Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 41)

PRINCIPLES OF OPERATION Page 23

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

REFERENCE SECTION -- PRINCIPLES OF OPERATION

Low-Pass Filter

Filter Type

The low-pass filters in the CyberAmp 380 are fourth-order Bessel filters.

The Bessel filter is sometimes called a linear-phase or constant delay filter. All filters alter the phase

of the sinusoidal components of the signal. In a Bessel filter, the change in phase with respect to

frequency is linear. Put differently, the amount of signal delay is constant in the pass band. This

means that pulse-like signals in the pass band are minimally distorted.

To illustrate the trade offs in the use of a Bessel filter, it is convenient to compare it to a Butterworth

filter. The Butterworth filter is sometimes called a maximally flat amplitude filter. All low-pass

filters attenuate sinusoidal components of the signal progressively more as the -3 dB cutoff frequency

is approached from DC. In a Butterworth filter, the attenuation in the pass band is as flat as possible

without having passband ripple. This means that the frequency spectrum is minimally distorted.

Time-Domain Considerations

The Bessel filter was chosen because it is excellent for pulse-like (i.e., time domain) data such as

evoked potentials, neuronal spikes, single-channel openings and other transient responses. The

overshoot after an input step is less than 1%. By comparison, the overshoot of a Butterworth filter is

much worse (typically 10-15%), as shown in Figure 8.

BUTTERWORTH

BESSEL

Figure 8. The overshoots of fourth-order Bessel and Butterworth filters are compared.

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Microsoft LPF-00004 Manuel de l'opérateur Page 41