External Frequency Reference - Infineon Cypress WICED CYW43903 Manual

3.2 External Frequency Reference

As an alternative to a crystal, an external precision frequency reference can be used, provided that it meets the phase noise require-
ments listed in Table 4.
If used, the external clock should be connected to the WRF_XTAL_XON pin through an external 1000 pF coupling capacitor, as shown
in Figure 6. The internal clock buffer connected to this pin will be turned off when the CYW43903 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_VDD1P35 pin.
Table 4. Crystal Oscillator and External Clock—Requirements and Performance
Parameter
Frequency
Frequency tolerance over the
lifetime of the equipment,
3
including temperature
Crystal load capacitance
ESR
Drive level
Input impedance
(WRF_XTAL_XON)
WRF_XTAL_XON
Input low level
WRF_XTAL_XON
Input high level
WRF_XTAL_XON
input voltage
(see Figure 6)
WRF_XTAL_XON
input voltage
(see Figure 6)
Duty cycle
4
Phase noise
(IEEE 802.11b/g)
4
Phase noise
(IEEE 802.11n, 2.4 GHz)
1. (Crystal) Use WRF_XTAL_XON and WRF_XTAL_XOP.
2. See External Frequency Reference
3. It is the responsibility of the equipment designer to select oscillator components that comply with these specifications.
4. Assumes that external clock has a flat phase noise response above 100 kHz.
Document Number: 002-14826 Rev. *G
PRELIMINARY
Figure 6. Recommended Circuit to Use With an External Reference Clock
Reference
Clock
Conditions/Notes
2.4 GHz and 5 GHz bands:
IEEE 802.11a/b/g/n operation
Without trimming
–
–
External crystal must be able to
tolerate this drive level.
Resistive
Capacitive
DC-coupled digital signal
DC-coupled digital signal
IEEE 802.11b/g operation only
IEEE 802.11n AC-coupled analog
input
37.4 MHz clock
37.4 MHz clock at 10 kHz offset
37.4 MHz clock at 100 kHz offset
37.4 MHz clock at 10 kHz offset
37.4 MHz clock at 100 kHz offset
for alternative connection methods.
1000 pF
WRF_XTAL_XON
NC
WRF_XTAL_XOP
Crystal
Min. Typ.
– 37.4
–20 –
– 16
– –
200 –
– –
– –
– –
– –
– –
– –
– –
– –
– –
– –
– –
External Frequency
1
Reference
Max. Min. Typ.
– – –37.4
20 –20 –
– – –
60 – –
– – –
– 30k 100k
7.5 – –
– 0 –
– 1.0 –
– 400 –
– 1 –
– 40 50
– – –
– – –
– – –
– – –
CYW43903
2
Max. Units
– MHz
20 ppm
– pF
– Ω
– µW
– Ω
7.5 pF
0.2 V
1.26 V
1200 mV
p-p
– V
p-p
60 %
–129 dBc/Hz
–136 dBc/Hz
–134 dBc/Hz
–141 dBc/Hz
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