SNAA342 September   2020 LMX2820

 

  1.   Trademarks
  2. 1 Overview of Frequency Hopping
  3. 2 Integrated VCO Overview
    1. 2.1 Integrated Silicon VCO Structure
    2. 2.2 Robustness and Consistency of VCO Calibration
  4. 3Components of Synthesizer Lock Time
    1. 3.1 Write Time
    2. 3.2 VCO Calibration Time
    3. 3.3 Analog Settling Time
  5. 4Improving VCO Calibration Time With the LMX2820
    1. 4.1 Improving LMX2820 VCO Calibration With Partial Assist
    2. 4.2 Full Assist Method of Improving VCO Calibration Time
    3. 4.3 Instant Calibration – The Ultimate to Blazing-Fast VCO Calibration
  6. 5Conclusion
  7. 6References

2.1 Integrated Silicon VCO Structure

Nowadays, PLLs with integrated VCOs on silicon technologies are very common for the reasons cost, size, wideband frequency coverage, and performance. To achieve good wideband performance while keeping good phase noise performance, it is common practice to use a series of switched capacitors and inductors.

GUID-20200728-CA0I-XTWJ-6X4D-LBRKH8K3WVZW-low.gifFigure 2-1 Simplified Tank Circuit for Silicon VCO.
For this particular example, there are eight capacitor combinations and two inductor combinations, which allow 16 possible frequency bands to be achieved as Figure 2-2 shows.

GUID-20200728-CA0I-KHZH-2FVT-HCVFJM7KH5VL-low.gifFigure 2-2 Frequency Bands.

Modern day synthesizers typically have more capacitors and inductors. For instance, the LMX2820 device has eight VCO cores and 192 different values in the capacitor bank to create 1536 total bands. Between these bands, there is overlap to allow for temperature drift and process variation. In addition to this, there is also an internal amplitude setting that varies as a function of frequency and temperature. So in summary, the three key parameters for the VCO Calibration to determine are:

  • VCO_ CORE: This is the inductor choice that greatly impacts the frequency band

  • VCO_CAPCODE: This is the capacitor bank setting that changes the frequency band

  • VCO_DACISET: This is the amplitude setting that optimizes phase noise

The purpose of the VCO calibration is to find the proper values for these three parameters. This calibration is activated by programmable serial commands when a change in frequency is desired.