External Frequency Reference - Infineon Cypress CYW43353 Manual

3.2 External Frequency Reference

As an alternative to a crystal, an external precision frequency reference can be used. The recommended default frequency is 37.4
MHz. This must meet the phase noise requirements listed in
If used, the external clock should be connected to the WRF_XTAL_IN pin through an external 1000 pF coupling capacitor, as shown
in Figure 4. The internal clock buffer connected to this pin will be turned off when the CYW43353 goes into sleep mode. When the
clock buffer turns on and off, there will be a small impedance variation. Power must be supplied to the WRF_XTAL_BUCK_VDD1P5
pin.
Table 2. Crystal Oscillator and External Clock—Requirements and Performance
Parameter
Frequency
Frequency
Frequency
Frequency tolerance
over the lifetime of the
equipment, including
5
temperature
Crystal load capacitance
ESR
Drive level
Input impedance
(WRF_XTAL_IN)
WRF_XTAL_IN
input low level
WRF_XTAL_IN
input high level
WRF_XTAL_IN
input voltage
(see Figure 4)
Duty cycle
6
Phase noise
(IEEE 802.11b/g)
Document Number: 002-14949 Rev. *G
PRELIMINARY
Figure 4. Recommended Circuit to Use with an External Reference Clock
Reference
Clock
Conditions/Notes
2.4 GHz and 5 GHz bands, IEEE 802.11ac
operation
5 GHz band, IEEE 802.11n operation only
2.4 GHz band IEEE 802.11n operation, and
both bands legacy 802.11a/b/g operation
only
Without trimming
–
–
External crystal must be able to tolerate this
drive level.
Resistive
Capacitive
DC-coupled digital signal
DC-coupled digital signal
AC-coupled analog signal
37.4 MHz clock
37.4 MHz clock at 10 kHz offset
37.4 MHz clock at 100 kHz offset
Table 2.
1000 pF
WRF_XTAL_IN
NC WRF_XTAL_OUT
1
Crystal
Min. Typ.
35 37.4
19 37.4
Ranges between 19 MHz and 38.4 MHz
–20 –
– 12
– –
200 –
– –
– –
– –
– –
– –
– –
– –
– –
External Frequency
2 3
Reference
Max. Min. Typ.
38.4 – 37.4
38.4 35 37.4
20 –20 –
– – –
60 – –
– – –
– 30k 100k
7.5 – –
– 0 –
– 1.0 –
– 1000 –
– 40 50
– – –
– – –
CYW43353
Max. Units
– MHz
38.4 MHz
4
20 ppm
– pF
– Ω
– µW
– Ω
7.5 pF
0.2 V
1.26 V
1200 mV
p-p
60 %
–129 dBc/Hz
–136 dBc/Hz
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