AN2451
Board description
Doc ID 12791 Rev 3
19/55
Figure 9.
Schematic of Rx and Tx filters
5.1.1
Tx active filter
The Tx active filter is based on the ST7540 internal power amplifier (PA), whose input and
output pins are available externally to allow a filtering network to be tailored around the
amplifier.
For the ST7540 reference design board, a 3-pole low-pass filter has been developed by
cascading a simple R-C low-pass stage and a Sallen-Key 2-pole cell with 9dB gain. The
R19-C32 low-pass stage is aimed at introducing attenuation starting from approximately an
octave above the transmission channel frequency.
The transfer function of the 2
nd
order Sallen-Key cell is:
Equation 1
The corner frequency may be calculated as:
Equation 2
represents the measured transfer function of the Tx active filter. It shows good
rejection on both the 2
nd
and 3
rd
harmonic frequencies for the 72 kHz signal.
A s
( )
A
0
s
2
ω
C
2
----------
s
ω
C
Q
•
------------------
1
+
+
---------------------------------------------
=
A
0
1
R
14
R
12
---------
+
⎝
⎠
⎛
⎞ ω
C
1
R
9
R
10
•
C
33
•
C
21
•
---------------------------------------------------------
=
,
=
Q
R
9
R
10
•
C
33
•
C
21
•
R
9
C
33
R
10
C
21
R
9
C
21
1
A
0
–
(
)
+
+
----------------------------------------------------------------------------------------
=
where
and
f
c
1
2
π
R
9
R
10
•
C
33
•
C
21
•
•
----------------------------------------------------------------------
135.7kHz
=
=
