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Vector Network Analyzer1 Objectives To get acquainted with a vector network analyzer (VNA).To learn how to characterize networks at high frequencies.To learn how to measure scattering parameters.To explore the characteristics of microwave devices.2 Background Theory2.1 Transmission BasicsElectronic circuits which operate at high frequencies present some unique challenges for a propercharacterization. At high frequencies the wavelengths of operation become similar in dimension to thephysical properties of circuit elements. This results in circuit performance that is distributed in naturerather than describing the voltage and current at a specific circuit node. It is more appropriate todescribe how waves in a transmission medium respond to a component in their path. Networkanalyzers are a class of instruments that have been developed to characterize Radio Frequency (RF)components accurately and efficiently as a function of frequency. Network analysis is the process ofcreating a data model of the transfer and/or impedance characteristics of a linear network throughstimulus-response testing over the frequency range of interest. At frequencies above 1MHz, lumpedelements actually become circuit consisting of the basic elements plus parasitic elements like straycapacitance, lead inductance and unknown absorptive losses. Since parasitic depend on the individualdevice and its construction, they are almost hard to predict. Above 1 GHz, component geometries arecomparable to a signal wavelength, intensifying the variance in circuit behavior due to deviceconstruction. Network analysis is generally limited to the definition of linear networks. Since linearityconstrains networks stimulated by a sine wave to produce a sine-wave output, sine-wavetesting is an ideal method for characterizing magnitude and phase response as a function offrequency. This chapter discusses the key parameters used to characterize RF components,the types of network analyzer techniques used to make measurements and theconsiderations to be made in obtaining the most accurate results.2.2 Component CharacteristicsRF (frequencies less than 3 GHz) or microwave (frequencies in the 3 to 30 GHz range) energy can belikened to a light wave. Incident energy on a Device Under Test (DUT) (for example, a lens) is eitherreflected from or transmitted through the device (see Figure 1). By measuring the amplitude ratios andphase differences between the two waves it is possible to characterize the reflection (impedance) andtransmission (gain) characteristics of the device.2.3 Reflection and TransmissionThere are many terms used to describe these characteristics. Some use only magnitude information(scalar) and others include both magnitude and phase information (vector). If an incident voltage andcurrent waves on a device are denoted by Vincient, Iincident, the ratio of Vincient and Iincident is called thecharacteristic impedance Z0, and a device terminating a transmission system has an impedance calleda load impedance ZL, then the device characteristics can be defined by:Transmission terms:Reflection terms:Γ VreflectVincident VtransVincidentInsertion loss(dB) 20 log10 τ , Gain(dB) 20log10 τ Z L Z0, -1 Γ 1.Z L Z0Transmisson coefficient: τ where Γ : reflection coefficientVreflect, Vincident: reflected, incident waves on DUTZL, Z0: load, characteristic impedancesReturn loss (dB) 20 log10 Γ 20 log10 ρStanding wave ratio: SWR Vtrans: transmitted wave on DUT1 ρ1 ρ1

2.4 Scattering (S) ParametersMany component measurements are two-port networks, such as amplifiers, filters, cablesand antennas. These component characteristics are typically used to determine how aparticular device would contribute as a part of a more complex system. To provide amethod that models and analyzes a full two ports device in the RF environment, scatteringparameters (S parameters) are defined as follows:Sij biajak 0 , k jwhere aj denotes the wave entering the jth port, and bi denote the wave exiting the ith port.Once all S parameters are obtained, the relationship between the incoming and outgoingwaves can be written in terms of scattering matrix, [S], as:[b] [S][a], where [a],[b] are incoming, outgoing wave vectors, respectively.This is a characterization technique similar to a lower-frequency Z or Y modeling, exceptthat it uses incident, transmitted and reflected waves to characterize the input and outputports of a device as opposed to using voltage and current terms which are impossible tomeasure at high frequencies. The S parameter terms are related to other parameters withcertain conditions. For instance, S11 is equivalent to a device input reflection coefficientunder the condition the device has a perfect Z0 match on its output. S parametercharacterization of devices plays a key role in the ability of measuring, modeling anddesigning complex systems with multiple components. By definition, S parameters can bemeasured with a network analyzer.3 Vector Network AnalyzerA network analyzer measurement system can be set of four major parts: a signal source providing theincident signal, signal separation devices to separate the incident, reflected and transmitted signals, areceiver to convert the microwave signals to a lower Intermediate Frequency (IF) signals, a signalprocessor and a display section to process the IF signals and display detected information, as shown inFigure 1.Figure 1: The major elements of a network analyzer.The receiver performs the full S parameters which defined as:S11 reflected wave Areflected backward wave B; S 22 ; incident wave R1incident backward wave R22

S 21 transmitted wave Btransmitted backward wave A. ; S12 incident waveR1incident backward waveR23.1 Signal SourceThe signal source (RF or microwave) produces the incident signal used to stimulate the Device UnderTest (DUT). The DUT responds by reflecting part of the incident energy and transmitting theremaining part. By sweeping the frequency of the source the frequency response of the device can bedetermined. Frequency range, frequency stability, signal purity, output power level and level controlare factors which may affect the accuracy of a measurement. The source used for the networkanalyzer measurements, is a synthesizer, which characterized by stable amplitude frequency and ahigh frequency resolution (less than 100 Hz at microwave range).3.2 Signal SeparationThe next step in the measurement process is to separate the incident, reflected and transmitted signals.Once separated, their individual magnitude and/or phase differences can be measured. This can beaccomplished through the use of wideband directional couplers, bridges or power splitters. Adirectional coupler is a device that consists of two coupled transmission lines that are configured tocouple energy to an auxiliary port if it goes through the main port in one direction and not in theopposite direction. Directional couplers usually have relatively low loss in the mainline path and thuspresent little loss to the incident power. In a directional coupler structure (see Figure 1) the coupledarm samples a signal traveling in one direction only. The coupled signal is at a reduced level and therelative amount of reduced level is called the coupling factor. For instance a -20dB directional couplermeans that the coupled port power level is 20 dB below the input, which is equivalent to 1% of theincident power. The remaining 99% travel through the main arm. The other key characteristic of adirectional coupler is directivity. Directivity is defined as the difference between a signal detected inthe forward direction and a signal detected in the reverse direction (isolation between the forward andreverse signals). A typical directional coupler will work over several octaves with 30 dB directivity.The two-resistor power splitter (see Figure 1) is used to sample either the incident signal or thetransmitted signal. The input signal is split equally between the two arms, with the output signal(power) from each arm being 6dB below the input (power). A primary application of the powersplitter is for producing a measurement with a very good source match. If one side of the splitteroutput is taken to a reference detector and the other side goes through a DUT to a transmissiondetector, a ratio display of transmitted to incident has the effect of making the resistor in the powersplitter determine the equivalent source match of the measurement. Power splitters are verybroadband, have excellent frequency response and present a good match at the DUT input. Separationof the incident and reflected signals can be accomplished using either a dual directional coupler or asplitter.3.3 ReceiverThe receiver provides the means of converting and detecting the RF or microwave signals to a lowerIF or DC signals. There are basically two receiver techniques used in network analysis (see Figure 2).Figure 2: Fundamental and harmonics mixing receiverThe receivers are broadband tuned receivers that use either a fundamental mixing or harmonic mixinginput structure to convert an RF signal to a lower-frequency IF signal. The tuned receivers provide a3