NXP Semiconductors FREESCALE MC13192, Справочное руководство, Страница: 26 / 34

RF Front End
MC13192EVB Reference Manual, Rev. 1.2
3-8 Freescale Semiconductor
This inverter type reference oscillator requires two external load capacitors to resonate the crystal. Because
the oscillator runs with a very low current draw (11 µA), crystals with a load capacitance of 6-8 pF are
recommended to ensure proper oscillation. The crystal should also have a low Co value.
A built-in switched capacitor bank allows fine tuning of the reference frequency. Each oscillator pin is
connected to a 0-5 pF capacitor bank. The default start up value is 2.5 pF at each pin. The pull ability of
the crystal determines the actual tuning range, but values of +/-20 to 50 ppm are typical.
The external capacitors together with the PCB capacitance and the internal switched capacitor bank makes
up the load capacitance of the crystal. The values of the external capacitors are:
Cext = 2(Cl -Cpcb-2.5pF)
A typical starting value would be 2Cl -8pF.
With the recommended ZD00882 crystal from KDS, typical external capacitors are 8.2 pF each, but may
require fine tuning depending on the actual PCB layout. The typical range of load capacitors for the crystal
(6-10 pF) are also efficient as 2.4 GHz coupling and decoupling capacitors, so the same value may be used
both for crystal loading and coupling/decoupling, saving components in the BOM.
To meet the requirements of the ZigBee specification, the oscillator should be within 40 ppm of 16 MHz.
This applies for initial spread, temperature drift, and aging. Crystal performance (temperature drift and
initial tolerance) are the main contributors to this tolerance.
3.7 Internal Power Supplies
VBATT and VDDINT are the battery supply to MC13192 and should be connected together. A decoupling
capacitor (1 µF recommended) should be placed close to the chip.
VDDA, VDDLO1, and VDDLO2 must be connected together and decoupled with a 100 nF capacitor to
ensure stability of the internal regulator. Bias for the power amplifier PA should also be connected to this
supply. VDDD and VDDVCO should each be decoupled with a 100 nF capacitor to ensure stability of the
internal regulators.
The capacitor values are not especially critical. 10% X5R/X7R capacitors are recommended. For room
temperature operation only, Y5V capacitors may be employed. For low-cost applications, the capacitors
may be reduced to 10 nF without stability loss, but decoupling will be less efficient.
3.8 RF Modifications
The MC13192EVB demonstrates that with high-performance solutions and some experimentation, the
performance requirements of several applications can be satisfied with a less complex front-end. This
section provides some general suggestions as to what modifications can be made and the resultant
performance changes.
3.8.1 LNA
Most applications will have more than adequate range without an LNA and not using an LNA leaves out
a considerable number of components and saves some current usage. Also, the board space used by the
LNA could be used for improving antenna efficiency.