CC2550 Datasheet, PDF (29/50 Pages) Texas Instruments – Single Chip Low Cost Low Power RF-Transmitter

CC2550
29 Crystal Oscillator
A crystal in the frequency range 26MHz-
27MHz 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 2.5pF.
The crystal oscillator circuit is shown in Figure
15. Typical component values for different
values of C_L are given in Table 22.
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 0.4Vpp 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
7 on page 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. By specifying
the total expected frequency accuracy in
SmartRF^® Studio together with data rate and
frequency deviation, the software calculates
the total bandwidth and compares this to the
chosen receiver channel filter bandwidth. The
software reports any contradictions, and a
more accurate crystal is recommended if
required.
XOSC_Q1
XOSC_Q2
XTAL
C51
C71
Figure 15: Crystal oscillator circuit
Component
C51
C71
C_L= 10pF
15pF
15pF
C_L=13pF
22pF
22pF
C_L=16pF
27pF
27pF
Table 22: Crystal oscillator component values
29.1 Reference signal
The chip can alternatively be operated with a
reference signal from 26 to 27MHz instead of a
crystal. This input clock should have an
amplitude of TBD. The reference signal must
30 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.
be connected to the XOSC_Q1 input and ac-
coupled using a serial cap. The XOSC_Q2 line
must be left un-connected. C51 and C71 can
be omitted when a reference signal is used.
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.
Preliminary Data Sheet (rev. 1.1) SWRS039
Page 29 of 51