WO2004112237A1

WO2004112237A1 - Amplifier impedance matching

Info

Publication number: WO2004112237A1
Application number: PCT/GB2004/002469
Authority: WO
Inventors: Dennis Culling
Original assignee: Sepura Ltd
Current assignee: Sepura Ltd
Priority date: 2003-06-13
Filing date: 2004-06-11
Publication date: 2004-12-23
Anticipated expiration: 2005-12-13
Also published as: EP1634369A1; GB0313781D0; GB2404088B; GB0413118D0; GB2404088A; CN1820408A

Abstract

A radio frequency transmitter comprising a Cartesian loop amplifier (2), a network portion (8) and an antenna (6). The network portion (8) includes a low pass LC filter that operates to transform the effective impedance presented to the amplifier (2) by the network portion (8) and the antenna (6) such that an increase in the effective impedance of the antenna (6) results in a decrease in the effective impedance presented to the amplifier (2). The network portion (8) also includes a buffering portion that comprises a resistor inside another low pass filter stage. The resistor acts as a buffer to limit the minimum impedance that is presented to the amplifier (2).

Description

Amplifier Impedance Matching

This invention relates to radio transmitter amplifiers, particularly but not exclusively linear amplifiers as used in digital radio (wireless communication) applications.

Linear amplifiers are frequently required in digital radio transmitters in order to provide reliable linear amplification of input signals which comprise at least an element of amplitude modulation. In such radio transmitters, linearity and power control of the amplifier are important in order to amplify the signal accurately. In one particular example, the signals used in TETRA (TErrestrial Trunked RAdio) comprise both phase and amplitude modulation. TETRA communications equipment uses a linear amplifier whose linearity is ensured by using Cartesian loop feedback. This arrangement is described in greater detail in WO 00/10247.

It is important in such amplifier arrangements that the average output power of the amplifier remains constant in order to maintain linearity. With a correctly connected antenna this is theoretically easy to achieve since the impedance presented to the amplifier by the antenna should remain constant. It is known, however, that the effective impedance of the antenna can, in practice, fluctuate depending upon the proximity of other objects, particularly metal objects. If this fluctuating impedance were to be presented to the amplifier, then there would be an increased danger of clipping (since if the impedance increases too far then the signal voltage could exceed its maximum design value) . Such clipping is undesirable since it degrades the quality of the transmitted signal and generates unwanted artefacts in adjacent channels.

In prior art transmitters of the type described above, therefore, it is common to use an isolator between the antenna and the amplifier.

An isolator is a device well known in this context It includes a puck-shaped ferrite member and a further, separate magnet. The isolator generally has two connection pins which are connected in use to the amplifier and the antenna respectively. The important characteristic of an isolator is that it presents a constant impedance to pin one, connected to the amplifier, regardless of fluctuations in impedance of the antenna connected to pin two.

In fact, the isolator is a special case of a wave manipulator device commonly known as a circulator in which an input signal is proportionately directed to one of two outputs depending upon the impedances of each output relative to the input. In the special case of the isolator, one of the outputs is terminated by a fixed value resistor so that in the event of an impedance mismatch,, the reflected signal is dissipated in this resistor, rather than being reflected back to the amplifier output. This results in a fixed impedance being presented at the input equal to the value of the resistor, regardless of the impedance at the output. By presenting a constant impedance to the amplifier, isolators are invaluable in digital radio applications in which linearity and constant output power are important to the operation of the amplifier in the face of a potentially changing antenna impedance. However, the ferrite member and separate magnet required for an isolator make such a component heavy and bulky. Furthermore, isolators are relatively costly components not only because of the materials required but because they are difficult to design and manufacture reliably. It is an object of the present invention to alleviate the above mentioned problems.

When viewed from a first aspect the present invention provides a radio frequency transmitter comprising: a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; an antenna connected to the output of said amplifier; and a network portion arranged between said amplifier and said antenna, said network portion comprising at least one inductance-capacitance filter arranged to transform the effective impedance presented to said amplifier by the antenna and the network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, the transmitter further comprising a resistor in series with the amplifier for limiting the minimum effective impedance presented to the amplifier; wherein no isolator is provided between the amplifier and the antenna.

