Network Analysis Basics

Transcription

1 Adolfo Del Solar Application Engineer MD1010 Network B2B

2 Agenda Overview What Measurements do we make? Network Analyzer Hardware Error Models and Calibration Example Measurements

3 What is a Network Analyzer? Vector network analyzers (VNAs) Are stimulus-response test systems Characterize forward and reverse reflection and transmission responses (S-parameters) of RF and microwave components Quantify linear magnitude and phase Are very fast for swept measurements Provide the highest level of measurement accuracy Reflection Transmission RF Source DUT S 21 S 11 S 22 S 12 Magnitude Phase R1 LO R2 A B Test port 1 Test port 2

4 Lightwave Analogy to RF Energy Incident Transmitted Reflected Lightwave DUT RF

5 Why do we need to test components? Verify specifications of building blocks for more complex RF systems Ensure distortionless transmission of communications signals linear: constant amplitude, linear phase / constant group delay nonlinear: harmonics, intermodulation, compression, AM-to-PM conversion Ensure good match when absorbing power (e.g., an antenna) KPWRFM97

6 1. Complete characterization of linear networks 2. Complex impedance needed to design matching circuits 3. Complex values needed for device modeling High-frequency transistor model 4. Time-domain characterization Mag Time 5. Vector-error correction Error Measured Actual

7 Agenda Overview What Measurements do we make? Network Analyzer Hardware Error Models and Calibration Example Measurements

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

9 normalized values 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) attenuation is lowest at 77 ohms ohm standard power handling capacity peaks at 30 ohms characteristic impedance for coaxial airlines (ohms)

10 Load Power (normalized) Power Transfer Efficiency RS RL For complex impedances, maximum power transfer occurs when ZL = ZS* (conjugate match) RS +jx jx RL RL / RS Maximum power is transferred when RL = RS

11 Transmission-line terminated with Zo Zs = Zo Zo = characteristic impedance transmission line of Zo V inc 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

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

13 Transmission-line Terminated with 25 W Zs = Zo ZL = 25 W V inc Vrefl Standing wave pattern does not go to zero as with short or open

14 High-Frequency Device Characterization Incident R Reflected A REFLECTION Transmitted B TRANSMISSION Reflected Incident = A R Transmitted Incident = B R SWR S-Parameters S 11, S 22 Reflection Coefficient G, r Return Loss Impedance, Admittance R+jX, G+jB Gain / Loss S-Parameters S 21, S 12 Transmission Coefficient T,t Insertion Phase Group Delay

15 Reflection Parameters Reflection Coefficient G = V reflected V incident = r Return loss = -20 log(r), r Emax Emin F = G = Z L - Z O Z L + ZO Voltage Standing Wave Ratio VSWR = Emax Emin = 1 + r 1 - r No reflection (ZL = Zo) Full reflection (ZL = open, short) 0 r 1 db RL 0 db 1 VSWR

16 Smith Chart Review +jx Polar plane 90 o R o o -jx 0 Rectilinear impedance plane -90 o Smith Chart maps rectilinear impedance plane onto polar plane Z L = Zo G = 0 (short Z = 0 ) Z L = G L = 1 ±180 O G Constant X Constant R = 1 0 O (open) Smith chart

17 Transmission Parameters V Incident DUT V Transmitted Transmission Coefficient = T = V Transmitted V Incident = t Insertion Loss (db) = - 20 Log V Trans V Inc = - 20 log t Gain (db) = 20 Log V Trans V Inc = 20 log t

18 Why Use S-Parameters? relatively easy to obtain at high frequencies measure voltage traveling waves with a vector network analyzer don't need shorts/opens which can cause active devices to oscillate or selfdestruct 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 Incident a 1 S11 Reflected b 1 Transmitted S21 DUT Port 1 Port 2 S12 b1 = S11a 1 + S12 a2 b 2 = S21 a1 + S22 a 2 Transmitted S22 Reflected a2 Incident b2

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

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21 Agenda Overview What Measurements do we make? Network Analyzer Hardware Error Models and Calibration Example Measurements

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

23 Source Supplies stimulus for system Swept frequency or power Traditionally NAs used separate source Most Agilent analyzers sold today have integrated, synthesized sources

24 Signal Separation Incident DUT Transmitted SOURCE Reflected SIGNAL SEPARATION REFLECTED TRANSMITTED measure incident signal for reference separate incident and reflected signals INCIDENT (R) (A) (B) RECEIVER / DETECTOR PROCESSOR / DISPLAY splitter directional coupler

25 Directivity Directivity is a measure of how well a coupler can separate signals moving in opposite directions (undesired leakage signal) (desired reflected signal) Test port Directional Coupler

26 Detector types Incident DUT Transmitted SOURCE Reflected SIGNAL SEPARATION Diode Scalar broadband (no phase information) INCIDENT (R) REFLECTED (A) TRANSMITTED (B) RECEIVER / DETECTOR RF DC AC PROCESSOR / DISPLAY Tuned Receiver RF IF = F LO F RF ADC / DSP Vector (magnitude and phase) IF Filter LO

27 T/R Versus S-Parameter Test Sets Transmission/Reflection Test Set S-Parameter Test Set Source Source Transfer switch A R B A R 1 B Port 1 Port 2 R 2 Port 1 Port 2 Fwd DUT Fwd DUT Rev RF always comes out port 1 port 2 is always receiver response, one-port cal available RF comes out port 1 or port 2 forward and reverse measurements two-port calibration possible

