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- Load Impedance The impedance that normally must be connected to the output terminal of the filter in order to meet filter specifications; The filter output will drive this load.
- Lowpass Filter A filter which passes low frequencies and rejects high frequencies.
- Overshoot The amount in percent by which a signal exceeds its steady-state output on its initial rise.
- Passband The frequency range passed by a filter.
- Passband Ripple Variations of attenuation with frequency within the passband of a filter.
- Phase Shift The changing of phase of a signal as it passes through a filter. A delay in time of the signal is referred to as phase lag and in normal networks, phase lag increases with frequency, producing a positive envelope delay (see envelope delay).
- Q The figure of merit of a capacitor or inductor. The ratio of its reactance to its equivalent series resistance. Also in bandpass filters “loaded Q” is a term used to define the percentage of 3 dB bandwidth.
- Loaded Q = Center Frequency (Fc) / 3 dB Bandwidth
- Relative Attenuation Attenuation measured with the point of minimum attenuation taken as zero dB, or: (Relative Attenuation = Attenuation minus Insertion Loss.)
- Return Loss The ratio in dB of maximum power sent down a transmission line to the power returned toward the source. Also equal to 20 times the log of the reciprocal of the reflection coefficient. If return loss is infinite, all power is absorbed in the circuit.
- Ringing The tendency of a filter to oscillate for a time when a transient waveform is applied to it.
- Ripple Generally referring to the wavelike variations in the amplitude response of a filter. Tchebycheff and elliptic function filters ideally have an equal-ripple characteristic, which means that the difference in peaks and valleys of the amplitude response in the passband are always the same. Butterworth, Gaussian, and Bessel functions have no ripple. Ripple is usually measured in dB.
- Rise Time The length of time it takes a step-function at the output of a filter to move from 10% to 90% of its steady state value on the initial rise.
- Shape Factor An important parameter of all filters:
- Bandpass: SF = Attenuation Bandwidth/ 3 dB Bandwidth
- Bandstop: SF = 3 dB Bandwidth/ Attenuation Bandwidth
- Lowpass: SF = Attenuation Frequency / Fco
- Highpass: SF = Fco / Attenuation Frequency
- Source Impedance The output impedance of the circuit that drives the filter. The impedance of the circuit the filter must work from or be tested in.
- Step Function A signal change in amplitude from one level to another which occurs in zero time. Usually refers to a rectangular front waveform used in testing transient response.
- Stopband The area of frequency where it is desirable to reject or attenuate all signals as much as practical.
- Tchebycheff Function A mathematical function that produces a curve that defines ripples within certain bounds (see ripple). This function produces a squarer amplitude response than the butterworth function but with less desirable phase, and time delay characteristics. There is a whole family of Tchebycheff functions (0.1 ripple, 0.5 ripple, etc.).
- Time Delay The amount of time it takes for certain signals to pass through a filter.
- Transient Response The response of a lowpass filter to a step function, or very low frequency square wave. If a sudden voltage rise is applied to a lowpass filter the output will respond some time later. Transient response can also apply to a bandpass filter responding to a sudden burst of signal within its passband.
- Voltage Standing Wave Ratio (VSWR) The ratio between the peak and valley of standing waves on a transmission time.
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