Crystal Oscillator; Reference Signal; External Rf Match - Texas Instruments CC2500 TK Manual

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 , specified for the crystal. The
L
total load capacitance seen between the
crystal terminals should equal C
crystal to oscillate at the specified frequency.
1

C
L
1

C C
81
The parasitic capacitance is constituted by pin
input capacitance and PCB stray capacitance.
Total parasitic capacitance is typically 2.5 pF.
Component
C81
C101
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

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
for the
L

C
parasitic
1
101
C = 10 pF
L
15 pF
15 pF
Table 32: Crystal Oscillator Component Values
SWRS040C
The crystal oscillator circuit is shown in Figure
27. Typical component values for different
values of C are given in Table 32.
L
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
C81
Figure 27: Crystal Oscillator Circuit
C =13 Pf C
L
22 pF
22 pF
XOSC_Q1 input. The sine wave must be
connected to
XOSC_Q1
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.
circuitry ensures match in both RX and TX
mode.
CC2500
Although
input/output, the chip can be connected to a
single-ended antenna with few external low
cost capacitors and inductors.
The passive matching/filtering
connected to CC2500 should have the following
differential impedance as seen from the RF-
port (RF_P and RF_N) towards the antenna:
CC2500
XOSC_Q2
XTAL
C101
=16 pF
L
27 pF
27 pF
using a
has a balanced
network
Page 50 of 89
serial
RF