Micrel
MICRF405
April 2006
21
M9999-041906
(408) 955-1690
Crystal Oscillator (XCO)
Adr
Data
A6..A0
D7
D6
D5
D4
D3
D2
D1
D0
0010101 ClkOut_1=0 ClkOut_0=0 XCO_Fast=1 XCOtune4=1 XCOtune3=0 XCOtune2=0
XCOtune1=0
XCOtune0=0
The crystal oscillator is a very critical block. As the
crystal oscillator is a reference for the RF output
frequency, very good phase and frequency stability
is required. When selecting crystal it should be paid
special attention to the total frequency tolerance and
load capacitance as these will directly influence on
the carrier frequency.
C2
C1
XTB, pin 7
XTA, pin 8
Figure 9. Crystal Oscillator Circuit.
The crystal should be connected between pins XTA
and XTB (pin 7 and 8). MICRF405 has an internal
crystal capacitor bank used for crystal tolerance
tuning during production. These internal capacitors
can be enabled using the XCOtune[4:0] bits. If
XCOtune[4:0]=0 then no internal capacitors are
connected to the crystal pins, while 18pF are
connected to each pin if XCOtune[4:0]=31. The unit
capacitance is about 0.6pF. The internal XCOtune
feature is optimized for a crystal with a load
capacitance of 9pF and will give the expected
oscillation frequency when no external capacitors
are connected and XCOtune[4:0]=16. If a crystal
requires higher load capacitance, additional
capacitors must be added off-chip (C1 and C2 in
Figure 9).
If XCOtune[4:0]=0, the loading capacitors can be
calculated by the following formula;
parasitic
L
C
C
C
C
+
+
=
2
1
1
1
1
The parasitic capacitance is the pin input
capacitance and PCB stray capacitance. Typically
the total parasitic capacitance is around 6pF. For
instance, for a 9pF load crystal the recommended
values of the external load capacitors are 5.6pF.
The start-up time of a crystal oscillator is typically
around a millisecond. Therefore, to save current
consumption, the XCO is turned on before any other
circuit block. During start-up the XCO amplitude will
eventually reach a sufficient level to trigger the M-
counter. After counting 2 M-counter output pulses
the rest of the circuit will be turned on. The current
consumption during the prestart period is typically
205礎. If the XCO_Fast bit is set, then XCO will
start up faster, typically in about 300祍. This comes
at the expense of a higher current consumption of
typically 2mA during the period from start up until the
first output pulse of the M-divider.
If an external reference shall be used instead of a
crystal, the signal shall be applied to pin 7, XTB.
Due to internal biasing, AC coupling is
recommended for use between the external
reference and the XTB-pin.
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