CC2550-RTY1 datasheet(32/62 Pages) TI2

32 / 62 page

CC2550

SWRS039B

Page 32 of 58

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 (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.

parasitic

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

20. Typical component values for different

values of CL are given in Table 21.

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

4.3 on page 7).

XOSC_Q1

XOSC_Q2

XTAL

C51

C71

Figure 20: Crystal Oscillator Circuit

Component

CL= 10 pF

CL=13 pF

CL=16 pF

C51

15 pF

22 pF

27 pF

C71

15 pF

22 pF

27 pF

Table 21: Crystal Oscillator Component Values

22.1

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. C51

and C71 can be omitted when using a

reference signal

External RF Match

The balanced RF output of

CC2550 is designed

for a simple, low-cost matching and balun

network on the printed circuit board. A few

passive external components ensure proper

matching.

Although

CC2550 has a balanced RF 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

CC2550 should have the following

differential impedance as seen from the RF-

port (RF_P and RF_N) towards the antenna:

Zout = 80 + j74 Ω

To ensure optimal matching of the

CC2550

differential output it is highly recommended to

follow the CC2550EM reference design [3] as

closely as possible. Gerber files for the

reference designs are available for download

from the TI website.

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