GB2536679A

GB2536679A - Impedance adjustment circuit for a negative impedance converter

Info

Publication number: GB2536679A
Application number: GB1505062.8A
Authority: GB
Inventors: Hu Sampson; Wan Liang
Original assignee: Smart Antenna Technologies Ltd
Current assignee: Smart Antenna Technologies Ltd
Priority date: 2015-03-25
Filing date: 2015-03-25
Publication date: 2016-09-28
Anticipated expiration: 2035-03-25
Also published as: GB2536679B; GB201505062D0

Abstract

An impedance matching circuit for an electrically small antenna 9 comprises a series-coupled negative impedance converter (NIC) 1-8 in parallel with a passive one-port network such as a resistor-capacitor circuit 14-17. The passive circuit improves impedance matching and circuit stability. The matching circuit further comprises a series inductor 10 in the antenna port and a shunt capacitor 11 in the other port. The passive circuit may comprise an inductor.

Description

IMPEDANCE ADJUSTMENT CIRCUIT FORA NEGATIVE IMPEDANCE CONVERTER

[0001] This invention relates to matching circuits to match antennas to RE sources and, in particular, to an impedance adjustment circuit for a negative impedance converter implemented in a matching circuit.

BACKGROUND

[0001] Electrically small antennas can be generally classified as TM (transverse magnetic) and TE (transverse electric) mode antennas. For a TM mode small antenna, which is widely used in wireless communication systems, the input impedance is considerably reactive with a small real part. It is therefore critical to match the antenna to the receiver or transmitter to maximise the total efficiency in the frequency range of interest.

[0002] Normally, an electrically small TM mode antenna can be characterised by or represented as a series connected combination of a resistor, a capacitor and an inductor.

Figures 1 and 2 show that, at low frequencies, the reactance can equally be represented by a series connected capacitor and inductor, with the capacitor playing a dominant role in the reactance. The resistor represents the resistance of the radiating element of the antenna.

[0003] There are two different ways to match a highly reactive antenna of this type. One approach is conventional passive matching, where a large series inductor Lex, is placed between the antenna and the signal port as a necessary component. However, the resistive loss that is introduced by the inductor Lext dramatically degrades the total efficiency. In fact, even with lossless inductors, the match is effective over only extremely small instant bandwidths because the reactive part of the electrically small antenna cannot be neutralised over a broad frequency band with passive components (the real part of the impedance is much smaller than the imaginary part). This is illustrated in Figure 3.

[0004] The other approach uses an NIC (negative impedance converter) to create a negative capacitor, which can then be configured to cancel the reactance of the antenna as much as possible. This is a type of non-Foster impedance matching, and is illustrated in Figure 4.

[0005] There is a relationship between antenna size and the realisable bandwidth as defined by the Chu limit (Chu, L. J.; "Physical limitations of omni-directional antennas"; Journal Applied Physics 19: 1163-1175; December 1948). The Chu limit gives the relationship between the radius of the circle that completely circumscribes an antenna and the Q of the antenna. However, McLean (McLean, J. S.; "A re-examination of the fundamental limits on the radiation Q of electrically small antennas"; IEEE Transactions on Antennas and Propagations; Vol. 44; No. 5; pp. 672-676; May 1996) redefined how the Q of an antenna should be calculated, and this is given in equation 1.1: (ka) where k is the wave number and a is the radius of a sphere that completely circumscribes the antenna as shown in Figure 5.

[0006] McLean's equation is a derivation from the original Chu limits equations. There has also been much research into ways of improving the gain of an antenna through the use of matching networks, but this is also bounded by the Harrington limits (Harrington, R. F.; "Effect of antenna size on gain, bandwidth and efficiency"; Journal of Research of the National Bureau of Standards -D. Radio Propagation; vol. 64D; p. 12; 29"' June 1959) on antennas as given in: G = (ka)2 (1.2) The Chu limit can be related to the antenna bandwidth by rewriting the Q of the antenna as shown in equation 1.3: Vea (ka) (1.3) where fe is the antenna centre frequency at resonance and af is the bandwidth of the antenna.

[0007] Comparing equation 1.1 with equation 1.3, it can be seen that reducing the radius of the sphere which translates to a physical reduction in the antenna size, the antenna bandwidth also reduces. The reduction in size means that the antenna radiation resistance also reduces, and this in turn leads to a reduction the antenna efficiency. From equation 1.2, it is clear that antenna gain is also proportional to the antenna size a.

