Microsoft LPF-00004 Manual del operador descargar pdf (Pagina 42)

Page 24 PRINCIPLES OF OPERATION

CYBERAMP 380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Amplitude Characteristics

For signals at the selected -3dB frequency the amplitude response of the low-pass filter is attenuated

by 3 dB. This means that the amplitude of the signal at the output of the filter is 1/√2 (i.e., 0.7071) of

the amplitude of the input signal. Equivalently, the -3 dB frequency is the frequency at which the

signal power at the output of the filter has fallen to half of the power of the input signal.

In addition to their type (Bessel, Butterworth, Elliptic, etc.), filters are described by their "order". The

higher the order of a filter, the steeper the slope of the attenuation in the stop band. In the case of a

low-pass filter, the stop band is defined as those frequencies above the -3 dB frequency.

A first-order filter (also called a single-pole filter) is formed with a single resistor and capacitor. In

the stop band, the attenuation increases at 6 dB/octave, which is equal to 20 dB/decade. In linear

terminology, this attenuation rate can be re-stated as a voltage attenuation increasing by 10 for each

decade increase in frequency. The dB attenuation increases linearly with the order of the filter. That

is, a fourth-order filter has 80 dB/decade attenuation in the stop band. The normalized frequency

responses of the fourth-order Bessel filter and Butterworth filter are shown in Figure 9.

BESSEL

BUTTERWORTH

10 0.1

Normalized

Frequency, f/f

-3dB

Figure 9. The frequency responses of fourth-order Bessel and Butterworth filters are compared. The spectra have

been normalized so that the signal magnitude in the pass band is 0 dB. The

3 dB frequency has been normalized to unity.

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Microsoft LPF-00004 Manual del operador Pagina 42