ADVANCED DIGITAL SYSTEMS

Advanced Digital Systems



Advanced Digital Systems

Introduction

The following describes filter types, what they do and how they perform. Along with definitions and detailed graphs, we are hopeful this information is both useful and informative.

Filter Types

Monolithic Crystal Filters

2 Quartz resonator internally coupled utilizing piezoelectric effect.

Discrete Crystal Filter

Single quartz resonator with external components utilizing the piezoelectric effect.

Notch filters

Crystal or Discrete component filter that passes all frequencies except those in a stop band centered on a center

frequency.

High Pass Filters

Discrete component filter that passes high frequency but alternates frequencies lower then the cut off frequency.

Low Pass Filters

Discrete component filter that passes low frequency signals but alternates signals with frequencies higher then the cut off frequency.

Filter Designs

Chebyshev

The transfer function of the filter is derived from a

chebychev equal ripple function in the passband only. These filters offer performance between that of Elliptic function filters and Butterworth filters. For the majority of applications, this is the preferred filter type since they offer improved selectivity, and the networks obtained by this approximation are the most easily realized.

Butterworth

The transfer function of the filter offers maximally flat amplitude. Selectivity is better then Gaussian or Bessel filters, but at the expense of delay and phase linearity. For most bandpass designs, the VSWR at center frequency is extremely good. Butterworth filters are usually the least sensitive to changes in element values.

Bessel/Linear Phase

The transfer function of the filter is derived from a Bessel polynomial. It produces filter with a flat delay around center frequency. The more poles used, the wider the flat region extends. The roll-off rate is poor. This type of filter is close to a Gaussian filter. It has poor VSWR and loses its maximally flat delay properties at wider bandwidths.

Elliptic

The passband ripple is similar to the Chebyshev but with greatly improved stopband selectivity due to the addition of finite attenuation peaks. The network complexity is increased over the Butterworth or Chebyshev, but still yields practical realizations over nearly the entire operating region.

Gaussian

The transfer function of the filter is derived from a Gaussian function. The step and impulse response of a Gaussian filter has zero overshoot. Rise times and delay are lowest of the traditional transfer functions. These characteristics are obtained at the expensive of poor selectivity, high element sensitivity, and a very wide spread of element values. Gaussian filter is very similar to the Bessel except that the delay has a slight “hump” at center frequency and the rate of roll-off is slower. Because of the delay response, the ringing characteristics are better then the Bessel. Realization restrictions also apply to these filters.



Gaussian to 6 (or 12) dB- This approximation has a passband response that follows the Gaussian shape and, at either the 6 or 12 dB point, the response changes and follows the Butterworth characteristic. The phase, or delay, response is somewhat improved over a strict Butterworth and the attenuation is better than the pure Gaussian and so it is a true compromise type of approximation, as ...