AN4068 datasheet(23/63 Pages) STMICROELECTRONICS

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AN4068

ST7580 reference design description

Doc ID 022923 Rev 2

23/63

7.1.2

Reception passive filter

The reception filter is made up of the series of a resistor and a parallel L-C resonant. The

transfer function of the filter can be written as:

Equation 5

where RL is the DC series resistance of the inductor (in this case, about 2 Ω).

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

Equation 6

Equation 7

It is quite evident that the quality factor and the filter selectivity depend not only on the R5

value, but also on RL. Higher RL leads to lower steepness of the resonance, while higher R5

gives higher selectivity.

The RL value impacts insertion losses in a more evident way. 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, there is 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 can be seen that a

higher Q can help to keep the losses small. However, a high Q would bring the response to

a higher sensitivity of the components' tolerance.

Figure 9 shows the measured frequency response of the Rx passive filter. The filter shows a

-3 dB bandwidth equal to 65 kHz and an attenuation of less than 1 dB at fC.

)

(

⋅

≅

⋅

=80 kHz

⋅

=1.3

()

)

(

⋅

≅

⋅

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