In the present invention a network portion comprising an inductance-capacitance (LC) filter to transform the impedance of the antenna and more particularly to decrease the effective impedance seen by the amplifier when the effective impedance of the antenna increases is arranged between the amplifier and the antenna. This obviates the need to include an isolator in the transmitter circuit. This accordingly provides benefits in terms of e.g., a reduction in cost and weight, particularly in the context of hand-held transmitters. Furthermore, the reduction in space required may be exploited by making the transmitter smaller or by adding additional features, a bigger battery, etc. It was previously believed in the art that the fluctuating impedance of an antenna in a radio transmitter requiring a linearised, constant average power amplifier, necessitated the provision of an isolator to isolate fluctuations in the impedance of the antenna from the amplifier in order to prevent impedance mismatch giving rise to clipping and the problems associated therewith which are set out above. However, the Applicant has realised that implicit in this belief is the assumption that the environmental context of the antenna may act to increase or decrease its effective impedance compared to its free space value. Further, the Applicant has appreciated that in fact in practical radio transmitter arrangements of this type, the effective impedance of the antenna is usually only increased compared to its free space value, rather than being increased or decreased. This means that by transforming the increase in effective impedance to result in a reduction in the impedance presented to the amplifier, clipping of the signal, which results from the amplifier being presented with an impedance higher than a matching impedance, may be avoided without requiring an isolator.

By providing a resistor in series with the amplifier output, a minimum impedance which may be presented to the amplifier is established. This ensures that if the effective impedance of the antenna increases too far (or in the worse case the antenna is disconnected completely) the impedance presented to the amplifier cannot correspondingly fall to less than the value of the resistor. This prevents an excessive output current . It will of course be appreciated that the principles of the invention may be beneficially applied when designing a linearised digital radio frequency power amplifier in which clipping arising from antenna impedance mismatch is to be avoided. In such a situation, the LC filter need not perform the impedance transformation of the invention as its sole function. For example, it may also act to filter out harmonics of the desired signal prior to transmission, or indeed perform any other signal function. It is simply necessary for the skilled person, by judicious design of the LC filter (consistent with any other roles it might be required to perform) to ensure that the impedance inversion required in accordance with the present invention is achieved.

When viewed from a second aspect the present invention provides a method of making a radio frequency transmitter comprising: providing a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; providing an antenna connected to the output of said amplifier; arranging a network portion between said amplifier and said antenna, said network portion comprising at least one inductance- capacitance filter, such that in use said network portion transforms the effective impedance presented to said amplifier by said antenna and said network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier; and providing a resistor in series with the amplifier arranged to limit the minimum effective impedance presented to the amplifier; wherein no isolator is provided between the amplifier and the antenna.

In fact it may not be apparent from the structure of an RF transmitter designed in accordance with the principles of the present invention that it is has been so designed except that an isolator will not be required. When viewed from a further broad aspect therefore the invention provides a radio transmitter comprising a linear amplifier, and an antenna connected to the amplifier, wherein the transmitter does not have an isolator between the amplifier and the antenna. As well as incorporating the principles of the invention in new transmitter designs, the network portion described in accordance with the transmitter of the first aspect of the invention may be employed in existing transmitter designs as a direct replacement for the isolator where it is currently used.

When viewed from a yet further aspect therefore, the invention provides a radio frequency network portion for insertion between a linear amplifier and an antenna, the network portion comprising at least one inductance- capacitance filter arranged to transform in use the effective impedance presented to said amplifier by the network portion and antenna such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, said network portion further comprising a resistor in series with the input for limiting the minimum effective impedance presented by the network portion.

Such a network portion could even be assembled as a discrete component to replace an isolator physically as well as electrically. However, this is not essential and the network portion may be incorporated into an existing transmitter design in any suitable way. As discussed above, the network portion should be arrangable in use such that an isolator is not required between the amplifier and the antenna to shield the amplifier from changes in the effective impedance presented by the antenna.

The precise form of the LC filter will be determined on an application by application basis, particularly where its function in accordance with the present invention is combined with another function.

The filter may be arranged as a high pass filter, a low pass filter or any more complex arrangement. Insofar as the present invention is concerned, it is only necessary that it performs the impedance inversion discussed herein. Indeed, unless required for other reasons, the

LC filter would be configured to pass signals throughout the ordinary operating band of the transmitter. A single LC stage may be provided, or multiple stages may be provided, again depending upon the application. The series resistor could be arranged directly in series with the amplifier. This would mean that its value would need to be chosen consistent with its function to limit the minimum impedance presented to the amplifier whilst minimising its effect during ordinary operation, i.e. where there is in fact no impedance mismatch between the antenna and the amplifier. In accordance with particularly preferred embodiments of the invention however, the resistor is provided within an LC filter - i.e. between a capacitor and an inductor. The Applicant has found with such an arrangement that by suitable choice of the values of the resistor, capacitor and inductor, the desired buffering effect at high impedance mismatch may be achieved whilst giving minimal loss under matched conditions.

