHz -0.17 -0.07 -0.06
Table 5-4 Total RMS Jitter 10MHz Input, 100MHz and 1GHz Output
Parameter Data Unit
Input PowerCondtion -10 -6 0 5 10 14 dBm
Trace Trace 1: Yellow Trace 2: Blue Trace 3: Green -
Case 1: RMS Jitter without Buffer FOUT = 100MHz 12kHz - 20MHz 1027 931.858 403.643 244.971 165.222 138.81 fs
10kHz - 50MHz 1108 968.048 431.324 266.197 185.035 159.823
FOUT = 1GHz 12kHz - 20MHz - 765.797 - 246.719 160.439 -
10kHz - 50MHz - 805.778 - 262.336 172.988 -
Trace Trace 4: Orange Trace 5: Dusty Blue Trace 3: Red -
Case 1: RMS Jitter without Buffer FOUT = 100MHz 12kHz - 20MHz 243.679 197.268 150.522 118.663 111.924 110.943 fs
10kHz - 50MHz 269.621 219.699 175.192 137.409 131.464 131.557
FOUT = 1GHz 12kHz - 20MHz - 195.569 - 110.807 104.213 -
10kHz - 50MHz - 212.638 - 120.615 114.175 -
RMS Jitter Delta: Case 2 - Case 1 FOUT = 100MHz 12kHz - 20MHz -783.321 -734.59 -253.121 -126.308 -53.298 -27.867 fs
10kHz - 50MHz -838.379 -748.349 -256.132 -128.788 -53.571 -28.266
FOUT = 1GHz 12kHz - 20MHz - -570.228 - -135.912 -56.226 -
10kHz - 50MHz - -593.140 - -141.721 -58.813 -
Table 5-5 Total RMS Jitter 100MHz Input, 1GHz Output
Parameter Data Unit
Input PowerCondtion -6 5 10 dBm
Trace Trace 1: Yellow Trace 2: Blue Trace 3: Green -
Case 1: RMS Jitter without Buffer FOUT = 1GHz 10kHz - 50MHz 80.840 48.762 45.407 fs
Trace Trace 4: Orange Trace 5: Dusty Blue Trace 3: Red -
Case 1: RMS Jitter without Buffer FOUT = 1GHz 10kHz - 50MHz 47.177 44.624 43.949 fs
RMS Jitter Delta: Case 2 - Case 1 FOUT = 1GHz 10kHz - 50MHz -33.663 -4.138 -2.458 fs