The Elliptic design is also noted for a predetermined amount of passband ripple along with ripple in the stop band. See fig.C. While this desing requires less components than the Butterworth of Chebyshev to realize a given shape factor, the quality factor required of its components is much higher than that of other filter types. This results in a reduced useful frequency range and an increase cost. Using this filter type, Allen Avionics has been able to achieve stop band ratios of less than 1.012 at 40dB. For example, a lowpass filter with its 3dB cutoff frequency at 10,000Hz would have its 40dB point at 10,120Hz.

     The amplitude, delay and phase response of the Bessel and Gaussian designs are very similar to each other. Both filter tpes have excellent flat delay characteristics in the pass band when they are used for lowpass or narrow band bandpass. In the wide band bandpass, speial designs must be used to keep the delay flat over the passband. Their shape factors are extremely poor (2 to 1 at 13dB) and for this reason their usefulness is somewhat limited. If better shape factors are 

needed with constant delay distortion in the passband., delay equalized filters have to be used. The Bessel filters are represented by curves A, B and C on the Linear Phase Lowpass page of the catalog. A much more desirable and useful design is the Transitional Gaussian. Curves D,E and F are representative of these types of Linear Phase Lowpass Filters. Linear Phase Filters of this type are extremely popular because of their cost/performance ratio.

     The communications industry has a high demand for Linear Phase Flat Delay Bandpass Filters. Allen Avionics has developed a series of synthesized bandpass designs which have exceptional flat delay characteristics. The advantage of this design over that of a delay equalized bandpass filter is that it utilizes less components. This results in economy, reliability and generally smaller sizes. Because of our computer modeling capabilities, fast deliveries (about one week) are achieved. They are available with center frequencies ranging from 1 KHz to 80 MHz and bandwidths from 2% to 50%. Filters of this type are tabulated later in these pages.