Abstract: Performance characteristics of Electrically Small Antennas are limited by fundamental physics. Using Non-Foster elements into antennas structures appears like an opportunity to overshoot those limits. In this paper, a Negative Impedance Converter circuit is designed for antenna applications at 868 MHz. These active circuits, that produce Non-Foster negative capacitance or inductance, are known as sensitive circuits due to their nonlinearities that can produce instabilities. Thus, designing Negative Impedance Converters at Ultra High Frequency bands becomes quite challenging. Accurate design is strongly recommended to predict the final behavior of the circuit. The Negative Impedance Converter sensitivity is investigated with respect to Printed Circuit Board design through both simulation and measurement results.

Keywords: Negative Impedance Converter, Non-Foster elements, negative impedance, electrically small

antennas. References

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Sensitivity of Negative Impedance Converter circuit with respect to PCB design effects

L. Batel1, L. Rudant1, J-F. Pintos1, K. Mahdjoubi 2, 1 CEA-LETI MINATEC Campus, Grenoble, France, 2IETR, University of Rennes 1,

France lotfi.batel@cea.fr

Forum for Electromagnetic Research Methods and Application Technologies (FERMAT)

*This use of this work is restricted solely for academic purposes. The author of this work owns the copyright and no reproduction in any form is permitted without written permission by the author.*

| 1

Sensitivity of Negative Impedance Converter

circuit with respect to PCB design effects

L. Batel1, L. Rudant1, J-F. Pintos1, K. Mahdjoubi 2

1 CEA-LETI MINATEC Campus, Grenoble, France

2IETR, University of Rennes 1, France

lotfi.batel@cea.fr

| BATEL Lotfi

| 2

• Motivations: Improve compact antennas performances for directivity

applications

Innovative antennas for Internet of Things .

Directive antennas issues.

State of the art : negative impedances.

Practical achievement of Non-Foster for antennas.

• Negative impedance converter sensitivity analysis

Theoretical aspects and preliminary investigations.

• NIC architecture.

• Schematic Simulations.

Practical and relevant studies.

• Full EM Co-Simulation.

• Measurement Results.

• Conclusion and future works

OUTLINE

| BATEL Lotfi

| 3

MOTIVATIONS: IMPROVE COMPACT ANTENNAS PERFORMANCES

FOR DIRECTIVITY APPLICATIONS.

| BATEL Lotfi

| 4

■ Compact and directive antennas for smart objects:

→ Focus the radiation in useful directions.

→ Spatial selectivity.

INNOVATIVE ANTENNAS FOR INTERNET OF THINGS

MOTIVATIONS:

IMPROVE COMPACT ANTENNAS PERFORMANCES FOR

DIRECTIVITY APPLICATIONS.

Control what you point at.

| BATEL Lotfi

| 5

■ Usual techniques to enhance antennas’ directivity:

→ Reflectors, directors, arrays, lenses.

■ Classical techniques that lead to:

→ Larger antennas.

→ Limited bandwidth.

DIRECTIVE ANTENNAS ISSUES

Trade-off: Directivity and antenna size.

MOTIVATIONS:

IMPROVE COMPACT ANTENNAS PERFORMANCES FOR

DIRECTIVITY APPLICATIONS.

| BATEL Lotfi

| 6

■ Non-Foster elements are classically used to enhance compact

antennas’ impedance matching bandwidth.

■ Negative impedance used here as an opportunity to enhance

compact and directive antennas performances:

Negative resistive elements → Superdirectivity [1].

Non-Foster elements → Directivity bandwidth of compact antennas

[1].

STATE OF THE ART : NEGATIVE IMPEDANCES

MOTIVATIONS:

IMPROVE COMPACT ANTENNAS PERFORMANCES FOR

DIRECTIVITY APPLICATIONS.

Solution: Negative impedance elements

| BATEL Lotfi [1] L.Batel et al. ”High Directive Compact Antenna with Non-Foster Elements”, IWAT 2015.

| 7

■ Non-Foster elements are achieved with Negative Impedance

Converters circuits (NIC).

→ Active and complex RF circuits.

→ Few practical realizations.

→ Often designed for VHF bands frequencies.

→ Suffer from difficulties with instability.