28 Achieving measurement flexibility Channel Trace Trace Trace Trace Trace Channel Sweep type Frequency Power IF bandwidth # of points Trigger state Averaging Calibration RF Performance Easy to learn and use Powerful measurement config. Advanced connectivity Flexible automation choices Rules of 4: 32 independent measurement channels 26 windows to view traces and channels 8 active and 8 memory traces per window 4-parameter display needs only one channel Global Trigger source Port extensions RF power on/off Trace Parameter Format Scale Markers Trace math Electrical delay Phase offset Smoothing Limit tests Time-domain transform Window Trace (CH1) Window Trace (CH3) Window Trace (CH1) Trace (CH2) Window Trace (CH2) Trace (CH4)

29 Agenda Overview What Measurements do we make? Network Analyzer Hardware Error Models and Calibration Example Measurements

30 The Need for Calibration Why do we have to calibrate? It is impossible to make perfect hardware It would be extremely difficult and expensive to make hardware good enough to entirely eliminate the need for error correction How do we get accuracy? With vector-error-corrected calibration Not the same as the yearly instrument calibration What does calibration do for us? Removes the largest contributor to measurement uncertainty: systematic errors Provides best picture of true performance of DUT Systematic error

31 Systematic Measurement Errors R Directivity A Crosstalk B DUT Frequency response reflection tracking (A/R) transmission tracking (B/R) Source Mismatch Load Mismatch Six forward and six reverse error terms yields 12 error terms for two-port devices

32 Types of Error Correction response (normalization) simple to perform only corrects for tracking errors stores reference trace in memory, then does data divided by memory thru vector requires more standards requires an analyzer that can measure phase accounts for all major sources of systematic error SHORT S 11a OPEN thru S 11m LOAD

33 Systematic Measurement Errors Measured Vector-error correction Is a process for characterizing systematic error terms Measures known electrical standards Removes effects of error terms from subsequent measurements Electrical standards Can be mechanical or electronic Are often an open, short, load, and thru, but can be arbitrary impedances as well Errors Actual

34 Reflection: One-Port Model RF in Ideal RF in Error Adapter 1 E D = Directivity S11 A E D E S S11A E RT E S = Reflection tracking = Source Match S11 M S11 M S11 M = Measured E RT S11 A = Actual To solve for error terms, we measure 3 standards to generate 3 equations and 3 unknowns S11M = ED + ERT S11A 1 - ES S11A Assumes good termination at port two if testing two-port devices If using port 2 of NA and DUT reverse isolation is low (e.g., filter passband): assumption of good termination is not valid two-port error correction yields better results

35 E D E S E RT E D' E S' Two-Port Error Correction a 1 ERT' b 1 E D E S E RT = fwd directivity = fwd source match = fwd reflection tracking = rev directivity = rev source match = rev reflection tracking Forward model Port 1 EX Port 2 S 11 A E L E TT E X E L' E TT' E X' S 21 A S 12 A S 22 A Each actual S-parameter is a function of all four measured S-parameters Analyzer must make forward and reverse sweep to update any one S-parameter Luckily, you don't need to know these equations to use network analyzers!!! E TT E L a 2 b 2 E TT' S11m - ED = fwd load match S11 a = fwd transmission tracking S m E D' = fwd isolation Port 1 Port 2 S 11 A = rev load match S m - E X = rev transmission tracking = rev isolation S21 a a 1 b 1 E L' Reverse model S 21 A S 12 A E X' E RT' S 22 ES' E D' A S m E D S E E RT E S E m E X S ' 21-12m - E X ' ( )( ' ) - L ( )( ) RT ' E TT E TT ' S E m E D S E S E RT E S E L E m E X S m E ' X ' ( )( ' ) ' L ( )( ) RT ' E TT E TT ' 21 S22m - E D ' ( )( 1 ( E E TT E S '-E L )) RT ' S m E D S E m E D E S ' S ' ( )( E RT E S ' ) - E L ' E ( 21 m - E X S )( 12 m - E X ) RT ' L E TT E TT ' 12 S12 a S22a S - E ' S - E ( m X )( 1 11m D ( E ' )) E TT ' E S - E L RT S ' ( m E D S ' E )( m E D S ' ) ' ( )( ) E S E RT E RT ' S E L E m E X S m E X - L E TT E TT ' S22m - ' ( E D S )( 11 m - E D S ' E ) ' ( )( ) E RT ' E S E 21 m - E X S 12 m - E X 1 - L RT E TT E TT ' S ( m E D S E m E D ' S E S E RT E S E L E m E X S m E X ' )( ' ) ' L ( 21 - )( ) RT ' E TT E TT ' b 2 a 2

36 Return Loss (db) VSWR Before and After One-Port Calibration 0 20 data before 1-port calibration data after 1-port calibration MHz

37 Errors and calibration standards UNCORRECTED RESPONSE 1-PORT FULL 2-PORT DUT Convenient Generally not accurate No errors removed thru DUT Easy to perform Use when highest accuracy is not required Removes frequency response error ENHANCED-RESPONSE Combines response and 1-port Corrects source match for transmission measurements SHORT OPEN LOAD DUT For reflection measurements Need good termination for high accuracy with two-port devices Removes these errors: Directivity Source match Reflection tracking SHORT OPEN LOAD thru DUT SHORT OPEN LOAD Highest accuracy Removes these errors: Directivity Source, load match Reflection tracking Transmission tracking Crosstalk

38 Agenda Overview What Measurements do we make? Network Analyzer Hardware Error Models and Calibration Example Measurements

39 Thank you! Adolfo Del Solar Application Engineer MD1010 Network B2B