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, 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.
parasitic
L
C
C
C
C
101
81
1
1
1
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 C
L
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
C
L
= 10 pF
C
L
=13 Pf
C
L
=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
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. 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:
Summary of Contents for CC2500
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