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SCALAR NETWORK ANALYSIS: an ATTRACTIVE and Cosl-EFFECTIVE TECHNIQUE for TESTING MICROWAVE COMPONENTS

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SCALAR NETWORK ANALYSIS: an ATTRACTIVE and Cosl-EFFECTIVE TECHNIQUE for TESTING MICROWAVE COMPONENTS SCALAR NETWORK ANALYSIS: AN ATTRACTIVE AND COSl-EFFECTIVE TECHNIQUE FOR TESTING MICROWAVE COMPONENTS Hugo Vitian Network Measurements Division 1400 Fountain Grove Parkway Santa Rosa, CA 95401 RF ~ Microwave Measurement Symposium and Exhibition Flio- HEWLETT ~~ PACKARD www.HPARCHIVE.com Scalar Network Analysis: an Attractive and Cost-Effective Technique for Testing Microwave Components ABSTRACT Scalar network characterization is an economic alternative to vector network analysis; in particular, in a productive environment where ease-of-use, throughput and low cost are key considerations. However, scalar measurements are not restricted to frequency response measurements on linear networks. Scalar network analyzers are very powerful analysis tools to characterize amplifiers, mixers, modulators, and other components as a function of signal level or control parameter, as well as for cable testing in frequency- and distance-domains. This presentation describes the fundamental measurement concepts and discusses some specific examples. Author: Hugo Vifian, Engineering Section Manager for economy vector network analyzers, HP Network Measurements Division, Santa Rosa, CA. Diplom-Ingenieur and PhD from Swiss Federal Institute of Technology, (ETH). With HP since 1969, projects include HP 8755 frequency response test set, HP 8505 RF vector network analyzer, HP 8756 and 8757 scalar network analyzers. 2 www.HPARCHIVE.com INTRODUCTION TO SCALAR NETWORK MEASUREMENTS Introduction: Even though scalar microwave measurements have been made ever since high frequencies were discovered, they were not really addressed, as such, but INTRODUCTION TO SCALAR rather called power measurements or NETWORK MEASUREMENTS VSWR measurements, etc. It was not until modern network analysis with vector error correction was intro­ duced that the rather obvious term magnitude or scalar measurements became more popular. 2617 The objectives of this presentation are: OBJECTIVES To explain the difference between Scalar and Vector Network Analyzer • Introduce Scalar Measurement concepts. Concepts Introduce a variety of network • Explain Network Characterization characterization techniques in the Technique in the Microwave microwave frequency range. Frequency Range Discuss practical scalar measure­ • Discuss Practical Measurement ment applications. Applications 2618 1. SCALAR MEASUREMENT CONCEPTS SCALAR The fundamental difference between Vector and Scalar Network Analyzers MEASUREMENT CONCEPTS stems from different receiver con­ cepts applied. All the other sys­ tems characteristics are a direct • Scalar and Vector Network Analyzer or indirect result of this. Comparison 2622 3 www.HPARCHIVE.com 1.1 SCALAR AND VECTOR NETWORK ANALYZER COMPARISON The Scalar Analyzer Receiver mea­ SCALAR AND VECTOR NETWORK sures the magni tude of the micro­ ANALYZER COMPARISON wave test signal to be measured by converting it into a low frequency SCALAR VECTOR AC or DC signal by means of a ther­ Receiver • Homodyne • Heterodyne mo electrical effect or a diode Concept Mostly Mostly homodyne process. (1) Single Converter Multi-Converter (MW-DC) (MW-IF1-IF2-DC) Front End • Diode Detector • Fundamental Most Vector Network Analyzers, on Frequency or or the other hand, are heterodyne Converter Thermo Couple Harmonic Mixer receivers with multiple frequency converters based on fundamental or harmonic mixing with a local oscil­ lator signal. (2) 2639 Subsequent filtering allows reduc­ ing the processing bandwidth and VECTOR NETWORK ANALYZER BLOCK DIAGRAMS therefore improving the signal-to­ noise ratio which ultimately leads r Source--., r-------------,Test Set and Receiver to improved sensitivity and dynamic I I Front End range. Furthermore, the narrowband : rv}-!----!..---l+! T I receiver characteristic suppresses Signal Processing harmonic and spurious signals at and Display other frequencies. :AI I The Vector Analyzer then measures I I the down converted microwave signal I characteristics in terms of ampli­ I I tude and phase at a low intermedi­ L __.J Swept ate frequency (IF). Source 2642 In contrast, Scalar Network An­ alyzers with diode detectors as SCALAR NETWORK ANALYZER BLOCK DIAGRAM frequency (down) converters are Source Test Set and Receiver inherently broadband receivers. (4) r - -, r - -0;;';.;'t.:-o,- ----- --, I 0 I Front End This gives them the capability to I rv~-!