Introduction to Vector Network Analyzer VNA Vector Network

Introduction to Vector Network Analyzer (VNA) Vector Network Analyzer 1

Vector Network Analyzer (VNA) • Introduction • Transmission line basics • Reflection and transmission parameters • S parameter definition Vector Network Analyzer 2

Introduction – Low Integration High Types of Devices Tested by VNA Duplexers Diplexers Filters Couplers Bridges Splitters, dividers Combiners Isolators Circulators Attenuators Adapters Opens, shorts, loads Delay lines Cables Transmission lines Waveguide Resonators Dielectrics R, L, C's Passive RFICs MMICs T/R modules Transceivers Receivers Tuners Converters Antennas Switches Multiplexers Mixers Samplers Multipliers Diodes Device type Vector Network Analyzer VCAs Amplifiers VCOs VTFs Oscillators Modulators VCAtten’s Transistors Active 3

Reasons for testing component • To verify specifications of building blocks in a complex RF systems such as amplifiers and filters in a transceiver • Measured hardware prototype compared to simulation model • To ensure component or circuit cause no distortion in the transmission of communications signals • linear: constant amplitude, linear phase / constant group delay versus frequency • nonlinear: harmonics, intermodulation, compression, AM-to-PM conversion • To ensure good matching for absorbing energy efficiently (such as good matching antenna) Vector Network Analyzer 4

Lightwave Analogy to RF Energy • Network analysis is concerned with the accurate measurement of the ratios of the reflected signal to the incident signal, and the transmitted signal to the incident signal. Incident Reflected Transmitted Lightwave DUT RF Vector Network Analyzer 5

Transmission Line Basics + I - Low frequencies l wavelengths >> wire length l current (I) travels down wires easily for efficient power transmission l measured voltage and current not dependent on position along wire High frequencies l wavelength » or << length of transmission medium l need transmission lines for efficient power transmission l matching to characteristic impedance (Zo) is very important for low reflection and maximum power transfer l measured envelope voltage dependent on position along line Vector Network Analyzer 6

Transmission Line Zo • • Zo determines relationship between voltage and current waves Zo is a function of physical dimensions and r Zo is usually a real impedance (e. g. 50 or 75 ohms) 1. 5 attenuation is lowest at 77 ohms 1. 4 1. 3 1. 2 normalized values • 50 ohm standard 1. 1 1. 0 0. 9 0. 8 power handling capacity peaks at 30 ohms 0. 7 0. 6 0. 5 10 20 30 40 50 60 70 80 90 100 characteristic impedance for coaxial airlines (ohms) Vector Network Analyzer 7

Power Transfer Efficiency RS RL For real impedances, maximum power transfer occurs when RL = RS Load Power (normalized) 1. 2 1 0. 8 0. 6 0. 4 0. 2 0 0 1 2 3 4 5 6 7 8 9 RL / R S Maximum power is transferred when RL = RS 10 For complex impedances, maximum power transfer occurs when ZL = ZS* (conjugate match) Vector Network Analyzer 8

Transmission Line Terminated with Zo Zs = Zo Zo = characteristic impedance of transmission line Zo Vinc Vrefl = 0! (all the incident power is absorbed in the load) For reflection, a transmission line terminated in Zo behaves like an infinitely long transmission line Vector Network Analyzer 9

Transmission Line Terminated with Short, Open Zs = Zo Vinc o Vrefl In-phase (0 ) for open, o out-of-phase (180 ) for short For reflection, a transmission line terminated in a short or open reflects all power back to source Vector Network Analyzer 10

Transmission Line Terminated with 25 W Zs = Zo ZL = 25 W Vinc Vrefl Standing wave pattern does not go to zero as with short or open Vector Network Analyzer 11

High Freq. Device Characterization Incident Transmitted R B Reflected A TRANSMISSION REFLECTION Reflected Incident = SWR S-Parameters S 11, S 22 Reflection Coefficient G, r A Transmitted R Incident Return Loss Impedance, Admittance R+j. X, G+j. B = B R Group Delay Gain / Loss S-Parameters S 21, S 12 Vector Network Analyzer Transmission Coefficient T, t Insertion Phase 12

Reflection Parameters Reflection Coefficient G Vreflected = = Vincident r Return loss, RL = -20 log (r), Emax Emin F = = G r ZL - ZO Z L + ZO VSWR = Emax Emin = 1+r 1 -r Full reflection (ZL = open, short) No reflection (ZL = Zo) 0 r 1 ¥ d. B RL 0 d. B 1 VSWR ¥ Vector Network Analyzer 13

Transmission Parameters V Incident DUT Transmission Coefficient = T = V Insertion Loss (d. B) = - 20 Log Gain (d. B) = 20 Log V Transmitted V Trans V Inc V V Transmitted V Incident Trans = - 20 log = tÐf t Inc = 20 log Vector Network Analyzer t 14

Smith Chart Review +j. X Polar plane 90 o 1. 0. 8. 6 0 +R ¥® + 180 o - . 4 o . 2 0 ¥ 0 -j. X -90 o Rectilinear impedance plane Constant X Constant R Z L = Zo Smith Chart maps rectilinear impedance plane onto polar plane G= 0 ZL = Z L = 0 (short) G= 1 ± 180 G =1 O (open) 0 O Smith chart Vector Network Analyzer 15