[0008] These two fundamental limits on the antenna make it difficult to provide a small antenna with a low 0 (wideband). However, more and more devices these days require smaller antennas and there is need for these antennas to still have wide usable bandwidths.

[0009] Passive matching networks help to match antennas, but because they involve resonating the reactive part of the antenna with passive elements, they only give a good match at specific frequencies. Away from the specific frequency, the antenna return loss decreases. This necessitates the use of multiple or reconfigurable matching networks to cover wide frequency bands. However, using non-Foster elements could help provide continuous wideband matching because unlike Foster elements, the slope of the reactance versus frequency of a non-Foster element is always negative as shown in Figure 4.

[0010] With these properties, non-Foster elements are able to cancel out completely the reactance of other elements and antennas because of the difference in slope and direction of rotation on the Smith chart.

[0011] One implementation of non-Foster elements is through the use of NICs (negative impedance converters). NICs were first proposed by Linvill (Linvill, J.G.; "Transistor negative-impedance converters"; Proc. IRE; vol 41, pp 725-729; 1953). The Linvill NIC consists of two transistors connected in a common base configuration. "Common base" or "common gate" refers to a specific input and output setup of a transistor in amplifier applications. In a Linvill type NIC, the RF input terminal is connected to the emitter or source of one transistor, and the RF output terminal to the emitter or source of the other transistor. The reactance to be inverted is connected between the two collectors or drains, and the base or gate of each transistor is connected to the collector or drain of the other transistor in the form of a feedback path. The emitters or sources form the two ports of the NIC.

[0012] The circuit schematic of a conventional Linvill type NIC is shown in Figure 6. The NIC comprises first and second biased transistors 1, 2 connected in a crossover configuration. It will be understood that the NIC may comprise field effect transistors, in which case the transistors 1, 2 will have a source 5, 5', a drain 4, 4' and a gate 3, 3'. Alternatively, the NIC may comprise bipolar junction transistors, in which case the transistors 1, 2 will have an emitter 5, 5', a collector 4, 4' and a base 3, 3'.

[0013] The collector or drain 4 of the first transistor 1 is connected to the base or gate 3' of the second transistor 2, and the collector or drain 4' of the second transistor 2 is connected to the base or gate 3 of the first transistor 1. A predetermined impedance is provided between the respective collectors or drains 4, 4' on the one hand, and the respective bases or gates 3, 3' on the other hand, of the first and second transistors 1, 2.

The predetermined impedance determines the negative impedance that the NIC applies to an RF signal passing between the input port 100 and the output port 101. The predetermined impedance may consist of a resistor 6, an inductor 7 and a capacitor 8 connected in series between the respective collectors or drains 4, 4'.

[0014] Figure 7 shows how the Linvill type NIC of Figure 6 can be implemented as a non-Foster matching circuit for an electrically small antenna 9. An external inductor 10 is connected between the antenna 9 and the source or emitter 5' of the second transistor 2 (the RF input of the NIC). The negative impedance generated by the NIC is used to neutralise the reactance of the external inductor 10 and the antenna 9. An external capacitor 11 is connected to the source or emitter 5 of the first transistor 1 so as to transform the neutralised impedance to 500 at the RF output port of the NIC.

[0015] However, the present Applicant has found that conventional Linvill type NIC matching circuits as shown in Figure 7 are not always ideal enough to generate the precise negative impedance that is required in particular antenna applications. This can result in lower total efficiency, higher noise figures and potential instability.

BRIEF SUMMARY OF THE DISCLOSURE

[0016] Viewed from a first aspect, there is provided a negative impedance converter for a matching circuit for matching an impedance of an antenna to an impedance of an RF source or load, the negative impedance converter comprising first and second transistors connected in a cross-over configuration, each transistor having a source or emitter, a drain or collector and a gate or base, wherein the source or emitter of one transistor is configured as an RF input port, the source or emitter of the other transistor is configured as an RF output port, the drain or collector of the first transistor is connected to the gate or base of the second transistor and the drain or collector of the second transistor is connected to the gate or base of the first transistor, and an impedance is connected between the drain or collector of the first transistor and the drain or collector of the second transistor, and further wherein the negative impedance converter is provided with a passive impedance adjustment network connected between the source or emitter of the first transistor and the source or emitter of the second transistor.