-_----;+f ¥ ~I----+::J----~ measure any signal in the entire I II I L __:J Signa' frequency band instantaneously. A ': L---------Kl-----+f ~;::,c.ss'ng feature which is important for A Display characterizing frequency translat­ I ing components (mixers), or when I I high frequency agility is required. I The trade offs are: a) less sen­ I sitivity mainly because of the IL __.J L ~ Swept Broadband broadband noise and lower conver­ Source Homodyne sion efficiency of the diode con­ Receiver verter and b) limited rejection of undesirable signal components. (5) 2643 4 www.HPARCHIVE.com A simple way to model such a con­ figuration is a mixer multiplier DIODE DETECTOR AS FREQUENCY CONVERTER circuit with both RF and La inputs connected together. At high RF power levels, the signal at the La Log Vo input is sufficient to convert the RF signal proportionally (with a OutputVoltage ! nominal conversion loss) to the Pro ortional to Power output of the mixer. This operating area is called linear (conversion) for the diode Log to Power !IL-+--+--+-+--+-+--+--+ detector and an incremental change PIN = V,Z Zo at the input generates a propor­ -10 dBm tional change in (detected) output signal. As the RF signal is reduced, the La component in the model drops and 2659 the conversion efficiency is re­ duced until the RF and La only con­ tribute in a mUltiplicative way to SCALAR AND VECTOR NETWORK the output signal. Therefore at ANALYZER COMPARISON low signal levels, an incremental RF input signal drop translates SCALAR VECTOR into twice the drop of detected Basic • Broad Band • Narrow Band signal at the output. This diode Characteristics • Very Frequency • High operating area is called square law Agile Selectivity (conversion) since the detected • Lower • High Sensitivity signal is proportional to the prod­ Sensitivity (~-100 dBm, (~-60 dBm) Wide Dynamic uct of RF and La which is RF­ Range) squared. • Self-Convolving • Synchronously When used as a Scalar Analyzer Mixer Without Tuned Local front end, the diode square law Local Oscillator Oscillator characteristic is extended by ap­ • Lower Cost • Higher Cost & Performance & Performance propriate signal processing over the entire operating range (dynamic range) in order to maintain a cali­ 2640 brated display. Having the signal vectors available in complex form (amplitude and SCALAR AND VECTOR NETWORK phase) allows the application ANALYZER COMPARISON sophisticated error correction techniques which dramatically re­ SCALAR VECTOR duces measurement ambiguities. (3) Fundamental • Signal • Signal Measurements Magnitude Amplitude However, the improved performance or and of a Vector Network Analyzer does Magnitude Phase Squared not come for free and the hardware (Power) cost of the heterodyne structure Accuracy • Scalar Error • Elaborate with the synchronously tuned local Correction Vector Error oscillator and the additional pro­ Only Correction cessing capability adds substan­ tially to the cost of this measure­ ment system. 2641 5 www.HPARCHIVE.com 1.2 FUNDAMENTAL SCALAR MEASUREMENTS Let's quickly review the two most common scalar measurement concepts: FUNDAMENTAL SCALAR MEASUREMENTS 1. Transmission Transmission (vs. Frequency) 2. Reflection as a function of frequency. (6)(7) T DUT A TRANSMISSION: 2647 The swept source provides a test signal over the frequency range of interest which is applied to the device under test (DUT) and the (T) Gain/Loss ; Transmitted transmitted signal is measured Incident by the Scalar Analyzer. Since we are usually interested in Transmis­ sion Gain or Loss, the incedent signal (I) is measured and compared with the transmitted signal and the signal ratio is computed as shown in the graph below. Frequency As an example, the transfer func­ tion or gain of a band-pass filter is displayed as a function of frequency. 2648 REFLECTION: In a reflection measurement, the Reflection Measurements (vs. Frequency) incident and reflected signal are Device Test Port separated by a directional device Directional Port I Device I (bridge coupler, etc.) and measured Source either separately, or as a super­ Slotted Line Incident Coupler ~~~=":""--I'" OUT position in form of a standing wave Bridge pattern (slotted line). 2649 6 www.HPARCHIVE.com The reflection coefficient, gamma is then computed by either forming the ratio of reflected (R) over r'M (~, V5WR, RL, 5,,) incident (I) signal or by convert­ ing the standing wave ratio as shown later. Frequency r, * r'M Measurement Errors are a Function of: Directivity (D), Test Port Match (rTP) and Device Match (r,) 2658 ACCURACY CONSIDERATIONS: INF -5 Unfortunately, a perfect separation r TP -4 of incident and reflected signal is not possible. A part of the inci­ III -3 "U dent signal, attenuated by the di­ - 2 h,,',5~~~i-------:7" rectivity (D) of the directional o0:: device will be present also. Fur­ 0:: thermore, since the directional t5 0 device does not have a perfect port L: match, some of the reflected signal ~ +2 h..e-~·-~-~~ H (gamma-L) gets re-reflected ~ +3 (gamma-TP). Mainly, these two er­ L: +4 ror terms cause the measured reflection coefficient (gamma-LM) +50 40 of the device under test to be dif­ ferent than the actual term 2650 (gamma-L). The chart and the table show how we can estimate the measurement ambi­ Examples: D rTP Error guity caused by the directivity (D) dB VSWR Ambiguity and the test port match (gamma-TP) 30 dB 40 * 30 +3.5 dB as a function of the value of -2.5 reflection coefficient to be mea­ 20 dB 40 * 20 + 1.0 dB sured (gamma-L). CD - .9 10 dB 40 ~2.0 10 + 1.3 dB CD -1.2 For high returnloss measurements 4 dB 30 4 +2.5 dB (gamma-L >20dB), the directivity CD -2.0 (D) dominates the ambiguity CD 4 dB 40 ~1.1 4 ±.4 dB and for lower returnloss (gamma-L (10dB),the test port match For High (~20 dB) Return Loss Measurements becomes the major contributor.
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