AN2744 datasheet(25/56 Pages) STMICROELECTRONICS

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AN2744

Board description

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higher selectivity. The values of the actual components give a Q of about 2.2 for the 72 kHz

channel and 1.8 for the 86 kHz channel.

The value of RL impacts more obviously on insertion losses. To evaluate the relationship

between RL and the losses on received signal, the following simplified expression of |R(s)| at

f = fc can be used:

Equation 4

With the chosen components, this formula gives a loss always lower than 1 dB. The same

calculation gives unitary transfer if RL is set to zero.

Looking at the first way to express the module of the transfer function, it can be noticed that

a higher Q can help to keep the losses small. A high Q would bring to a higher sensitivity of

the filter to tolerance of the components.

Figure 14

and Figure 15 show the measured frequency response of the Rx passive filter for

the two channels. The filter has an attenuation of about 5 dB at center frequency. This loss

is mostly due to the Tx filter topology, in particular to the R21-C27 branch that is in parallel to

the Rx path.

)

(

⋅

≅

⋅

Figure 14.

Measured frequency response of the Rx passive filter for 72 kHz channel (typical)

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1E+04

1E+05

1E+06

Freq (Hz)

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1E+04

1E+05

1E+06

Freq (Hz)

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