As discussed above the type of amplifier for which the present invention is beneficial is one that is required to amplify signals with at least an element of amplitude modulation. Typically, although not invariably, the amplifier will be designed to have a constant average output power. Such an amplifier would normally be at least partially linear. Preferably the transmitter comprises linearising means. For example Cartesian loop feedback may be employed. Most preferably the transmitter incorporates the loop phase error correction technique described in WO 00/10247. In preferred embodiments the antenna is commonly mounted on a single housing with the amplifier and network portion - i.e. it is not a remote antenna.

It is believed that the principles of the present invention may be implementable in other ways. Thus, according to another aspect of the present invention, there is provided a radio frequency transmitter comprising: a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; an antenna connected to the output of said amplifier; and a network portion arranged between said amplifier and said antenna, said network portion comprising means for transforming the effective impedance presented to said amplifier by the antenna and the network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, the transmitter further comprising means for limiting the minimum effective impedance presented to the amplifier.

According to a further aspect of the invention, there is provided a method of making a radio frequency transmitter comprising: providing a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; providing an antenna connected to the output of said amplifier; arranging a network portion between said amplifier and said antenna, said network portion comprising means for transforming the effective impedance presented to said amplifier by said antenna and said network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier; and providing means for limiting the minimum effective impedance presented to the amplifier. According to another aspect of the invention, there is provided a radio frequency network portion for insertion between a linear amplifier and an antenna, the network portion comprising means for transforming in use the effective impedance presented to said amplifier by the antenna and the network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, the network portion further comprising means for limiting the minimum effective impedance presented to the amplifier in use by the network portion and the antenna.

These aspects of the present invention can include any one or more or all of the preferred and optional features of the invention described herein. Thus, for example, the impedance transforming means preferably comprises an LC filter circuit, and the means for limiting the minimum impedance presented to the amplifier preferably comprises a resistor. However, other arrangements would be possible, if desired. For example, a transformer could be used in place of the LC filter to provide the impedance transformation, and although the lossy part of the network (i.e. the minimum impedance limiting means) should be resistive, it does not necessarily have to be provided by a discreet resistor, but could, e.g., be provided by the resistive loss in an inductor, capacitor or other component. As discussed above, the arrangement should be such that no isolator is required between the amplifier and the antenna to shield the amplifier from changes in the effective impedance presented by the antenna (and thus, most preferably, no isolator is provided between the amplifier and antenna) .

As discussed above, the present invention is particularly applicable to radio transmitters for use in mobile communications systems, such as the TETRA system. Thus the present invention also extends to a mobile communications terminal that includes a transmitter arranged in accordance with the present invention. Such a terminal could be a mobile or base station, as is known in the art .

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic block diagram of a radio transmitter using an isolator according to a known arrangement ; Figure 2 is a schematic diagram similar to Figure 1 with a network portion in accordance with the invention in which the isolator is omitted;

Figure 3 is a schematic diagram showing part of a network portion in accordance with the present invention;

Figure 4 is a schematic diagram of another part of the network portion; Figure 5 is a schematic diagram of an overall network portion in accordance with the present invention; and

Figure 6 is a diagram identical to Figure 4 except that the matched impedance situation is simulated.

Turning to Figure 1 , there may be seen a highly schematic diagram showing a Cartesian loop amplifier 2, an isolator 4 and an antenna 6. The Cartesian loop amplifier may be of any convenient design. A suitable amplifier is disclosed and described fully in WO

00/10247, with particular reference to Figure 3 thereof. The isolator 4 is provided between the amplifier 2 and antenna 6 and serves to dissipate any signal reflected from the antenna 6 arising from impedance mismatch between it and the antenna 2 thereby ensuring a constant predetermined impedance is presented to the amplifier 2.

Figure 2 shows a schematic diagram similar to Figure 1 except that no isolator is provided. Instead the isolator has been replaced by a network portion 8, shown more clearly in Figures 3 and 4. By omitting the isolator and its associated ferrite member and magnet, a significant space, weight and cost saving may be achieved.

