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Glossary Of RF & Microwave Terms

Amplifier

An active device where a signal is fed to the input of the amplifier and an amplified, higher power level, signal of the same frequency as the input signal, exits the amplifier. A measure of how much the signal was amplified is called the amplifier gain.

Amplitude Balance

The maximum amplitude difference between the output ports, in dB, of a power divider or 90 degree hybrid coupler across the range of frequency of operation.

Attenuation

Loss of signal amplitude or power, measured in dB.

Bandpass Filter

A filter that passes a band of frequencies and rejects higher and lower frequencies outside this band.

Bandwidth

The width of the pass band of a bandpass filter. It is bounded by the upper and lower frequencies of the pass band of the band pass filter.

Center Frequency

It is the center frequency (mid point) of the pass band of a band pass filter.

Coupling

Is a measure of the ratio, in dB, of the input power of a coupler to power picked up by the coupled port.

Cutoff Frequency

The upper frequency edge of the pass band of low pass filters.

Dual Directional Coupler

A passive 4 port device where the power enters through the main line from port 1 to port 2 and a fraction of this power is picked up by the coupled port, port 3. A very small fraction is also picked up by port 4, in case of matched load, the smaller this amount of power picked up by port 4, is a measure of a good coupler directivity. If port 2 of the directional coupler is not terminated with the characteristic impedance of the coupler, mismatch load condition, some power will be reflected back to port 1, and is picked up by the isolated port, port 4.

Directivity

The directivity of a directional coupler is a measure of how well a directional coupler isolates the coupled and isolated ports. The higher the directivity, the more accurate the coupler measurements will be.

Gain

Gain is the ratio of the output power to the input power of the amplifier in dB. Gain = 20*log(S21).

High pass Filter

A filter which passes high frequencies and rejects low frequencies.

Insertion Loss

It is the signal loss is dB as the signal passes from the input to the output ports through a passive circuit component like a filter, power divider/combiner, directional coupler, or a hybrid coupler. It is equal to 10 * LOG10 (Output Power/Incident Power) dB.

Isolation

It measures the amount of isolation, power leaking, from one port to another. The higher the number in dB means a better isolation between ports. For example, in a power combiner, this is a measure of how much power can travel between the input ports of the combiner.

Linear Phase Filter

A filter where the phase response versus frequency is a straight line. This filter displays a constant delay in its passband.

Low pass Filter

A filter which passes low frequencies and rejects high frequencies.

Microstrip

A transmission line where the current flows in the conductor located above a dielectric layer. The ground plane is located directly below the dielectric layer. Microstrip transmission lines have even and odd wave modes of propagation travelling with different phase velocity due to asymmetry.

1-dB Compression Point

It is amount of output power from an amplifier where in the output power versus input power linear gain curve, the gain is decreased by 1 dB from the linear gain asymptote.

Passband

The frequency range of the passband of a filter.

Phase Balance

It is the measure of maximum phase shift between the outputs of a power divider or a hybrid coupler relative to input port, across the frequency of operation.

Phase Velocity

This is the velocity at which the phase of any one frequency component of the wave travels.

Power Combiner

A passive RF/Microwave device that has multiple inputs and a single output. The input signals enter the input ports and are combined together into a single output at the output port. Some signal loss occurs in the process due to the insertion loss of the combiner.

Power Divider

A passive RF/Microwave device that has a single input and multiple outputs. The input signal enters the input ports and is divided into equal signals at the multiple output ports. Some signal loss occurs in the process due to the insertion loss of the divider. A power divider can be used a power combiner and vice versa, by reversing the input and output ports.

Radio Frequency, RF

It is frequency range of electromagnetic wave transmission, this is normally considered between high HF frequency range, 30 MHz, to 1GHz. Above that range is considered Microwave frequency.

Return Loss

Is the ratio in dB, between the incoming signal to the reflected signal as it enters a component. It is equal to 10 * LOG10(Reflected Power/Incident Power)

Ripple

Generally referring to the wavelike variations in the amplitude response of a filter. Chebyshev and elliptic function filters ideally have equi-ripple characteristics, which means that the difference in peaks and valleys of the amplitude response in the passband are always the same. Butterworth, has no ripple and is maximally flat. Ripple is usually measured in dB.

Stop band

The area of frequency where it is desirable to reject or attenuate all signals as much as possible.

Stripline

A transmission line where the current flows in the conductor centered between two dielectrics on top and bottom of the conductor. Ground is above and below the two dielectrics. Stripline transmission line has even and odd wave modes of propagation travelling with the same phase velocity due to symmetry.

3-dB Compression Point

It is amount of output power from an amplifier where in the Pin versus Pout linear gain curve, the linear gain asymptote is decreased by 3 dB.

3-dB Hybrid Coupler

It is a passive four-port device that is used either to equally split an input signal with a resultant 90° phase shift between outputs or to combine two 90-degree out-of-phase signals in-phase while maintaining high isolation between them

Time Delay

Is the delay between the time the signal enters the input port and exits the output port of a flter, or other microwave components.

Uni Directional Coupler

A passive 3 port device where the power enters through the main line from port 1 to port 2 and a fraction of this power is picked up by the coupled port, port 3.

Voltage Standing Wave Ratio, VSWR

The ratio between the max and minimum of standing waves on a transmission line.