BCM4339 Preliminary Data Sheet
External Frequency Reference
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 Table 2.
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 5. The internal clock buffer connected to this pin will be turned off when the
BCM4339 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.
Figure 5: Recommended Circuit to Use with an External Reference Clock
1000 pF
Reference
WRF_XTAL_IN
Clock
NC
WRF_XTAL_OUT
Table 2: Crystal Oscillator and External Clock—Requirements and Performance
External Frequency
Crystala
Referenceb c
Parameter
Conditions/Notes
Min. Typ. Max. Min. Typ. Max. Units
Frequency
2.4 GHz and 5 GHz bands,
IEEE 802.11ac operation
35
37.4 38.4
37.4 38.4 35
Ranges between 19 MHz and 38.4 MHzd
–
37.4
–
MHz
Frequency
Frequency
5 GHz band, IEEE 802.11n
operation only
19
37.4 38.4 MHz
2.4 GHz band IEEE 802.11n
operation, and both bands
legacy 802.11a/b/g operation
only
Frequency tolerance Without trimming
over the lifetime of the
equipment, including
temperaturee
–20
–
20
–20
–
20
ppm
pF
Crystal load
capacitance
–
–
–
–
12
–
–
–
–
ESR
–
–
60
–
–
–
–
–
–
–
Ω
Drive level
External crystal must be able to 200
tolerate this drive level.
µW
Input impedance
(WRF_XTAL_IN)
Resistive
–
–
–
–
–
–
–
30k
–
100k
–
Ω
Capacitive
7.5
–
–
–
7.5
0.2
pF
V
WRF_XTAL_IN
input low level
DC-coupled digital signal
0
Broadcom®
November 17, 2014 • 4339-DS106-R
Page 31
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