Evaluating SOLT calibration performance at RF using an LA Techniques VNA

Presentation by Nick Ridler 1

and Nils Nazoa 2

1National Physical Laboratory, UK2LA Techniques Ltd, UK

25th ANAMET meeting, 13 March 2006, NPL

• LA Techniques VNA

• SOLT with a characterised load

• Ripple plots – residual directivity and test port match

• Uncertainty/Best Measurement Capability (BMC)

• Conclusions

Outline

LA19-13-01 - a new type of VNA:

• PC-driven

• Built-in transmission & reflection test set (3 MHz to 3 GHz)

• Small size/portable

• Low cost

• Precision performance from non-precision cal kits (!)

www.www.latechniqueslatechniques.com.com

LA Techniques VNA

LA19-13-01 VNA

SOLT calibration

• Usually assume ‘perfect’ standards:

Short with |Γ| = 1

Open with |Γ| = 1

Load with |Γ| = 0

• However, for non-precision load, |Γ| ≠ 0

• This causes significant post-calibration errors (directivity, test port match, etc)

Solution:

• Measure complex voltage reflection coefficient (VRC) for a candidate cal load using a ‘reference calibration’reference calibration’

• NPL’s ‘national standard’ - Primary IMpedance Measurement System (PIMMS) - provides the ‘reference calibration’reference calibration’

• PIMMS uses TRL with an air line to provide traceability to SI

• Impedance renormalisation is used to give (50 + j0) ohms as the reference impedance (correcting for LF skin depth effects, etc)

SOLT calibration

Characterised SOLT

For each candidate cal load:

• Fit a curve to:measured complex VRC datameasured DC ‘VRC’ (i.e. resistance)

• Use fit to provide cal load complex VRC values during calibration at all required frequencies

(This procedure follows the method used for NPL’s national standard RF System.)

Residual errors - ripple plots

Let’s look at three different cases

Case 1 – SOLT assuming ‘perfect’ cal load

Case 2 – SOLT using ‘characterised’ cal load

Case 3 – SOLT using ‘characterised’ 2.0 VSWR mismatch (as the cal load)

{Use a 150 mm long ripple line – 2.92 mm connectors throughout}

Case 1 – SOLT assuming ‘perfect’ cal load

Residual directivity ripple plot DD ≈≈ 0.015 0.015 ≡≡ --36 dB36 dB

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 500 1000 1500 2000 2500 3000

Freq (MHz)

Lin

ea

r V

RC

Case 1 – SOLT assuming ‘perfect’ cal load

Residual test port match ripple plot MM ≈≈ 0.021 0.021 ≡≡ --34 dB34 dB

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

0 500 1000 1500 2000 2500 3000

Freq (MHz)

Lin

ea

r V

RC

Case 2 – SOLT using characterised cal load

Residual directivity ripple plot DD ≈≈ 0.002 5 0.002 5 ≡≡ --52 dB52 dB

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 500 1000 1500 2000 2500 3000

Freq (MHz)

Lin

ear

VR

C

Case 2 – SOLT using characterised cal load

Residual test port match ripple plot MM ≈≈ 0.010 0.010 ≡≡ --40 dB40 dB

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

0 500 1000 1500 2000 2500 3000

Freq (MHz)

Lin

ear

VR

C m

ag

nit

ud

e

Case 3 – SOLT using characterised mismatch

Residual directivity ripple plot DD ≈≈ 0.006 0.006 ≡≡ --44 dB44 dB

0

0.005

0.01

0.015

0.02

0.025

0 500 1000 1500 2000 2500 3000

Freq (MHz)

Lin

ea

r V

RC

ma

gn

itu

de

Case 3 – SOLT using characterised mismatch

Residual test port match – ripple plot MM ≈≈ 0.016 0.016 ≡≡ --36 dB36 dB

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

0 500 1000 1500 2000 2500 3000

Freq (MHz)

Lin

ea

r V

RC

ma

gn

itu

de

Summary performance

(By comparison, some previously published values for SOLT cals (using fixed high precision loads) in 7 mm to 18 GHz are: D = 0.01 and M = 0.03.)

0.0100.010

((--40 dB)40 dB)

0.016

(-36 dB)

0.021

(-34 dB)

Residual

test port match

0.002 50.002 5

((--52 dB)52 dB)

0.006

(-44 dB)

0.015

(-36 dB)

Residual

Directivity

Characterised Characterised load calload cal

Characterised mismatch cal

(VSWR = 2.0)

Perfect load cal

Uncertainty/BMC

Reflection coefficient – linear magnitude uncertainty

Using the EA Guide

approach with:

D = 0.002 5M = 0.010

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Measured magnitude VRC

VR

C m

ag

nit

ud

e u

nc

ert

ain

ty

Uncertainty/BMC

Reflection coefficient – phase uncertainty

Using:

U(φ) = sin-1(U(|Γ|)/|Γ|)

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Measured magnitude VRC

VR

C p

has

e u

nce

rtai

nty

(d

egs)

Summary

• An assumed ‘perfect’ cal load can be the major source of systematic error in a calibrated VNA

• The cal load can be ‘characterised’ in terms of its measured VRC

• The LA Techniques VNA enables characterised cal load data to be stored for use during calibration

• Residual directivity improvement (from -36 dB to -52 dB)

• Residual Test Port Match improvement (from -34 dB to -40 dB)

Bibliography

Nils Nazoa and Nick Ridler, “LA19-13-01 3 GHz VNA calibration and measurement uncertainty”, LA Techniques Ltd, Technical Note Ref LAP02, 2006.

M G Cox, M P Dainton and N M Ridler, “An interpolation scheme for precision reflection coefficient measurements at intermediate frequencies. Part 1: theoretical development”, IMTC'2001 Proceedings of the 18th IEEE Instrumentation and Measurement Technology Conference, Budapest, Hungary, 21-23 May 2001, pp 1720-1725.

N M Ridler, M J Salter and P R Young, “An interpolation scheme for precision reflection coefficient measurements at intermediate frequencies. Part 2: practical implementation”, IMTC'2001 Proceedings of the 18th IEEE Instrumentation and Measurement Technology Conference, Budapest, Hungary, 21-23 May 2001, pp 1731-1735.

“EA guidelines on the evaluation of vector network analysers (VNA)” European co-operation for Accreditation, EA-10/12, May 2000.

N M Ridler and C Graham, “Some typical values for the residual error terms of a calibrated vector automatic network analyser (ANA)”, BEMC 99, 9th International Conference on Electromagnetic Measurement Conference Digest, Brighton Metropole Hotel, UK, 2-4 November 1999, pp 45/1-45/4.