[0017] The impedance connected between the respective drains or collectors of the first and second transistors determines the negative impedance that is presented by the negative impedance converter as a whole when an RF signal is input to the source or emitter of the first transistor and output from the source or emitter of the second transistor.

[0018] The passive impedance adjustment network is effectively connected in parallel with the negative impedance converter, across the respective sources or emitters of the first and second transistors.

[0019] The passive impedance adjustment network may comprise at least one resistor connected in parallel with a capacitor.

[0020] In certain embodiments, the impedance adjustment network comprises a first resistor connected in parallel with a first capacitor, followed by a second resistor connected in parallel with a second capacitor. In these embodiments, the passive impedance network is intended to adjust the impedance converted by the negative impedance converter around the in-band frequencies. This adjustment helps to improve the in-band matching performance (total efficiency and return loss) and to avoid excessive negative resistance (which can cause instability).

[0021] In other embodiments, the passive impedance adjustment network may comprise just a single capacitor connected in parallel with the negative impedance converter, across the respective sources or emitters of the first and second transistors. This is the simplest form of passive impedance adjustment network. A resistor may be placed in parallel or in series with the capacitor in order to strengthen the adjustment effect. Alternatively or in addition, an inductor and a series-connected resistor can be placed in parallel with the capacitor.

[0022] The first and second transistors of the negative impedance converter may be biased as required.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 shows an electrically small antenna connected to a 50 ohm signal port; Figure 2 shows the antenna of Figure 1 represented as an equivalent series connected resistor, capacitor and inductor; Figure 3 shows the arrangement of Figure 2 provided with a passive impedance matching network, together with a plot of reactance against angular frequency; Figure 4 shows the arrangement of Figure 2 provided with a non-Foster matching network comprising a negative capacitance, together with a plot of reactance against angular frequency; Figure 5 illustrates an antenna circumscribed by a sphere of radius a; Figure 6 is a schematic of a conventional Linvill type negative impedance converter (N IC); Figure 7 shows a conventional NIC arrangement for matching an antenna to a transceiver; Figure 8 shows an NIC with an impedance adjustment network of a first embodiment; Figure 9 shows an implementation of the NIC of Figure 8; Figure 10 is an alternative schematic showing the NIC of Figure 8 being used to match an antenna to a feeding port; Figure 11 is a plot comparing the input impedance of a conventional NIC with the input impedance of an NIC of the first embodiment; and Figure 12 is a plot comparing the matching performance of a conventional NIC with the matching performance of an NIC of the first embodiment; and

DETAILED DESCRIPTION

[0024] Figure 8 shows an NIC similar to that of Figure 6, but provided with an impedance adjustment network in accordance with a first embodiment. The NIC comprises first and second biased transistors 1, 2 connected in a crossover configuration. The NIC may comprise field effect transistors, in which case the transistors 1, 2 will have a source 5, 5', a drain 4, 4' and a gate 3, 3'. Alternatively, the NIC may comprise bipolar junction transistors, in which case the transistors 1, 2 will have an emitter 5, 5', a collector 4, 4' and a base 3, 3'.

[0025] The collector or drain 4 of the first transistor 1 is connected to the base or gate 3' of the second transistor 2, and the collector or drain 4' of the second transistor 2 is connected to the base or gate 3 of the first transistor 1. A predetermined impedance is provided between the respective collectors or drains 4, 4' on the one hand, and the respective bases or gates 3, 3' on the other hand, of the first and second transistors 1, 2.

The predetermined impedance determines the negative impedance that the NIC applies to an RF signal passing between the input port 100 and the output port 101. The predetermined impedance is represented by a resistor 6, an inductor 7 and a capacitor 8 connected in series between the respective collectors or drains 4, 4'.

[0026] An impedance adjustment network is provided in the form of first and second parallel resistor-capacitor banks 12, 13 connected in series to form a two terminal network. The impedance adjustment network is connected between the sources or emitters 5, 5' of the first and second transistors 1, 2 as shown. The first resistor-capacitor bank 12 comprises a resistor 14 and a capacitor 15 connected in parallel, and the second resistor-capacitor bank 13 comprises a resistor 16 and a capacitor 17 connected in parallel.

