micrf507ymltr Micrel Semiconductor, micrf507ymltr Datasheet - Page 16

• Manufacturer Part Number: micrf507ymltr
• Description: 470mhz To 510mhz Low-power Fsk Transceiver With +10dbm Power Amplifier
• Manufacturer: Micrel Semiconductor

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.
The load capacitance C
terminals is:
Where C
adjustable capacitor bank, and C
internal chip capacitance when XCOtune bits are all
zeros, plus PCB stray capacitance across pins 23 and
24. The value of C
values required depend on the total C
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
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:
March 2010
Micrel, Inc.
•
•
•
•
•
XtalOut
Pin 24
Package TSX-10A,
Nominal frequency 16.000000MHz
Frequency tolerance ±10ppm
Frequency stability ±9ppm, load capacitance
9pF
Pulling sensitivity 15ppm/pF
C
XCOtune
L
Figure 7. Crystal Oscillator Circuit
=
C
1
8
is the capacitance of the internal
+
1
pin
C
is about 6pF. The loading capacitor
1.5pF
1
9
C9
+
L
C
XCOtune
seen between the crystal
TN4-26011
Y1
pin
+
C
1.5pF
is defined as the
L
C8
pin
specified for the
Pin 23
XtalIn
16
To achieve 9pF load capacitance required to center
TN4-26011 at 16MHz, set the external capacitors to
1.5pF and XCOtune=16
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.
The start-up time of the cystal oscillator, given in Table
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 280μA (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.
Table 4. Typical Crystal Oscillator Start-up Time
100,0
-20,0
-40,0
-60,0
80,0
60,0
40,0
20,0
Figure 8. XCO Tuning with the XCOtune Field
0,0
0
XCOtune
16
31
0
1
2
4
8
8
with C8 = C9 = 1.5pF
XCO bitvalue
dec
16
. Figure 8 shows the tuning
Start-up Time (µs)
24
1140
2050
590
590
700
700
810
M9999-032210-B
32
MICRF507
2x1.5pF
2x0pF