Network Analyzer Terminologies | Network Analyzer Glossary

This page of T&M World covers Scalar and Vector Network Analyzer Terminologies. This Network Analyzer Glossary include transmission coefficient, Insertion loss, gain, reflection coefficient, Return loss, S-parameters etc.

The figure-1 depicts network analyzer (8720D from keysight) used for testing 2 port DUT. As shown a1 is the incident wave, b2 is the transmitted wave, b1 and a2 is reflected waves.

transmission coefficient

It is a measure of how much power gets through from the DUT. It is expressed as follows:
Transmission coefficient = (2*ZL)/(ZL + Zin)

Insertion loss

It is the loss to the incident power when it travels through the portion of passive circuit or cable. It is measured in dB. It is difference of output power and input power fed to the DUT. It is negative value. It is expressed as follows:
Insertion Loss (dB) = Output Power - Input Power
Output power is less than input power.

Gain

It is the boost to the incident power when it travels through active circuit i.e. amplifier. It is measured in dB. It is difference of output power and input power. It is positive value and expressed as follows:
Gain (dB) = Output Power - Input Power
Output power is greater than input power.

reflection coefficient

Reflection coefficient is the measure of how much power gets reflected from DUT when coupled with network analyzer as shown at input and output ports.
b1 = Γin * a1
a2 = ΓL * b2

This is similar to V1 = Zin*I1 and V2= ZL*I2 impedance equations.

From the similarities between above equations,
Reflection Coefficient at Input = ( Zin - Z0 ) / (Zin + Z0)
Reflection Coefficient at Output = ( ZL - Z0 ) / (ZL + Z0)

Return loss

It is other way of representing reflection coefficient. It is expressed in dB as follows:
Return Loss = -20 Log[abs(Reflection coefficient)]

S-parameters

From the figure above, following S-parameter equations can be obtained:
b1= (S11 x a1) + (S12 x a2)
b2= (S21 x a1) + (S22 x a2)

Here S11 and S22 refers to input and output reflection coefficients consecutively. S12 is isolation and S21 is insertion loss.

For actual measurements of S-parameters output port is terminated with matched load. Hence ZL = Z0 and a2 = 0
In this condition, S11 = b1/a1 and S22 = b2/a1

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