[0027] Figure 9 shows how the NIC with impedance adjustment network of Figure 8 can be implemented as a non-Foster matching circuit for an electrically small antenna 9. An external inductor 10 is connected between the antenna 9 and the source or emitter 5' of the second transistor 2. The negative impedance generated by the NIC is used to neutralise the reactance of the external inductor 10 and the antenna 9. An external capacitor 11 is connected to the source or emitter 5 of the first transistor 1 so as to transform the neutralised impedance to 500 at the RF output port 101 of the NIC. The first and second parallel resistor-capacitor banks 13, 14 constitute the passive impedance adjustment network.

[0028] Figure 10 shows the arrangement of Figure 9 in more general block form. An NIC 200 includes an impedance represented here by a series inductor 201, capacitor 202 and resistor 203, this impedance determining the negative impedance applied by the NIC 200.

An impedance adjustment network 204 as described above is connected in parallel with the NIC 200. An electrically small antenna 205 is connected to the RF output 206 of the NIC 200 by way of a passive impedance transformation network 207. An antenna feeding port 208 is connected to the RF input 209 of the NIC 200 by way of another passive impedance transformation network 210.

[0029] To demonstrate the surprising technical benefits obtained by the impedance adjustment network of present embodiments, reference shall now be made to Figure 11, which shows the input impedance of the conventional NIC of Figure 6 compared to the input impedance of the NIC with the impedance adjustment network of Figure 8 across a range of frequencies. From Figure 11, it can be seen that in the conventional NIC, the real part of the impedance decreases monotonically from a value of 54.370 (significantly higher than 500) at 856.9MHz to a value of 46.120 (significantly lower than 500) at 1030MHz. This means that the total efficiency is degraded at lower frequencies and that stability is a potential problem. In contrast, when the impedance adjustment network is implemented, the real part of the impedance has a local minimum of 48.730 at around 900MHz, and is 500 at both 856.9MHz and 976.6MHz. At 1030MHz, the real part of the impedance is 53.560. It can also be seen that the imaginary part of the impedance has much slower variation when the impedance adjustment network is implemented. As a consequence, the impedance adjustment network improves the matching performance and stability of the whole circuit.

[0030] Figure 12 shows how the matching performance of the circuit of Figure 7 is improved by provision of the impedance adjustment network as shown in Figure 9. In the conventional NIC circuit of Figure 7, the S-parameter curves have uneven shapes, and at some frequency points, both high total efficiency and high return loss occur simultaneously. By providing an impedance adjustment network as hereinbefore described, the shapes of the S-parameter curves assume more well-behaved shapes, and high total efficiency corresponds appropriately to a low return loss.

[0031] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0032] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments.

The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0033] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS: 1. A negative impedance converter for a matching circuit for matching an impedance of an antenna to an impedance of an RF source or load, the negative impedance converter comprising first and second transistors connected in a cross-over configuration, each transistor having a source or emitter, a drain or collector and a gate or base, wherein the source or emitter of one transistor is configured as an RF input port, the source or emitter of the other transistor is configured as an RF output port, the drain or collector of the first transistor is connected to the gate or base of the second transistor and the drain or collector of the second transistor is connected to the gate or base of the first transistor, and an impedance is connected between the drain or collector of the first transistor and the drain or collector of the second transistor, and further wherein the negative impedance converter is provided with a passive impedance adjustment network connected between the source or emitter of the first transistor and the source or emitter of the second transistor.
2. A negative impedance converter as claimed in claim 1, wherein the impedance adjustment network comprises at least one resistor connected in parallel with a capacitor.
3. A negative impedance converter as claimed in claim 1, wherein the impedance adjustment network comprises a first parallel resistor-capacitor bank connected in series with a second parallel resistor-capacitor bank.
4. A negative impedance converter as claimed in claim 1, wherein the impedance adjustment network comprises a capacitor.
5. A negative impedance converter as claimed in claim 4, further comprising a resistor connected in series with the capacitor.
6. A negative impedance converter as claimed in claim 4, further comprising a resistor connected in parallel with the capacitor.
7. A negative impedance converter as claimed in claim 4, further comprising a resistor and an inductor connected in parallel with the capacitor.
8. A negative impedance converter substantially as hereinbefore described with reference to or as shown in Figures 8 to 12 of the accompanying drawings.