PRACTICAL ACHIEVEMENT OF NON-FOSTER FOR ANTENNAS

MOTIVATIONS:

IMPROVE COMPACT ANTENNAS PERFORMANCES FOR

DIRECTIVITY APPLICATIONS.

Sensitivity issues at UHF band and above

| BATEL Lotfi

| 8

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

| 9

NIC architecture

NIC converts a passive load ZL to a negative impedance ZNIC.

Two cross-coupled Bipolar Junction Transistors (BJT).

Load (ZL ) inside the feedback loop.

Differential negative impedance (ZNIC ).

Easy to connect at balanced port of an antenna (e.g. dipole).

THEORETICAL ASPECTS AND PRELIMINARY INVESTIGATIONS

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

Analytical transfer function:

ℎ11 : BJT’s input resistance.

𝛽 : BJT’s current gain.

Implementation difficulties and sensitivity issues. | BATEL Lotfi

10

Schematic Simulations (1/2)

THEORETICAL ASPECTS AND PRELIMINARY INVESTIGATIONS

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

ZNIC

ZL

BJT BJT

L1

An extra microstrip line L1 is

introduced in different braches of the

NIC’s ideal model.

Identification of the critical lines and

the more sensitive area: feedback

loop.

Sensitivity study to transmission

lines and substrate nature for a

capacitive load ZL=10pF in the

feedback loop.

| BATEL Lotfi

|11

Schematic Simulations (2/2)

Sensitivity study to transmission lines and substrate nature for a

capacitive load ZL=10pF.

THEORETICAL ASPECTS AND PRELIMINARY INVESTIGATIONS

700 750 800 850 900 950 1000-25

-20

-15

-10

-5

0

Frequency (MHz)

C (

pF

)

Ideal model

Extra line 3mm

Extra line 6mm

Extra line 9mm

700 750 800 850 900 950 1000-25

-20

-15

-10

-5

0

Frequency (MHz)C

(pF

)

ideal model

Substrate RO4350B

Substrate FR4

■ Effect of microstrip lines’ length L1: ■ L1 substrate nature impact:

NIC behavior is significantly impacted by substrate nature and

transmission line lengths especially in the feedback loop.

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

| BATEL Lotfi

|12

Full EM Co-Simulation (1/2)

PRACTICAL AND RELEVANT STUDIES

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

RF ports location.

Transmission line lengths.

Physical dimensions.

Coupling effects.

Accurate model of the NIC with a full EM simulation of the PCB.

Three PCB configurations to check the preliminary investigations

on RO4350B™:

| BATEL Lotfi

|13

Full EM Co-Simulation (2/2)

PRACTICAL AND RELEVANT STUDIES

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

Different results compared with preliminary investigations.

The shortest NIC (1) has the closest behavior to the NIC’s ideal model.

Full EM co-simulation is required for an accurate NIC model.

NIC miniaturization leads to a better control of its behavior.

700 750 800 850 900 950 1000-15

-10

-5

0

Frequency (MHz)

C (

pF

)

Ideal model

Circuit1

Circuit2

Circuit3

| BATEL Lotfi

|14

Measurement Results

PRACTICAL AND RELEVANT STUDIES

NEGATIVE IMPEDANCE CONVERTER

SENSITIVITY ANALYSIS

700 750 800 850 900 950 1000-15

-10

-5

0

Frequency (MHz)

C (

pF

)

Circuit1 simulation results

Circuit3 simulation results

Circuit1 measurement results

Circuit3 measurement results

Measurement of RF circuits (1) and (3).

Stability ensured during measurement using a Spectrum analyzer.

Good agreements between full EM co-simulations and

measurements considering the negative capacitance value.

| BATEL Lotfi

|15

A sensitivity investigation to physical PCB effects on the NIC behavior is studied

from ideal schematic to full EM co-simulations and measurements.

Significant impact of the PCB is observed on the NIC’s Non-Foster response :

feedback loop.

Good agreements of measurement results with EM co-simulation but not with

theory:

→ A full EM co-simulation is strongly required to tune Non-Foster component PCB

design.

Negative capacitance measured with a good agreement at UHF band: circuit

candidate for compact antennas applications

Future work:

NIC circuit integration with compact antennas co-design for UHF band

applications.

Innovative compact and superdirective antennas with Non-Foster elements.

| BATEL Lotfi

CONCLUSION