Network Analyser

A network analyser is a network system that measures both amplitude and phase of a microwave signal over a wide frequency range in a reasonably small time.We may mention here that the slotted line could measure the amplitude and phase of a microwave signal only at a signal frequency.Moreover to make measurements at broad band frequencies using slotted line is both time consuming and costly in terms of manpower.

In a network analyser,the basic principle of measurement is to generate an accurate reference signal and compare this with respect to the test signal whose amplitude and phase are to be measured.A block schematic of a network analyser is shown in fig.

A sweep signal generator feeds a power divider or splitter that converts into two signals the test signal and the reference signal.The device under test (DUT) is fed with the test signal and length equalizer (phase equalising line) takes in the reference signal.These test and reference signals are than converted to standard IF frequency by a harmonic frequency converter.The output of the harmonic frequency converter is then used to determine the amplitude and phase of the test signal.

The harmonic frequency converter consists of a phase locked loop shown in fig 2 The local oscillator tracks the reference signal frequency making error possible swept frequency measurement.The double mixer arrangement  first converts the memory RF test signal to a first IF of 10 MHz and then a second IF of 278 kHz.

A network analyser is quite useful for measurement of both passive as well as active microwave components or network parameters.It is used for measurement of both  impedance (reflection) and gain (transmission) characteristics of microwave devices.It uses a stimulus response   method for testing over the frequency range of interest.As these parameters are a function of frequency with complex variables(having both magnitude and phase), a swept frequency measurement becomes imminent.

A network analyser can be scalar or vector.A scalar network analyser provides only magnitude characteristics of microwave devices as a function of frequency. A vector network analyser can of measurement in both these methods is the same in that they compare the incident or input power with the transmitter or reflected waves depending upon the parameter to be measured.The radio of the relevant signals is used to determine the scattering matrix of the DUT.

As explained earlier,we   have a signal source,power splitter,receiver/detector and a processor/display components.The DUT transmits or reflects the stimulating or incident signal from the signal sources which are used in the measurement of magnitude and phase of individual components of the test signal.The receiver and display could be a harmonic generator and the amplitude phase meter which have already been described.However in place of power splitter,we might have as well used a directional coupler or a higher impedance probe.