SNAA361A april   2022  – may 2023 LMX2820

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Reference Frequency
  6. 3Standard LMX2820 Phase Noise Performance
  7. 4 Pre-multiplier Stage Performance
    1. 4.1 External VCO Loop Filter Design
    2. 4.2 Active Loop Filter Implementation
    3. 4.3 External VCO Performance Results
  8. 5External PFD Topology
    1. 5.1 External PFD Loop Filter Modification
    2. 5.2 External PFD Measurement Results
  9. 6Putting it Together in the Real World
    1. 6.1 Real-World Performance Results
    2. 6.2 Why use the Dual Approach?
  10. 7Conclusion
  11. 8Revision History

2 Reference Frequency

The foundation of any RF synthesizer is the reference frequency. Achieving the best synthesizer phase noise starts with a high-quality reference source. Not all sources or generators are equivalent. Some show good low offset performance while sacrificing high offset noise floor. Depending on the application, one source can prove more beneficial than another based on the phase noise profile. The following signal generator sources are compared to find the best reference source for the subsequent measurement analysis.

  • Rohde & Schwarz (R&S®) SMA100B
  • Wenzel oscillator
  • Agilent PSG (with UNR enhanced phase noise option)
  • R&S SMA100A
  • Agilent ESG

Figure 2-1 shows the phase noise comparison between the sources operating at 307.2 MHz. Table 2-1 outlines the RMS jitter integrated from 1 kHz to 40 MHz. The source output power is over 10 dBm and the phase noise measurement were conducted with the Holzworth phase noise analyzer.

GUID-20220321-SS0I-7L30-0FRM-HFCWDT99PXBR-low.pngFigure 2-1 Phase Noise Comparison of Reference Sources
Table 2-1 RMS Jitter Comparison of Reference Sources (1 kHz–40 MHz)
InstrumentJitter
R&S SMA100B19.5 fs
Wenzel Oscillator10.4 fs
Agilent PSG113.4 fs
R&S SMA100A70.8 fs
Agilent ESG150.0 fs

The Wenzel oscillator is the premier standard. The oscillator provides excellent close-in and far-out phase noise performance and is ideal for the reference, but it is limited to just one frequency. The Rohde and Schwarz SMA100B rivals the Wenzel. The close-in phase noise is almost as good (or in some offset cases slightly better) than the Wenzel. Only at very high frequency offsets past 3 MHz does the SMA100B significantly deviate from the Wenzel.

The Agilent PSG has good very low frequency offset performance that is close to the Wenzel out to about 300 Hz. After that, there is a significant deviation from the standard. The Rohde SMA100A and the Agilent ESG have noticeable degradation at low-frequency offsets.

Overall, the SMA100B is a good substitute for the Wenzel oscillator for use as a reference to provide flexibility in selecting the reference frequency for the synthesizer.