SDAA607 November   2026 LMX2581

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Hardware Design
  6. 3Loop Filter Components Selection
    1. 3.1 Design Example
    2. 3.2 Simulation in TI PLLatinum Sim
    3. 3.3 Test Verification
      1. 3.3.1 Setup
      2. 3.3.2 Test Results
  7. 4References

3.3.2 Test Results

Figure 3-4 shows an example of a phase noise plot with an inappropriate loop filter. A 3rd order filter is used while a 100pF capacitor, which is far smaller than recommended, is placed near the Vtune pin C3. Therefore, the phase noise from 100kHz to 1MHz increases by approximately 10dBc, which leads to the RMS jitter (12kHz-20MHz) of 364fs.

LMX2581 Phase Noise Plot With C3 = 100pF Figure 3-4 Phase Noise Plot With C3 = 100pF

Higher phase detector frequency and higher charge pump gain can reduce the jitter. This relationship is proven by adjusting the settings in TICS PRO. Loop filter components are C2 = 47nF, R2 = 357Ω, and C3 = 1.5nF (C1 is NC as C3 is close to Vtune pin). Table 3-1 shows the results of each test. Furthermore, the results indicate that the output power also influences the jitter performance, even though the output power needs to be selected based on the output swing and slew rate required by the receiver.

Table 3-1
Test Doubler Kpd/uA Output Pwr Jitter/fs
1 2x 3100 25 137.4
2 1x 3100 25 188.7
3 2x 2500 25 151.1
4 2x 2100 25 168.6
5 2x 3100 30 137.7
6 2x 3100 20 139
LMX2581 TICS PRO LMX2581 View Figure 3-5 TICS PRO LMX2581 View

Different capacitance values for C2 and C3 have also been tried to figure out the best combinations. R2 is kept at 357Ohm for simplicity. The PLL setting is Doubler = 2x, Kpd = 3100uA, Output Pwr = 25. Along with the phase noise plot, the results prove that wider open loop bandwidth (smaller C3) helps suppress the VCO noise though the VCO phase noise can be slightly raised.

Test C2/nF C3/nF Jitter/fs
1 47 3.3 159
2 47 2.2 147.8
3 47 1.5 137.4
4 15 1.5 143
5 68 1.5 138
LMX2581 Phase Noise Plot With Optimized Loop Filter Figure 3-6 Phase Noise Plot With Optimized Loop Filter