Figure 10: LFXO assembly
By default the LED_1 and LED_2 functions of the module are used.
To use the LFXO function, the two solder bridges near the LFXO place holder
shall be disconnected using a cutter.
The input capacitance of the pad LED_1 and LED_2 are 4 pF. The values of C9 and C10
can be calculated as follows.
The load capacitance of LFXO is given by
C
l
=
C
9
l
∗
C
10
l
C
9
l
+
C
10
l
(1)
If
C
9
l
=
C
10
l
= C, then
C
l
=
C
2
(2)
whereas,
C
9 =
C
−
C
XL
1
−
C
P CB
(3)
C
10 =
C
−
C
XL
2
−
C
P CB
(4)
C
l
= Load capacitance of LFXO crystal.
C
XL
1
= Input capacitance of Pad LED_1 (4 pF)
C
XL
2
= Input capacitance of Pad LED_2 (4 pF)
C
P CB
= Parasitic capacitance of PCB Parasitic capacitance of the PCB can vary depending
on design and track length. It can vary from 0.5 pF to 2 pF.
For the crystal
with load capacitance of 9 pF and parasitic capacitance of 2 pF.
The value of C9 and C10 results in 12 pF which was also tested on the evaluation board.
Depending on parasitic capacitance of PCB, a capacitance of 12 pF may be a good starting
value for C9 and C10.
Mini evaluation board user manual version 1.3
© June 2021
www.we-online.com/wireless-connectivity
22