Figure 3 shows a schematic diagram of an exemplary impedance transformation part of a network portion 8 in accordance with the invention. As will be seen from the diagram, this is in the form of a low pass LC filter comprising a parallel arrangement of a 3.3 pico-Farad (pF) capacitor Cl and 12.55 nano-Henry (nH) inductor Ll between a pair of parallel capacitors C2 , C3 of values 6.8 pF and 8.2 pF respectively. In series with this arrangement is a transmission line 12 of 50 Ohm characteristic impedance (Zo = 50 Ohm, v = 169 Mm/s, I = 0.02 m) which merely represents the electrical length of the PCB tracks, etc.

In order to carry out a simulation of the operation of this part of the network, a terminating impedance of 650 Ohms was defined, as may be seen at the right hand end of Figure 3. This is a typical empirically determined value for a shielded antenna whose nominal impedance is 50 Ohms. When the simulation was run on a Touchstone network simulator package, it was found that at 400 mega-Hertz (MHz) the series equivalent of the network shown in Figure 3 was a resistance of 3.08 Ohms and an inductance of 3.87 nH. Thus, put simply, the increase in impedance of the antenna from its nominal value of 50 Ohms to 650 Ohms is transformed by the network of Figure 3 to approximately 3 Ohms . Although this adequately demonstrates the required impedance transformation, an impedance as low as 3 Ohms would represent too high a load on the amplifier and thus buffering is required.

The buffering part of the network is shown in Figure 4. In use the left hand end of this network would be connected to the left hand end of the network with the right hand end being connected to the output of the RF amplifier. This arrangement is shown in Figure

5, which shows schematically the arrangement of the buffering part of the network shown in Figure 4 and the impedance transformation part of the network shown in Figure 3 between the power amplifier 2 and the antenna 6.

The buffering part of the network portion shown in Figure 4 comprises two inductors L2 , L3 of 9.85 nH and 3.85 nH respectively and two capacitors C4 and C5 of capacitance 4.5 pF and 6.8 pF, respectively. The form of the arrangement is that of two low-pass LC filter stages. However, the values of L2 , L3 , C4 and C5 are chosen such that the characteristic cut-off frequency of the network is above the ordinary transmission band.

It will also be seen that a 10 Ohm resistor Rl is provided between the first inductor L2 and the first capacitor C4 in series with the latter. In other words the resistor Rl is inside the first low pass filter stage. The resistor Rl acts as a buffer to limit the minimum impedance which may be presented to the amplifier 2. However by being inside the filter, the effect of the buffering resistor Rl under matched conditions is minimised (as will be demonstrated below with reference to Figure 6) .

The network portion of Figure 4 was simulated with a 2.5 Ohm termination at its left hand end - i.e from the impedance transformation part as shown in Figure 3. This value is marginally worse than the 3 Ohm result from the previous simulation. It was found that at 400 MHz, the series equivalent of this part of the network was a 33 Ohm resistor and a 16 nH inductor. Thus the buffering network shown in Figure 4 presents an impedance of approximately 33 Ohms to the amplifier.

This is to be contrasted with the 3 Ohm impedance in the absence of this part of the network. Although the impedance is still mismatched, the mismatch is relatively small. More importantly the impedance presented is too low rather than too high. As explained earlier, the amplifier is far better able to cope with the mismatch being in this sense since this does not result in voltage clipping. In summary the overall network portion of the described embodiment has transformed an antenna impedance rise from 50 Ohms to 650 Ohms to a reduction of the impedance presented to the amplifier from 50 Ohms to 33 Ohms.

Figure 6 is identical to Figure 4 except that it simulates the matched impedance condition. In these circumstances it was found that the insertion loss of the buffering part of the network shown is -1.4 decibels (dB) . This compares favourably with the typical insertion loss of an isolator which is similar at -1.1 dB. Thus it will be seen that even without an isolator the buffering network buffers the impedance inverting effect of the low pass filter in the event of an impedance mismatch between the amplifier and the antenna whilst avoiding the detrimental effects of clipping.

Of course it will be appreciated that the particular network design and component values given in the above illustration are purely exemplary and will vary depending upon the particular application. In any particular application, the principles of the present invention may be employed by using the standard techniques of network analysis to obtain the required impedance inversion and buffering effects suitable for that application.

