Definitions

Definitions / RF Spectrum / VSWR to Return Loss / Watts to dBm / Frequency to Wavelength

Bandreject Filter - A Bandreject or notch filter is designed to reject a specific frequency band while passing all other frequencies.

Bandpass Filter - A filter designed to pass a specific frequency band while rejecting all other frequencies.

Bandwidth (e.g., 3 dB Bandwidth) - The width in frequency of a passband or reject band. The typical use of bandwidth is in reference to the 3 dB bandwidth points of a bandpass filters passband.

Bessel Transfer Function - A mathematical function used to design filter circuits. This function is used in designs requiring provide constant time delay without concern for amplitude response.

Butterworth Transfer Function - A mathematical function used to design filter circuits. This function is used in designs requiring constant amplitude response without concern for time delay or phase response.

Center Frequency (Fc) - The center or reference point for describing a bandpass filter's characteristics - including passband and rejection.

Chebychev Transfer Function - A mathematical function used to design filter circuits. This function is used in designs that require relatively steep rejection skirts without significant concern for passband ripple or phase flatness.

Cutoff Frequency (Fco) - A filter performance characteristic providing the end or start of the passband in Lowpass and Highpass filters. For Lowpass filters, F_co specifies the upper passband edge or knee. For Highpass filters, F_cospecifies the lower passband edge.

Dissipation - Energy that is lost by absorption in the filter circuit.

Elliptic Transfer Function - A mathematical function used to design filter circuits. This function is used in designs that require the sharpest possible amplitude filter response without concern for the transient response.

Group Delay - A filter performance characteristic that specifies the time delay encountered by a signal passing through a filter circuit.

Gaussian Transfer Function - A mathematical function used to design filter circuits. This function is used in design requiring maximum phase delay flatness across the passband without significant concern for a steep rejection skirt.

Highpass Filter - A filter designed to pass high frequencies while rejecting all lower frequencies.

Input Impedance - The impedance observed at the circuit input while the output is properly terminated.

Insertion Loss - The amount of loss to a signal after passing through the filter circuit. This loss comes in two forms:

Dissipative Loss - Input signal that is lost through absorption by filter circuit components. The magnitude of this loss is determined by the Q of the filter circuit. Mathematically, Dissipative Loss = 8.6*(Sections-1.5)*F_c / (Q* BW_3dB)
Return Loss - See definition.

Linear Phase - A filter performance characteristic providing a constant change in degrees for a constant change in frequency. This type of filter ideally displays a constant delay in its passband.

Load Impedance - The impedance of the device or circuit provided to the output of the filter circuit. Normally, the load impedance must match the input impedance.

Lowpass Filter - A filter designed to pass low frequencies while rejecting all higher frequencies.

Passband - The bandwidth were a filter passes a signal. Typically, passbands are defined by the loss or rejection relative to the center frequency or cutoff frequency.

Phase Shift - A filter performance characteristic providing the relative change in phase of an input signal after passing through a circuit.

Loaded Q - Provides an estimate of the circuit's Q required to achieve a 3dBc bandwidth. Mathematically, Loaded Q = Fc / BW_3dB.

Return Loss - A filter performance characteristic providing the relative amount of power reflected by an input signal. Mathematically, Return Loss = 20log [(VSWR + 1)/(VSWR-1)]. In a perfect transmission line, there is no reflected power so Return Loss is essentially infinite. In a perfectly reflective circuit, Return Loss is zero.

Rejection - A filter performance characteristic defining the relative attenuation from the desired signal passed by the filter. Units are provided in dBc.

Ripple - A filter performance characteristic defining the fine grain variations a filter's amplitude output over a specific bandwidth.

Rise Time - A filter performance characteristic of the time delay for a step-function to arrive at the output. Typically, this is further defined as the time delay for a signal to change from 10% to 90% of its final amplitude value.

S-Parameters

S11 - Power reflected back to Port 1 from Port 1 (Port 1 Return Loss)
S22 - Power reflected back to Port 2 from Port 2 (Port 2 Return Loss)
S21 - Power loss through device from Port 1 to Port 2 (Insertion Loss)
S12 - Power loss through device from Port 2 to Port 1 (Insertion Loss)

Shape Factor (SF) - A rough description of a filter's performance requirements. The following are the Shape Factors for four basic single-signal filter types:

SF_Bandpass = Rejection BW / 3dB Bandwidth
SF_Bandreject = BW_3dB / Rejection BW
SF_Lowpass = Rejection Frequency / Fco
SF_Highpass = Fco / Rejection BW

Source Impedance - The impedance of the circuit provided to the filter circuit.

Voltage Standing Wave Ratio (VSWR) - A filter performance characteristic defining the ratio of the maximum and minimum of standing waves in the filter circuit. Alternatively, this can be specified using return loss using the following conversion formula: Return Loss = 20log [(VSWR + 1)/(VSWR-1)]