In many cases it is important to know more about the passband of a filter around the transition region between the passband and the stopband. The information provided serves as a design aid where passband flatness is an important criteria.
The dissipative losses are greater at the bandedges than at center frequency. The passband of the filter becomes rounded at the bandedges. Since both the dissipative loss and the reflective losses are present in each filter, the ripple becomes superimposed on the rounded passband created by the dissipative losses. Because of this it is more useful to specify a relative bandwidth as shown than the equi-ripple bandwidth.
The relationship between center frequency insertion loss and the +/- 5 degree phase linearity bandwidth is shown. This bandwidth is defined as the maximum deviation from a best-fit line drawn between two points on either side of the passband. The relationship between center frequency insertion loss and the 1.5:1 VSWR bandwidth is also given. The VSWR corresponds to a 14 dB Return Loss in a 50 Ohm system.
A 4 pole filter with a 3 dB bandwidth of 60 MHz and 3.5 dB insertion loss:
0.5 dB bandwidth is .64 x 60 = 38.4 MHz
1.0 dB bandwidth is .77 x 60 = 46.2 MHz
±5 degrees phase bandwidth is .62 x 60 = 37.2 MHz
1.5:1 VSWR bandwidth is .85 x 60 = 51 MHz
Note: When out-of-band attenuation is not specified, a 3 dB bandwidth tolerance of -0 / + 10% will be used.
Filter Circuit Topology
Modern filter synthesis allows the placement of transmission zeroes by the designer. Smiths Interconnect incorporates the use of the latest software to design our filters to each unique application. Filters may be designed with asymmetrical responses to most efficiently attenuate low side or high side signals. Symmetrical responses are used where both lower and upper attenuations are important. Smiths Interconnect utilizes elliptic or pole-placed functions where finite zeros are required. The schematics and response curves below show just a few of the filter networks used.
In addition to the Low Ripple Chebyshev Responses shown, other transfer functions such as Bessel, Gaussian, Butterworth and Linear phase designs are available.
This is the simplest form of a ladder network. The lowpass filter response extends from D.C. to a specified cutoff frequency. The passband insertion loss is measured at .90 times the 3 dB cutoff. Stopband response may extend to 100 times the cutoff frequency.
This is the inverse of the lowpass circuit shown. The highpass filter is specified with a 3 dB cutoff as well as an upper passband limit. Because of parasitic elements inherent in the design, the passband cannot extend to infinite frequencies. Responses are available to 20 times the specified cutoff frequency.
This is a cascade of the lowpass and highpass circuits shown above. This configuration is generally used for bandwidths of an octave or greater. The response may be tailored to meet the upper and lower stopband requirements as needed.
This is the classical "resonant ladder" used in wideband applications. The circuit is obtained by a lowpass to bandpass transform. Its advantages are geometric symmetry and a small spread of element values when used in circuit transforms.
The capacitively coupled nodal circuit provides an excellent configuration for narrowband use. The highside response may be sharpened by the use of a variety of transforming networks.
This is the "dual" of the nodal circuit shown. It provides a steeper high side response due to the greater number of zeroes at infinity. This circuit may also use a variety of transforming networks to provide symmetry to the response.
The Elliptic filter (also known as a Cauer response) provides the steepest out of band attenuation of any filter response. This is achieved by adding anti-resonance, or notch sections to the filter. These responses are available in Lowpass, Highpass, Bandpass and Bandstop.
Unlike the Elliptic filter where the finite attenuation poles are determined by the mathematical function, the Pole Placed filter allows the designer to specify where these points fall. This design is useful where there are specific single frequencies to remove.
The standard environmental conditions are listed throughout the catalog in the corresponding section for each product series.
Most products offered by Smiths Interconnect may be designed to meet any of the extended environmental specifications shown in the following table. Conditions not listed may also be acceptable. Smiths Interconnect has the capability to test out products in accordance with these or similar environmental test methods. Please contact the sales department for your specific requirements.