CC1150-RTR1 datasheet(54/67 Pages) TI1

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XOSC_Q1

XOSC_Q2

XTAL

C51

C71

parasitic

CC1150

SWRS037B – JANUARY 2006 – REVISED MARCH 2015

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6.2.3

Crystal

A crystal in the frequency range of 26 to 27 MHz must be connected between the XOSC_Q1 and

XOSC_Q2 pins. The oscillator is designed for parallel mode operation of the crystal. In addition, loading

capacitors (C51 and C71) for the crystal are required. The loading capacitor values depend on the total

load capacitance, CL , specified for the crystal. The total load capacitance seen between the crystal

terminals should equal CL for the crystal to oscillate at the specified frequency.

(10)

The parasitic capacitance is constituted by pin input capacitance and PCB stray capacitance. Total

parasitic capacitance is typically 2.5 pF.

The crystal oscillator circuit is shown in Figure 6-3. Typical component values for different values of CL are

given in Table 6-3.

Figure 6-3. Crystal Oscillator Circuit

The crystal oscillator is amplitude regulated. This means that a high current is used to start up the

oscillations. When the amplitude builds up, the current is reduced to what is necessary to maintain

approximately 0.4 Vpp signal swing. This ensures a fast start up, and keeps the drive level to a minimum.

The ESR of the crystal should be within the specification in order to ensure a reliable start up (see

Section 4.7).

The initial tolerance, temperature drift, aging and load pulling should be carefully specified in order to meet

the required frequency accuracy in a certain application.

Table 6-3. Crystal Oscillator Component Values

COMPONENT

CL= 10 pF

CL= 13 pF

CL= 16 pF

C51

15 pF

22 pF

27 pF

C71

15 pF

22 pF

27 pF

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