CC2500-RTY1 datasheet(50/96 Pages) TI

50 / 96 page

CC2500

SWRS040C

Page 50 of 89

26 Crystal Oscillator

A crystal in the frequency range 26-27 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 (C81 and C101)

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





101

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

27. Typical component values for different

values of CL are given in Table 32.

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

4.4 on page 12).

XOSC_Q1

XOSC_Q2

XTAL

C81

C101

Figure 27: Crystal Oscillator Circuit

Component

CL= 10 pF

CL=13 Pf

CL=16 pF

C81

15 pF

22 pF

27 pF

C101

15 pF

22 pF

27 pF

Table 32: Crystal Oscillator Component Values

26.1

Reference Signal

The chip can alternatively be operated with a

reference signal from 26 to 27 MHz instead of

a crystal. This input clock can either be a full-

swing digital signal (0 V to VDD) or a sine

wave of maximum 1 V peak-peak amplitude.

The reference signal must be connected to the

XOSC_Q1

input. The sine wave must be

connected

XOSC_Q1

using

serial

capacitor. When using a full-swing digital

signal this capacitor can be omitted. The

XOSC_Q2

line must be left un-connected. C81

and C101 can be omitted when using a

reference signal.

27 External RF Match

The balanced RF input and output of

CC2500

share two common pins and are designed for

a simple, low-cost matching and balun network

on the printed circuit board. The receive- and

transmit switching at the

CC2500 front-end is

controlled by a dedicated on-chip function,

eliminating the need for an external RX/TX-

switch.

A few passive external components combined

with the internal RX/TX switch/termination

circuitry ensures match in both RX and TX

mode.

Although

CC2500 has a balanced RF

input/output, the chip can be connected to a

single-ended antenna with few external low

cost capacitors and inductors.

The

passive

matching/filtering

network

connected to

CC2500 should have the following

differential impedance as seen from the RF-

port (RF_P and RF_N) towards the antenna:

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