Glossary of Microwave and RF Filter Terms

Amplitude Match: The absolute difference in the amplitude response between a reference filter and a filter under test.

Amplitude tracking: The relative difference in the amplitude response between a reference filter and a filter under test.

Attenuation: The decrease in amplitude of a signal at a specified frequency during its transmission through a filter.

Bandwidth: (1) The limits of a band of frequencies. In a bandpass filter these limits, or passband edges, are generally the frequencies at which -3dB of attenuation is measured, relative to the attenuation at the maximum transmission point of the passband. (2) The number of hertz expressing the difference between the upper 3dB frequency and lower 3dB frequency of a bandpass filter (F"-3dB - F'-3dB).

Bessel Filter: A filter network designed to exhibit constant time delay.

Butterworth Filter: A filter network that exhibits the flattest possible response in its passband. The response is monotonic, rolling off smoothly at the rate of 6dB per octave, per pole.

Center Frequency - F_o: The midpoint of the bandpass filter passband, expressed as the arithmetic mean of the two -3dB frequencies.

Chebyshev Filter: A filter network that is designed to exhibit a predetermined passband ripple, in exchange for which, it provides more rapid attenuation above the cut-off (-3dB) frequency.

Cut-Off frequency: The frequency that marks the edge of the filters passband and the beginning of the transition to the stopband, usually -3dB relative to filters reference frequency.

Diplexer or Duplexer: A form of a multiplexer consisting of a common input with two outputs.

Distortion: Undesired changes in the purity of a signal, so that a spurious element or elements are added. These changes can be exhibited as amplitude distortion where the output does not bear the same proportion to the input at all frequencies or phase distortion or non linear phase shift.

Dynamic Range: The difference between the overload level and the minimum acceptable signal level, expressed in dB.

Envelope Delay: The propagation time delay undergone by the envelope of an amplitude modulated signal as it passes through a filter. Envelope delay is proportional to the slope of the curve of phase shift as a function of frequency.

Filter: A selective network comprised of capacitors, inductors and / or resistors which passes a specific band of frequencies and attenuates the out of band frequencies.

Fourier Analysis: The process of analyzing a complex wave by separating it into a plurality of component wave, each of a particular frequency, amplitude and phase displacement.

Gaussian Filter: A filter network designed to pass a step function with zero overshoot and minimum rise time. This is similar to a Bessel Filter.

Highpass Filter: A filter that passes high frequencies and attenuates low frequencies.

Impedance (As it applies to filters): The resistive source and load terminations required to insure proper filter performance.

Insertion Loss: While many definitions exist this one applies to filters. The difference between the power received at the load, before, as compared with after the insertion of a filter network. This loss is usually expressed in dB.

Linear Phase Response: A filter that exhibits a constant change in degrees per unit of frequency. The resultant plot of phase versus frequency is a straight line.

Loaded Q (Working Q): A term that defines the percentage of the 3db bandwidth of a Bandpass Filter. Q=Center Frequency (F_c) in Hertz/ 3dB Bandwidth in Hertz

Lowpass Filter: A filter that passes low frequencies and attenuates high frequencies, sometimes referred to as an anti-aliasing filter.

Overshoot: The amount expressed in percent, by which a signal exceeds its steady state output on its initial rise.

Passband: The band of frequencies that passes through a filter with essentially no attenuation.

Passband Ripple: The variation of attenuation in the passband of a filter, usually measured in dB.

Phase Shift: The changing of a signal as it passes through a filter. A delay in time of the signal referred to as phase lag and in normal network phase lag increases with frequency, producing a positive envelope delay. Please see envelope delay.

Q: The Q or figure merit of a filter, is a measure of the sharpness of response or its frequency selectivity. Please see "Loaded Q".

Relative Attenuation: Attenuation measured with the point of minimum attenuation taken at 0dB. Relative attenuation=attenuation minus insertion loss.

Ripple: The wavelike variations in the amplitude response of a filter. Chebyshev and Elliptic function filters have characteristics such that the differences in peaks and valleys of the amplitude response in the passband are always the same. Butterworth, Bessel and Gaussian filters do not have ripple. Ripple is usually measured in dB.

Shape Factor (Bandwidth Ratio): In a filter, the ratio comparing the high-attenuation level bandwidth and low attenuation level bandwidth. Or the ratio of the 3dB bandwidth to the stopband bandwidth.

Step Function: A signal characterized by instantaneous changes between amplitude levels. The term usually refers to a rectangular front wave-form used for making tests of transient responses.

Stopband: That part of the frequency spectrum that is subjected to a specified amount of attenuation by a filter.

Time Delay: The slope of the phase versus the frequency curve. In a loose sense, this is the time it takes a designated point in a wave to pass through a filter. It is also called envelope delay.

Transition Band: The range of frequencies that bound a passband / stopband interface.

Voltage Standing Wave Ratio ( VSWR): In a stationary wave system, the ratio of the amplitude of the electric field or voltage at a voltage maximum to that at the adjacent voltage minimum.