CC2420 Datasheet, PDF (53/87 Pages) List of Unclassifed Manufacturers – 2.4 GHz IEEE 802.15.4 / ZigBee-ready RF Transceiver

SmartRF ^® CC2420
operation of the crystal. In addition,
loading capacitors (C₃₈₁ and C₃₉₁) 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
C₃₈₁ C₃₉₁
The parasitic capacitance is constituted by
pin input capacitance and PCB stray
capacitance. The total parasitic
capacitance is typically 2 pF - 5 pF. A
trimming capacitor may be placed across
C₃₈₁ for initial tuning if necessary.
The crystal oscillator circuit is shown in
Figure 29. Typical component values for
different values of C_L are given in Table
10.
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
a stable oscillation. This ensures a fast
start-up and keeps the drive level to a
minimum. The ESR of the crystal must be
within the specification in order to ensure
a reliable start-up (see the Electrical
Specifications section).
XOSC16_Q1
C391
XTAL
XOSC16_Q2
C381
Figure 29. Crystal oscillator circuit
Item
C_L= 16 pF
C381
27 pF
C391
27 pF
Table 10. Crystal oscillator component values
Input / Output Matching
The RF input / output is differential (RF_N
and RF_P). In addition there is supply
switch output pin (TXRX_SWITCH) that
must have an external DC path to RF_N
and RF_P.
In RX mode the TXRX_SWITCH pin is at
ground and will bias the LNA. In TX mode
the TXRX_SWITCH pin is at supply rail
voltage and will properly bias the internal
PA.
The RF output and DC bias can be done
using different topologies. Some are
shown in Figure 3 and Figure 4.
Component values are given in Table 2.
Using a differential antenna, no balun is
required.
If a single ended output is required (for a
single ended connector or a single ended
antenna), a balun should be used for
optimum performance.
Chipcon AS SmartRF^® CC2420 Preliminary Datasheet (rev 1.2), 2004-06-09
Page 53 of 87