A111 Pulsed Coherent Radar (PCR)
Datasheet, v1.8
Page 23 of 34
2019-06-12
© 2019 Copyright by Acconeer
8.2 XTAL
The input clock can origin from a crystal (XTAL), connected to XIN and XOUT.
The A111 sensor has a built-in XTAL oscillator and by adding an external XTAL component, as
shown in the below figure 8.1, a reference design without any external clock reference supplied is
possible. Note however, that the external clock reference still is supported and if used instead of an
external XTAL, it is connected to XIN.
C7
C8
GND
GND
GND
XOU T
XIN
TSX-3225
XTAL 26 MHz
I/O
GND
I/O
GND
Figure 8.1. External XTAL schematics.
To enable the internal XTAL oscillator to drive the external resonator, the relation in equation 1 must
be fulfilled.
Equation 1
𝑓 ∗ 𝐶
𝑝𝑖𝑛
0.8
∗ 𝑅
𝐸𝑆𝑅
0.61
< 0.7
Equation 2
𝐶 = 2(𝐶
𝐿
− 𝐶
𝑠𝑡𝑟𝑎𝑦
)
Equation 3
𝐶
𝑝𝑖𝑛
= 𝐶 + 𝐶
𝑠𝑡𝑟𝑎𝑦
∗ 2
The capacitance values are calculated in equation 2. C
L
and R
ESR
are XTAL parameters and vary from
XTAL to XTAL. The stray capacitance is the sum of the capacitance between XIN and XOUT, which
are found in the traces on PCB and in the package; 2 to 5 pF is a general estimation.
Example:
•
f = 26 MHz
•
C
L
= 9 pF
•
R
ESR
= 40 ohm
Assuming that C
stray
= 5 pF gives C7, C8 = 8 pF and that the condition is met with the result 0.63 <
0.7.