SWRA495K December   2015  – April 2024 CC1310 , CC1350 , CC2620 , CC2630 , CC2640 , CC2640R2F , CC2640R2F-Q1 , CC2642R-Q1 , CC2650 , CC2662R-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Oscillator and Crystal Basics
    1. 1.1 Oscillator Operation
    2. 1.2 Quartz Crystal Electrical Model
      1. 1.2.1 Frequency of Oscillation
      2. 1.2.2 Equivalent Series Resistance
      3. 1.2.3 Drive Level
      4. 1.2.4 Crystal Pulling
    3. 1.3 Negative Resistance
    4. 1.4 Time Constant of the Oscillator
  5. 2Overview of Crystal Oscillators for CC devices
    1. 2.1 24-MHz and 48-MHz Crystal Oscillator
    2. 2.2 24-MHz and 48-MHz Crystal Control Loop
    3. 2.3 32.768-kHz Crystal Oscillator
  6. 3Selecting Crystals for the CC devices
    1. 3.1 Mode of Operation
    2. 3.2 Frequency Accuracy
      1. 3.2.1 24-MHz and 48-MHz Crystal
      2. 3.2.2 32.768-kHz Crystal
    3. 3.3 Load Capacitance
    4. 3.4 ESR and Start-Up Time
    5. 3.5 Drive Level and Power Consumption
    6. 3.6 Crystal Package Size
  7. 4PCB Layout of the Crystal
  8. 5Measuring the Amplitude of the Oscillations of Your Crystal
    1. 5.1 Measuring Start-Up Time to Determine HPMRAMP1_TH and XOSC_HF_FAST_START
  9. 6Crystals for CC13xx, CC26xx and CC23xx
  10. 7High Performance BAW Oscillator
  11. 8References
  12. 9Revision History

1.4 Time Constant of the Oscillator

The start-up time of a crystal oscillator is determined by transient conditions at turn-on, small-signal envelope expansion due to negative resistance, and large-signal amplitude limiting. The envelope expansion is a function of the total negative resistance and the motional inductance of the crystal. The time constant of the envelope expansion is proportional to the start-up time of the oscillator given by Equation 8.

Equation 8. GUID-1C0C3B9D-0B9A-4BE6-816C-0DF28FC01D97-low.gif

A crystal with a low LM gives a shorter start-up time and so does a high-magnitude RN (low CL). A trade-off exists between pull-ability due to low-motional capacitance (CM) and fast start-up time due to low-motional inductance (LM), because the frequency of the crystal is dependent on the both CM and LM. Crystals in smaller package sizes have larger LM, and start more slowly than those in larger package sizes (see Section 1.2.1).

Table 1-1 summarizes crystal parameters and their values for the reference crystals recommended by TI for use with the CC devices.

Table 1-1 Crystal Parameters
Parameters Description 24-MHz Crystal
Used in TI CC26x0 Characterization		 TI-Assumed Default
32.768-kHz Crystal
Motional Inductance (LM) Partly determines crystal response time (how quickly the crystal responds to a change from the oscillator). Lower Lm → crystal responds more quickly to changes from the oscillator. Along with CM, a major determiner of the crystal quality factor 12.6 mH 5.0 kH
Motional Capacitance (CM) Partly determines crystal response time. Lower CM → crystal responds more slowly to changes from the oscillator. 3.4 fF 4.718 fF
Motional Resistance (RM) At resonance, Lm and CM cancel and RM is presented to the oscillator. RM ~ ESR assuming CL >> CO. 20 Ω (60-Ω maximum) 37 kΩ (70-kΩ maximum)
Load Capacitance (CL) The amount of load capacitor to tune the crystal to the correct frequency. This load capacitance also helps determine drive level. 9 pF 7 pF
Shunt Capacitance (C0) This is a parasitic capacitance due to crystal packaging. It helps determine the acceptable drive level. 1.2 pF 1 pF
ESR Equivalent Series Resistance. If CL >> CO, then ESR ~ RM 20 Ω (60-Ω maximum) 37 kΩ
Drive Level The maximum level of power in the crystal for reliable long-term operation, see Equation 5 200 µW <500 nW