AN4444 datasheet(31/68 Pages) STMICROELECTRONICS

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AN4444

ST7590 reference design description

8.1.3

Reception filter

Reception passive filter

The reception filter is made of a resistor in series with a parallel L-C resonant. The transfer

function of the filter can be written as:

Equation 3

where RL is the DC series resistance of the inductor (with L11 = B82462G4474M000,

RL = 2.7  max).

The center frequency and the quality factor of the filter can be expressed as:

Equation 4

The quality factor and the filter selectivity depend mainly on R5 value. Lower R5 leads to the

lower steepness of the resonance, while higher R5 gives higher selectivity.

RL value may impact insertion losses. To evaluate the relationship between the RL and the

received signal loss, the following simplified expression can be used:

Equation 5

With actual values of the components, we get almost unitary transfer at center frequency.

By looking the transfer function formula:

Equation 6

It can be noticed that a higher Q can help keeping the losses small. But a high Q would also

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

)

(











kHz

















0.56











)

(

















C 





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