CC2550_13 Datasheet, PDF (32/61 Pages) Texas Instruments – Low-Cost Low-Power 2.4 GHz RF Transmitter

CC2550
22 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,
C_L, specified for the crystal. The total load
capacitance seen between the crystal
terminals should equal C_L for the crystal to
oscillate at the specified frequency.
C_L =
1
1
+
1
+ C _parasitic
C₅₁ C₇₁
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 C_L 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
C51
C71
C_L= 10 pF
15 pF
15 pF
C_L=13 pF
22 pF
22 pF
C_L=16 pF
27 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 to XOSC_Q1 using a 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
23 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:
Z_out = 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.
SWRS039B
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