Impedance level: Is the value specified in Ohms, of the filter source impedance (input) and the terminating impedance (output).

Generally the input and output impedance are the same.

Insertion Loss (IL): Insertion Loss is a measure of power loss in a device, and at any frequency is defined as: IL = 10Log(Pl/Pin)
Where Pl is the load power and Pin is the power from the generator.

Return Loss (RL): The return loss is a measure of the filter performance. It is an indicator of how close the input and output impedance of the filter is to an ideal impedance value.

The return Loss at any frequency is defined as: RL = -10Log(Pr/Pin).
Where Pr is the power reflected back to the generator.

VSWR: Voltage standing wave ratio. It is the ratio between the maximum value and the minimum value of a standing wave.
It is also a measure of the filter performance indicating an impedance mismatch ratio between an ideal impedance value to the actual filter impedance.

The RL and VSWR are related by: RL = -20Log[(VSWR-1)/(VSWR+1)]

Group delay (GD): The group delay is a measure of the phase linearity of a device. Since a phase delay occurs at the output of a filter, it is important to know if this phase shift is linear with frequency. If the phase shift is nonlinear with frequency the output waveform will be distorted. The group delay is defined as the derivative of the phase shift versus frequency. A linear phase shift will result in a constant group delay, since the derivative of a linear function is a constant.

Relative attenuation: The attenuation difference measured from the minimum attenuation point to the
wanted rejection point. Relative attenuation is usually specified in dBc.

Diplexers and Multiplexers: Diplexers and multiplexers are characterized as having one input and two or more outputs. Diplexers are extensively used in the wireless industry and consists of a common (antenna) port, receive (Rx) port and transmit (Tx) port.

Isolation: In diplexers the ability to reject the transmit (Tx) frequency while looking at the receive (Rx) channel and the ability to reject the receive (Rx) frequency while looking at the transmit (Tx) frequency is called Rx/Tx isolation. The more isolation we have the better the filter can isolate the Rx form the Tx and vise versa. The result translates into cleaner transmitting and receiving signals.

Shape factor (SF): The shape factor of a filter is typically the ratio of the stopband bandwidth(BW) to the 3dB bandwidth. It is a measure of the steepness of the filter skirt.

For example if the 40dB BW is 40MHz and the 3dB BW is 10MHz, the shape factor will be 40/10=4.

Ripple(Ar): Is a measure of the flatness of the passband in a filter, and is normally expressed in decibels. The amount of ripple in a filter will effect the return loss. The greater the ripple, the worse the return loss, and vice versa.

Power handling: Is the rated average power in watts beyond which the performance of the filter may degrade or fail.

Rejection: Specific frequency or frequencies where the signal is attenuated at some specified value or set of values. The region outside the desired passband is sometime defined as the rejection frequency or frequencies, and the attenuation as the rejection .

Passband (PB): The minimum attenuation band of a filter. The passband is usually defined at the 0.5dB, 1dB, or at the 3dB point, or points as it is the case in a bandpass filter.

Center frequency (Fo): Is usually defined as the midpoint between the two 3dB point of a bandpass filter (or bandstop filter), and is normally expressed as the arithmetic mean of the 3dB points.

Cut-Off frequency (Fc): Is the transition point from the passband to the start of the stopband in a lowpass or highpass filter. That transition point is normally the 3dB point.

Dissipation: Is the energy loss in a filters due to non ideal elements such as the resistivity in an inductor or capacitor, core saturation, resistance of connecting wires, and metal conductivity. It is especially important in high power filters where energy loss or high dissipation can cause the filter to fail.