Glossary of RF Terms

Below you will find a list of terms commonly used in radio frequency (RF) product manufacturing.

The reduction of RF power through a device, usually measured in decibels (dB), and given mathematically by Attenuation (dB) = 10 log (Pout/Pin).
Band Reject (Notch) Filter:
A filter that rejects one band of frequencies and passes both higher and lower frequencies.
Bandpass Filter:
A filter that passes one band of frequencies and rejects both higher and lower frequencies.
The width of the passband of a bandpass filter. This is usually expressed as the frequency difference between lower and upper relative 3dB points.
Center Frequency (Fc):
A measure of a central frequency between the upper and lower cutoff frequencies, usually taken as 3dB relative attenuation - an important parameter of bandpass and bandstop filters.
Ceramic Filter:
Filters that are designed and manufactured from ceramics that exhibit the piezoelectric effect.
Characteristic Impedance:
The characteristic impedance of a filter is usually taken as equal to L/C where L is the total series inductance in henries and C is the total shunt capacity in farads (Measured in ohms).
Cut-Off Frequency (Fco):
The upper passband edge in lowpass filters or the lower passband edge in highpass filter. Specified by relative attenuation level below the mid band insertion loss.
Decibel (dB):
A unit used to express the ratio between two power levels existing at two points. Measured as: dB = 10 LOG10 ( P1 / P2 ).
Energy losses in a filter due to resistive, dielectric or core losses.
The non-uniform, undesirable modification of signals which produce an undesirable end effect. Can be related to phase, amplitude, pulse, delay, etc.
Dynamic Range:
The range, from the minimum, which is at a level 3 dB above the amplifier's internally generated floor, to a maximum input signal level that a component can accept and amplify without distortion.
Elliptic Function:
A mathematical function used to yield the squarest possible amplitude filter response with a given number of circuit elements.
Filter Temperature Drift:
Filters have a tendency to drift with variations in temperature which is why the best filter designs utilize materials that maintain frequency drift. Exposure to cold temperature = an increase in frequency response, decrease in insertion loss.
Gain is the ratio of the power output to the power input of the amplifier in db.
Group Delay:
The group/time delay of the envelope of an amplitude modulated signal as it passes through a filter.
Highpass Filter:
A filter which passes high frequencies and rejects low frequencies.
Input Impedance:
The impedance measured at the input terminal of a filter when it is properly terminated at its output terminal.
Insertion Loss:
The loss of signal caused by a filter being inserted in a circuit. It has many different definitions and is usually measured in db.
The ratio (expressed in dB) of the power level at one port compared to the resulting power level of the output port.
Limiting Level:
This is the input power level when the output power is goes into compression and no longer becomes linear.
Linear Phase Filter:
A filter where the phase response is a linear function of frequency.
Load Impedance:
The impedance that normally must be connected to the output terminal of the filter in order to meet filter specifications. The filter will drive this load.
Lowpass Filter:
A filter which allows or passes low frequencies and rejects high frequencies.
Lumped Element Filter:
Lumped Element filters are produced with inductors and capacitors. The term Lumped is derived from the fact that the values (inductive or capacitive) are lumped into a component. These filters can be designed with a very small footprint even at lower frequencies.
The frequency range where a filter is designed to pass signals.
Passband Ripple:
Variations of attenuation 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. In normal networks, phase lag increases with frequency, producing a positive envelope delay (see envelope delay).
Relative Attenuation:
Attenuation measured with the point of minimum attenuation taken as zero dB, or (relative attenuation = attenuation insertion loss).
The ratio of the input signal compared to the output signal.
Return Loss:
The ratio, in dB, of maximum power sent down a transmission line to the power returned toward the source.
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.
Scattering Parameters:
Scattering parameters or S-parameters define the performance of several variables at various frequencies of linear electrical networks when undergoing various steady state stimuli. The following information must be defined when specifying a set of S-parameters:
  • The frequency.
  • The nominal characteristic impedance (often 50 Ω).
  • The allocation of port numbers.
  • Conditions which may affect the network.
Shape Factor:
An important parameter of all filters:
  • BANDPASS: SF = Attenuation Bandwidth / 3dB Bandwidth
  • BANDSTOP: SF = 3dB Bandwidth / Attenuation Bandwidth
  • LOWPASS: SF = Attenuation Frequency / Fco
  • HIGHPASS: SF = Fco / Attenuation Bandwidth
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.
The area of frequency where it is desirable to reject or attenuate all signals, as much as is practical.
Suspended Substrate Stripline (SSS):
These filters are based upon printed circuit board technology that can be designed for both broadband and narrowband filters; typical frequency range 500MHz to 26GHz. The wide range of possible impedance values makes this Stripline technology ultimately suitable for high pass and low pass designs that can combined be to form multiplexers and broadband band pass filtered.
Time Delay:
The amount of time it takes for certain signals to pass through a filter.
Transfer Function:
A filter's transfer function determines both its frequency and time domain characteristics that are commonly used in the analysis of systems such as single-input single-output filters.
Transient Response:
The response of a system to a change from a steady state. Examples would be the response of a lowpass filter to a very low frequency square wave or a sudden voltage rise or a sudden burst of signal within a passband.
Voltage Standing Wave Ratio (VSWR):
The ratio between the maximum and minimum of standing waves on a transmission time.
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