CC1000-RTR2 datasheet(35/55 Pages) TI

35 / 55 page

CC1000

SWRS048A

Page 35 of 55

24. Crystal oscillator

CC1000 has an advanced amplitude

regulated crystal oscillator. 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

a 600 mVpp amplitude. This ensures a

fast

start-up,

keeps

the

current

consumption as well as the drive level to a

minimum

and

makes

the

oscillator

insensitive to ESR variations.

An external clock signal or the internal

crystal oscillator can be used as main

frequency reference. An external clock

signal should be connected to XOSC_Q1,

while XOSC_Q2 should be left open. The

XOSC_BYPASS bit in the FRONT_END

clock signal is used.

The crystal frequency should be in the

range 3-4, 6-8 or 9-16 MHz. Because the

crystal frequency is used as reference for

the data rate (as well as other internal

functions), the following frequencies are

recommended: 3.6864, 7.3728, 11.0592

or 14.7456 MHz. These frequencies will

give accurate data rates. The crystal

frequency

range

selected

XOSC_FREQ1:0 in the MODEM0 register.

To operate in synchronous mode at data

rates different from the standards at 1.2,

2.4, 4.8 kBaud and so on, the crystal

frequency can be scaled. The data rate

(DR) will change proportionally to the new

crystal frequency (f). To calculate the new

crystal frequency:

new

xtal

new

xtal

Using the internal crystal oscillator, the

crystal

must

connected

between

XOSC_Q1 and XOSC_Q2. The oscillator

is designed for parallel mode operation of

the crystal. In addition loading capacitors

(C171 and C181) 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.

parasitic

181

171

The parasitic capacitance is constituted by

pin input capacitance and PCB stray

capacitance. Typically the total parasitic

capacitance is 8 pF. A trimming capacitor

may be placed across C171 for initial

tuning if necessary.

The crystal oscillator circuit is shown in

Figure 24. Typical component values for

different values of CL are given in Table

12.

The initial tolerance, temperature drift,

ageing and load pulling should be carefully

specified in order to meet the required

frequency

accuracy

certain

application.

specifying

the

total

expected

frequency

accuracy

SmartRF

® Studio together with data rate

and frequency separation, the software

will calculate the total bandwidth and

compare to the available IF bandwidth.

C171

C181

XTAL

XOSC_Q1

XOSC_Q2

C171

C181

XTAL

XOSC_Q1

XOSC_Q2

Figure 24. Crystal oscillator circuit

Item

CL= 12 pF

CL= 16 pF

CL= 22 pF

C171

6.8 pF

18 pF

33 pF

C181

6.8 pF

18 pF

33 pF

Table 12. Crystal oscillator component values

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