SWRA495L December   2015  – April 2025 CC1310 , CC1350 , CC2620 , CC2630 , CC2640 , CC2640R2F , CC2640R2F-Q1 , CC2642R-Q1 , CC2650 , CC2662R-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Oscillator 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. 2 Overview of Crystal Oscillators for CC devices
    1. 2.1 24MHz and 48MHz Crystal Oscillator
    2. 2.2 24MHz and 48MHz Crystal Control Loop
    3. 2.3 32.768kHz Crystal Oscillator
  6. 3 Selecting Crystals for the CC devices
    1. 3.1 Mode of Operation
    2. 3.2 Frequency Accuracy
      1. 3.2.1 24MHz and 48MHz Crystal
      2. 3.2.2 32.768kHz 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. 4 PCB Layout of the Crystal
  8. 5 Measuring 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. 6 Crystals for CC13xx, CC26xx, CC23xx and CC27xx
  10. 7 High Performance BAW Oscillator
  11. 8 CC23XX and CC27XX Software Amplitude Compensation
  12. 9 Internal Capacitor Array for CC23XX and CC27XX
  13. 10Internal Capacitor Array for CC13xx and CC26xx
  14. 11Summary
  15. 12References
  16. 13Revision History

3.3 Load Capacitance

The crystal oscillator frequency is dependent on the capacitive loading of the crystal. The crystal data sheet provides the required load capacitance for the crystal, CL, for the oscillation to be at the correct frequency. The total CL consists of the loading capacitors and the parasitic capacitance of the layout and packaging. CL1 and CL2 are in series with respect to the crystal. Therefore, the effective load capacitance CL1 and CL2 present is CL1/2, assuming CL1=CL2. Extra capacitance between board traces that connect to the crystal increases the effective CL.

Using external capacitors to get the correct frequency means that the internal caps must be set to minimum. For example, an application can use near minimum on-chip capacitance of approximately 2-3pF and an off chip capacitance of 7pF to provide CL = 9pF to the crystal.

Only for CC13xx, Table 3-1 shows using external caps this way gives slightly worse frequency stability with temperature than using internal capacitors. Certain sub-1GHz users can need to use external load capacitors to reduce spurs at an offset of twice the crystal frequency from the RF carrier frequency.

Table 3-1 Using External Capacitor Results in Worse Frequency Stability Over Temperature
9pF Internal CLMinimum Internal CL, External CL
Frequency variation –40°C to +90°CSet by crystalSet by crystal + 5ppm
Voltage accuracy, ppm/V6.99

The following presents the relative advantages of crystals with different CL values.

The disadvantages of lower CL are as follows:

  • Crystals with < 7pF CL are more difficult to source with short lead times.
  • Frequency becomes more sensitive to changes in board capacitance as CL decreases. This is possible to meet frequency stability specifications with a CL as low as 3pF.
  • Lowering CL results in degraded RF phase noise.

Advantages of lower CL are as follows:

  • Lower CL causes a much faster start-up time. (Start-up time is proportional to CL2.)
  • Lower CL causes a faster amplitude control loop response time.
  • Lower CL is easier to use small size crystals (2.0 × 1.6 and so on) and maintain a start-up time at or less than 400µs. Start-up time worsens with smaller crystals due to an increase in LM.

The internal load capacitance has no appreciable impact on the shape of the frequency vs temperature of the high frequency crystal. This can be seen by looking at the following two plots. Figure 3-2 shows the frequency versus temperature curve for the crystal using 13 different but closely spaced load capacitance. Each different load capacitance shifts the curve up or down, but does not change the overall shape of the curve. This can be seen by removing the offset of each curve, as shown in Figure 3-3.

The Frequency vs Temperature Curve for the High Frequency Crystal for 13 Closely Spaced Load Capacitance ValuesFigure 3-2 The Frequency vs Temperature Curve for the High Frequency Crystal for 13 Closely Spaced Load Capacitance Values
Removing the Offset of the Frequency vs Temperature CurvesFigure 3-3 Removing the Offset of the Frequency vs Temperature Curves

Figure 3-3 shows that a change in the internal load capacitance does not influence the shape of the frequency vs temperature curve. This indicates that the internal load capacitors have minimal impact on this curve.

A method to change the on-chip load caps of the crystal is discussed in CC13xx/CC26xx Hardware Configuration and PCB Design Considerations application note.