TN Vector Network Analyzer Calibrations and Their Effect on Measurement Results. Bernie McDermott. May 2010

Transcription

1 TN Vector Network Analyzer Calibrations and Their Effect on Measurement Results May 2010 Bernie McDermott Lead Metrologist Calibration Products and Services

2 Measurement Examples and Technical Notes All measurement examples are performed using All Technical Notes are available at 2

3 What is a Vector Network Analyzer (VNA) VNA is an error corrected measurement system A VNA is capable of measuring: Behavior of linear electrical networks to steady state stimuli Especially useful in RF/Microwave frequencies Properties are expressed in Scattering (S) Parameters : scattering refers to the way in which the traveling currents and voltages in a transmission line are affected when they meet a discontinuity Voltage and current terms are replaced by S Parameter terms: Gain, return loss, VSWR, reflection coefficient S11, S12, S21, S22 3

4 What is a VNA Calibration The VNA calibration process requires measuring several types of known devices (conditions) These known devices establish the performance limits of the VNA A Vector Network Analyzer (VNA) Measurement is only as good as the Calibration 4

5 Types of VNA Calibrations Major Types of Calibration SOLT (Short/Open/Load/Thru) ecal (Electronic Calibration) TRL (Thru/Reflection/Line) / 5

6 SOLT Calibration Short/Open/Load/Thru (SOLT) Short 100% reflection of transmitted signal Open 100% reflection of transmitted signal -180 deg of short Load 100% absorption of transmitted signal Thru Theoretical 100% transmission of signal Pros Excellent for coax devices Cons 4 standards are required Non coax standards are difficult to build Sliding load calibrations can be time consuming 6

7 Sample Cal Kit File Sample Cal Kit values -Nominal values from specifications table 7

8 Test Set Data Files Sample Test Set values -Nominal values from specifications table Cal Kit and Test Set determine VNA performance limits 8

9 Broadband/Sliding Load SOLT Calibrations Sliding Load Calibration Kits Broadband Load Calibration Kits 9

10 ECAL Calibration ECAL Accurately characterized multi-port device (substrate) Built in memory for storage of characterization data Interface to allow communication with VNA/PC Excellent uncertainties & ease of use Pros Fast! minimal operator interaction during cal process Excellent Uncertainties ECAL can store custom user characterizations Cons VNA capability required May be restricted to VNA OEM s ECAL modules 10

11 ECAL Calibration 4 Port ECAL Module 2 Port ECAL Module 11

12 ECAL Calibrations Fast & Simple One connection Reasonable cost when compared to Sliding Load Kits 2 and 4 port modules available Improved performance reliability NIST is introducing an ECAL based VNA verification program 12

13 TRL Calibration TRL Pros Thru standard Reflection standard Line standard 1/4 wavelength Cons Excellent for non coax devices Two port calibration only Line requirements may be impractical at low frequencies 13

14 TRL Calibration TRL/LRL Calibration Kit 14

15 Broadband Load SOLT Calibrations Simple to Implement Kit Devices Male and female Opens Male and Female Shorts Male and Female Fixed Broadband Loads covering the entire frequency range Less expensive Cal Kit cost Reduced directivity 30dB or less to 18GHz 15

16 Sliding Load SOLT Calibrations More difficult to Implement Kit devices Male and female Opens Male and Female Shorts Male and Female limited range <2GHz Fixed Terminations Male and Female Sliding Termination >2GHz Two Step Load Calibration process Broadband Loads 5 positions minimum for Sliding Load Cal Kit can be expensive Excellent Directivity >45dB to 18GHz 16

17 Why various positions for Sliding Loads? Moving the load element allows the system to sample the load at various increments over the entire 360 degree phase range The best system directivity can be realized 5 unequally spaced positions minimum required for a reasonable representation of the perfect load Technote for implementing i Sliding Load Calibrations i tn_ pdf 17

18 One Port Measurement Example Making a one port measurement on a Power Sensor S11 Magnitude S11 Phase Circuit Configuration S21 S11 S22 S12 18

19 One Port Uncertainty Budget Cal Kit Error Terms Directivity Error Source Match Error Test Set Error Terms Reflection Tracking Directivity is the single largest contributor of uncertainty Type of Cal Kit determines Directivity 19

20 Sample Sensor SWR Uncertainty Budget 20

21 Evaluation of Results 1 Port S11 Magnitude of Agilent 8481A-H84 Sliding Load Calibration 21

22 Evaluation of Results 1 Port S11 Magnitude of Agilent 8481A-H84 Broadband Load Calibration 22

23 Evaluation of Results 1 Port S11 Phase of Agilent 8481A-H84 Broadband & Sliding Load Calibration 23

24 2 Port Measurement Example Making a 2 Port Measurement on a Power Splitter S11 Magnitude & Phase S22 Magnitude & Phase Real and Imaginary pairs for Source Match Full 2 Port Insertable Calibration i Adapter Substitution for Non-insertable connectors details - tn_ pdf Circuit Configuration S21 S11 S22 S12 24

25 A-B Tracking Uncertainty Budget A-B Tracking Cal Kit Error Terms Load Match Error Source Match Error Test Set Error Terms Transmission Tracking Error Isolation Error Note the Correlation Coefficient in the sample budget Why the straight line in our measurements? 25

26 Sample A-B Tracking Uncertainty Budget 26

27 Evaluation of Results A-B Tracking Uncertainty bars are for the Measured trace 27

28 Source Match Uncertainty Budget Source Match Cal Kit Error Terms Directivity Error Source Match Error Test Set Error Terms Reflection Tracking Error Power Splitter Technote for connection details tn_ pdf 28

29 Sample Source Match Uncertainty Budget 29

30 Evaluation of Results Source Match Port 2 Uncertainty bars are for standard magnitude data 30

31 Evaluation of Results Source Match Port 2 Uncertainty bars are for standard phase data 31

32 Evaluation of Results Source Match Port 2 Uncertainty bars are for standard data 32

33 Evaluation of Results Source Match Port Uncertainty bars are for standard phase data 33

34 Conclusions & Recommendations Changes to the reference plane will compromise most measurements Careful use of adapter substitution can yield usable results Stable, high quality cabling and adapters are a must! Protect your Cal Kits They are expensive They are essential Practice good connector techniques Clean carefully Rotate only the portions of the connector meant to do so 34

35