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Micrel, Inc.
Crystal Oscillator (XCO)
The crystal oscillator’s role as the synthesizer reference
demands very good phase and frequency stability. As
shown in Figure 7, the external components required for
the oscillator are a crystal, connected between pins 23
and 24, and loading capacitors.
MICRF507
To achieve 9pF load capacitance required to center
TN4-26011 at 16MHz, set the external capacitors to
1.5pF and XCOtune=16 dec . Figure 8 shows the tuning
range for two different capacitor values, 1.5pF and zero
(external capacitors omitted). External capacitor values
will strongly affect the tuning range. Using 1.5pF with the
above crystal gives a tuning range that is approximately
symmetrical about the center frequency.
Pin 24
XtalOut
C9
1.5pF
Y1
TN4-26011
C8
1.5pF
Pin 23
XtalIn
Figure 7. Crystal Oscillator Circuit
The load capacitance C L seen between the crystal
terminals is:
C L =
1
C 8
1
+
1
C 9
+ C XCOtune + C pin
Figure 8. XCO Tuning with the XCOtune Field
The start-up time of the cystal oscillator, given in Table
Where C XCOtune is the capacitance of the internal
adjustable capacitor bank, and C pin is defined as the
internal chip capacitance when XCOtune bits are all
zeros, plus PCB stray capacitance across pins 23 and
24. The value of C pin is about 6pF. The loading capacitor
values required depend on the total C L specified for the
crystal for oscillation at the desired frequency.
It is possible to tune the crystal oscillator internally by
giving the 5-bit register field XCOtune a non-zero value,
which causes internal capacitors to be switched across
4, is about a millisecond and increases with capacitance.
When the MICRF507’s main mode is switched from
Power down mode to Transmit mode via Standby mode,
or to Receive mode via Standby mode, only the XCO is
energized at first. Current consumption during this
prestart period is approximately 280 μ A (the same as for
Standby mode). After the XCO amplitude is sufficient to
trigger the M-counter and produce two pulses at its
output, the remaining circuits on the chip are powered
on.
the crystal. As this capacitance increases, frequency
decreases. When XCOtune is set to its maximum value
of 31, approximately 4.5pF additional capacitance is
connected across the crystal pins.
The XCO tuning can be used to cancel crystal resonant
frequency error, both initial and with temperature. It can
be used in combination with the Frequency Error
Estimator (FEE). See “FEE” section.
The crystal used is a TN4-26011 from Toyocom.
Specification:
? Package TSX-10A,
XCOtune
0
1
2
4
8
16
31
Start-up Time (μs)
590
590
700
700
810
1140
2050
?
?
?
?
Nominal frequency 16.000000MHz
Frequency tolerance ±10ppm
Frequency stability ±9ppm, load capacitance
9pF
Pulling sensitivity 15ppm/pF
Table 4. Typical Crystal Oscillator Start-up Time
with C8 = C9 = 1.5pF
October 2, 2013
16
Revision 2.2
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