Characterizing Unknown Linear 2 -port Devices Using parameters (H, Y, Z, S) to characterize devices at low frequency: l l gives linear behavioral model of our device (or network) measure parameters (e. g. voltage and current) versus frequency under source and load conditions (e. g. short and open circuits) compute device parameters from measured data predict circuit performance under any source and load conditions H-parameters V 1 = h 11 I 1 + h 12 V 2 I 2 = h 21 I 1 + h 22 V 2 Y-parameters I 1 = y 11 V 1 + y 12 V 2 I 2 = y 21 V 1 + y 22 V 2 various Z-parameters V 1 = z 11 I 1 + z 12 I 2 V 2 = z 21 I 1 + z 22 I 2 h 11 = V 1 I 1 V 2=0 (requires short circuit) h 12 = V 1 V 2 I 1=0 (requires open circuit) Extending measurements of these parameters to high frequencies is not very practical ! Vector Network Analyzer 16

Why Use S-Parameters? l l l relatively easy to obtain at high frequencies n hard to measure total voltage & current at the device ports at high frequency n measure voltage traveling waves with a vector network analyzer n don't need shorts/opens which can cause active devices to oscillate or self-destruct relate to familiar measurements (gain, loss, reflection coefficient. . . ) can cascade S-parameters of multiple devices to predict system performance can compute H, Y, or Z parameters from S-parameters if desired can easily import and use S-parameter files in our electronic-simulation tools a 1 Incident S 11 Reflected b 1 S 21 Transmitted DUT b 2 S 22 Port 2 Reflected Port 1 a 2 Incident S 12 Transmitted b 1 = S 11 a 1 + S 12 a 2 b 2 = S 21 a 1 + S 22 a 2 Vector Network Analyzer 17

Measuring S-Parameters a 1 Forward S 21 = b 1 Incident a 2 = 0 b 1 = a 1 b 2 = a 1 a 2 = 0 S 22 = S 12 = a 1 = 0 Z 0 DUT Load b 1 Load DUT Reflected Transmitted b 2 Transmitted 21 Z 0 S 11 Reflected Incident S 11 = S Incident Transmitted S 12 Vector Network Analyzer Reflected Incident Transmitted Incident S 22 b 2 = a 2 b a 1 = 0 b 2 Reverse Reflected Incident 1 = a 2 a 1 = 0 a 2 18

Equating S-Parameters with Common Measurement Terms S 11 = forward reflection coefficient (input match) S 22 = reverse reflection coefficient (output match) S 21 = forward transmission coefficient (gain or loss) S 12 = reverse transmission coefficient (isolation) Remember, S-parameters are inherently complex, linear quantities. They are expressed as real and imaginary or magnitude and phase pairs However, we often express them in a log magnitude format Vector Network Analyzer 19

Network Analyzers Vs Spectrum Analyzers. Measures known signal Amplitud e Amplitude Ratio 8563 A Frequency Network analyzers: l measure components, devices, circuits, sub-assemblies l contain source and receiver l display ratioed amplitude and phase (frequency or power sweeps) l offer advanced error correction SPECTRUM ANALYZER GHz Measures unknown signals Frequency Spectrum analyzers: l measure signal amplitude characteristics carrier level, sidebands, harmonics. . . ) l can demodulate (& measure) complex signals l are receivers only (single channel) l can be used for scalar component test (no phase) with tracking gen. or ext. source(s) Vector Network Analyzer 9 k. Hz - 26. 5 20

Network Analyzer Hardware Generalized Network Analyzer Block Diagram Incident Transmitted DUT SOURCE Reflected SIGNAL SEPARATION INCIDENT (R) REFLECTED (A) TRANSMITTED (B) RECEIVER / DETECTOR PROCESSOR / DISPLAY Vector Network Analyzer 21

Types of Network Analyzer Scalar Vector • Magnitude only • Phase and Magnitude • Broadband Detector with higher noise floor • Tuned Detector with lower noise floor • Lower Price • Higher Price • Normalization – Less Accurate • Complete Error Correction – More Accurate • Measures RL, SWR, Gain/Loss • Measures all Vector Network Analyzer 22

Basic Antenna Parameters 23

Antenna Parameters • Introduction • Antenna - Reflection Coefficient / Return Loss ? • Bandwidth • VSWR • Impedance Matching • Other Parameters? in class • Demo and Hands On Antenna Parameters 24

Introduction Antenna Parameters 25

Reflection Coefficient If 50 % of the signal is absorbed by the antenna and 50 % is reflected back, we say that the Reflection Coefficient. is 3 d. B. A very good antenna might have a value of -10 d. B (90 % absorbed & 10 % reflected). Antenna Parameters 26

Bandwidth Typically, bandwidth is measured by looking at SWR, i. e. , by finding the frequency range over which the SWR is less than 2. Bandwidth also can be measured by looking at the frequency range where reflection coefficient value dropped below than -10 d. B. Antenna Parameters 27

VSWR is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i. e. , that which corresponds to a perfect impedance match, is unity. The result is presented as a figure describing the power absorption of the antenna. A value of 2. 0: 1 VSWR, which is equal to 90 % power absorption, is considered very good for a small antenna. Antenna Parameters 28

Impedance An ideal antenna solution has an impedance of 50 ohm all the way from the transceiver to the antenna, to get the best possible impedance match between transceiver, transmission line and antenna. Since ideal conditions do not exist in reality, the impedance in the antenna interface often must be compensated by means of a matching network, i. e. a net built with inductive and/or capacitive components. Antenna Parameters 29

Hands On 1 - Antenna Characteristic Measurements You will be given few types of antennas, please measure the following : • Reflection Coefficient, S 11 • VSWR • 10 d. B bandwidth and % bandwidth • Impedance at resonance Antenna Parameters 30

Hands On 2 – Antenna Measurement Environment Display the S 11 of your antenna between your frequency range of interest. Place the marker at the minimum. Observe the trace when the following objects are brought close to it from different directions: • metallic object (e. g. steel rule, metal rod) • human hand • insulator (e. g. book, plastic) Observe the trace when the antenna is rotated by 90 degrees and when it is moved around relative to surrounding. Antenna Parameters 31