AN3213 datasheet(29/64 Pages) STMICROELECTRONICS

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AN3213

ST7570 demonstration board description

Doc ID 17429 Rev 1

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It is quite evident that the quality factor and the filter selectivity depend not only on the R3

value, but also on RL. A higher RL leads to lower steepness of the resonance, while a higher

R3 gives higher selectivity.

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

between RL and the received signal loss, the following simplified expression of

at f=fC

can be used:

Equation 8

With actual values of the components, a loss of about 1 dB is obtained. The same

calculation gives unitary transfer if RL is set to zero.

When looking at the first way to express the module of the transfer function, it can be seen

that a higher Q can help to keep the losses to a minimum. On the other hand, it would bring

the response to a higher sensitivity to component tolerance.

Figure 19 shows the measured frequency response of the Rx passive filter. The filter shows

a -3 dB bandwidth equal to 50 kHz and an attenuation of less than 1 dB at fC.

Figure 19.

Measured frequency response of the reception passive filter (typical)

Figure 20 shows the simulation of the response of the Rx passive filter with the components

tolerance effect. The shift on the center frequency gives a worst-case loss of nearly 0.5 dB

around 68 kHz.

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