ST ST7538Q, Application Note, Page: 24 / 56

Board description AN2744
24/56
5.1.3 Dual channel Rx passive filter
The dual channel Rx passive filter is made of a resistor in series with a parallel L-C resonant
circuit. The transfer function of the filter can be written as:
Equation 2
where R
L
is the dc series resistance of the inductor L7 (in the worst case, 2
Ω
) and C
P
is the
equivalent capacitance for the two channels: C16 + C20 for 72 kHz, only C20 for 86 kHz.
The center frequency and the quality factor of the filter can be expressed as:
Equation 3
The simplification done in Equation 3 is possible because R
18
>> R
L
. It's evident that the
quality factor, and then the filter selectivity, depends not only on the value of R
18
, but also on
R
L
. A higher R
L
means a lower steepness of the resonance, while a higher R
18
gives a
Figure 13. Simulated frequency response of the Tx passive filter for 86 kHz channel with
tolerance effect
1E5 1E4 1E6
-35
-30
-25
-20
-15
-10
-5
0
5
-40
10
T
P 7 18
L 18
P 7 18
7 P L 18
2
P 7 18
L 7
C L R
R R
s
C L R
L C R R
s
C L R
R L s
) s ( R
⋅ ⋅
+
+ ⋅
⋅ ⋅
+ ⋅ ⋅
+
⋅ ⋅
+ ⋅
=
P 7
P 7 18
L 18
C
C L 2
1
C L R
R R
2
1
2
1
fc
⋅ π
≅
⋅ ⋅
+
π
= ω ⋅
π
=