Claims

1. A radio frequency transmitter comprising: a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; an antenna connected to the output of said amplifier; and a network portion arranged between said amplifier and said antenna, said network portion comprising at least one inductance- capacitance filter arranged to transform the effective impedance presented to said amplifier by the antenna and the network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, the transmitter further comprising a resistor in series with the amplifier for limiting the minimum effective impedance presented to the amplifier; wherein no isolator is provided between the amplifier and the antenna.

2. The transmitter of claim 1, wherein the inductance-capacitance filter of the network portion is arranged to perform a function in addition to the impedance transformation.

3. The transmitter of claim 1 or 2 , wherein the inductance-capacitance filter of the network portion is configured to pass signals throughout the ordinary operating band of the transmitter.

4. The transmitter of claim 1, 2 or 3, wherein the resistor in series with the amplifier is provided within an inductance-capacitance filter.

5. The transmitter of claim 1, 2, 3 or 4 , wherein the linear amplifier employs Cartesian loop feedback.

6. The transmitter of claim 1, 2, 3, 4 or 5, wherein the antenna is commonly mounted on a single housing with the amplifier and network portion.

7. A radio transmitter comprising a linear amplifier, and an antenna connected to the amplifier, wherein the transmitter does not have an isolator between the amplifier and the transmitter.

8. A radio frequency transmitter comprising: a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; an antenna connected to the output of said amplifier; and a network portion arranged between said amplifier and said antenna, said network portion comprising means for transforming the effective impedance presented to said amplifier by the antenna and the network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, the transmitter further comprising means for limiting the minimum effective impedance presented to the amplifier.

9. A radio frequency network portion for insertion between a linear amplifier and an antenna, the network portion comprising at least one inductance-capacitance filter arranged to transform in use the effective impedance presented to said amplifier by the network portion and antenna such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, said network portion further comprising a resistor in series with the input to the network portion for limiting the minimum effective impedance presented to the amplifier in use by the network portion and the antenna.

10. The network portion of claim 9, wherein the resistor in series with the input is provided within an inductance-capacitance filter.

11. A radio frequency network portion for insertion between a linear amplifier and an antenna, the network portion comprising means for transforming in use the effective impedance presented to said amplifier by the antenna and the network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier, the network portion further comprising means for limiting the minimum effective impedance presented to the amplifier in use by the network portion and the antenna.

12. A terminal for a mobile communications system comprising a transmitter or radio frequency network portion in accordance with any one of claims 1 to 11.

13. A method of making a radio frequency transmitter comprising: providing a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; providing an antenna connected to the output of said amplifier; arranging a network portion between said amplifier and said antenna, said network portion comprising at least one inductance-capacitance filter, such that in use said network portion transforms the effective impedance presented to said amplifier by said antenna and said network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier; and providing a resistor in series with the amplifier arranged to limit the minimum effective impedance presented to the amplifier; wherein no isolator is provided between the amplifier and the antenna.

14. The method of claim 13, comprising arranging the inductance-capacitance filter of the network portion to perform a function in addition to the impedance transformation.

15. The method of claim 13 or 14, comprising configuring the inductance-capacitance filter of the network portion to pass signals throughout the ordinary- operating band of the transmitter.

16. The method of claim 13, 14 or 15, comprising providing the resistor in series with the amplifier within an inductance-capacitance filter.

17. The method of claim 13, 14, 15 or 16, wherein the linear amplifier employs Cartesian loop feedback.

18. The method of claim 13, 14 15, 16 or 17, comprising mounting the antenna commonly on a single housing with the amplifier and network portion.

19. A method of making a radio frequency transmitter comprising: providing a linear amplifier for amplifying signals comprising at least an element of amplitude modulation; providing an antenna connected to the output of said amplifier; arranging a network portion between said amplifier and said antenna, said network portion comprising means for transforming the effective impedance presented to said amplifier by said antenna and said network portion such that an increase in the effective impedance of said antenna results in a decrease in the effective impedance presented to the amplifier; and providing means for limiting the minimum effective impedance presented to the amplifier.

20. A radio frequency transmitter substantially as hereinbefore described with reference to any one of Figures 2 to 5 of the accompanying drawings .

21. A radio frequency network portion substantially as hereinbefore described with reference to any one of Figures 2 to 5 of the accompanying drawings .

22. A method of making a ratio frequency transmitter substantially as hereinbefore described with reference to any one of Figures 2 to 5 of the accompanying drawings.