AN2451 datasheet(23/55 Pages) STMICROELECTRONICS

23 / 55 page

AN2451

Board description

Doc ID 12791 Rev 3

23/55

Equation 5

The simplification made on fc formula is possible because R17>RL. Consequently, the

quality factor and filter selectivity depend not only on R17, but also on RL. A higher RL

produces a lower steepness of the resonance, while a higher R17 gives a higher selectivity.

Actual values of the components give a Q equal to 4.3.

The RL value impacts in a clearer way on insertion losses. To evaluate the relationship

between RL and the losses on the received signal, the following simplified expression of

at f=fc may be used:

Equation 6

With actual values of the components, we get a loss of about 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 is possible to notice

that a higher value of Q can help keeping the losses small. Nevertheless, a high value of Q

would bring a higher sensitivity of the filter to the components tolerance.

Figure 14 shows the measured frequency response of the Rx passive filter. The filter has an

actual -3 dB bandwidth equal to 17 kHz and an attenuation of about 1 dB at center

frequency, just as expected.

Figure 14.

Measured frequency response of the Rx passive filter (typical curve)

Figure 15 represents a simulation of the response of the Rx passive filter with the

components tolerance effect. A worst case loss of nearly 1.5 dB can be observed at 72 kHz

due to a shift on center frequency.

------

•

------

17L6C26

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

π L

6C26

--------------------------- Q

17L6C26

17RLC26

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

•

≅

()

πf

•

()

•

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

•

---------

•

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

≅

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