AN3213 datasheet(27/64 Pages) STMICROELECTRONICS

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AN3213

ST7570 demonstration board description

Doc ID 17429 Rev 1

27/64

7.1.1

Transmission active filter

The transmission active filter is based on the ST7570 internal Power Amplifier (PA), with

input and output pins externally available to allow a filtering network tailored around the

amplifier. For the ST7570 demonstration board, a 3-pole low-pass filter has been realized by

cascading a simple R-C low-pass stage and a Sallen-Key 2-pole cell. The R12-C9 low-pass

stage is aimed at having a corner frequency of nearly 80 kHz, just above the higher S-FSK

tone, for a first filtering of the TX_OUT harmonics. The 1 nF value of C9 has been found to

be the optimal value to obtain a good filtering action without yielding unwanted capacitive

load distortion on the TX_OUT line. The transfer function of the 2nd order Sallen-Key cell is:

Equation 1

Where

Equation 2

Equation 3

Equation 4

Figure 17 represents measured transfer function of the transmission active filter. It shows

good rejection at signal harmonic frequencies.

Figure 17.

Measured frequency response of the transmission active filter (typical)

()

-------

⋅

----------------

-----------------------------------------

---------

⎝⎠

⎛⎞

4.3

12.7dB

⋅⋅

⋅

---------------------------------------------------------------

125kHz

⋅⋅

⋅

10C12

9C6

10C6 1A0

–

()

-------------------------------------------------------------------------------------

1.03

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