Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
  • H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers
  • H03F3/24—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers of transmitter output stages
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
  • H03F1/02—Modifications of amplifiers to raise the efficiency, e.g. gliding Class A stages, use of an auxiliary oscillation
  • H03F1/0205—Modifications of amplifiers to raise the efficiency, e.g. gliding Class A stages, use of an auxiliary oscillation in transistor amplifiers
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
  • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
  • H03F1/56—Modifications of input or output impedances, not otherwise provided for
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
  • H03F3/189—High frequency amplifiers, e.g. radio frequency amplifiers
  • H03F3/19—High frequency amplifiers, e.g. radio frequency amplifiers with semiconductor devices only
  • H03F3/191—Tuned amplifiers
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H7/00—Multiple-port networks comprising only passive electrical elements as network components
  • H03H7/38—Impedance-matching networks
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/02—Transmitters
  • H04B1/04—Circuits
  • H04B1/0458—Arrangements for matching and coupling between power amplifier and antenna or between amplifying stages
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/387—A circuit being added at the output of an amplifier to adapt the output impedance of the amplifier
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/391—Indexing scheme relating to amplifiers the output circuit of an amplifying stage comprising an LC-network
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/451—Indexing scheme relating to amplifiers the amplifier being a radio frequency amplifier
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/504—Indexing scheme relating to amplifiers the supply voltage or current being continuously controlled by a controlling signal, e.g. the controlling signal of a transistor implemented as variable resistor in a supply path for, an IC-block showed amplifier

Definitions

• 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.
• linearity and power control of the amplifier are important in order to amplify the signal accurately.
• the signals used in TETRA 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.
• 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.
• 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.
• 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.
• 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.
• the ferrite member and separate magnet required for an isolator make such a component heavy and bulky.
• 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.
• 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.
• 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.
• LC inductance-capacitance
• 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.
• 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.
• 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.
• 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.
• 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.
• LC filter 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.
• 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.
• 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.
• the amplifier will be designed to have a constant average output power.
• Such an amplifier would normally be at least partially linear.
• the transmitter comprises linearising means.
• Cartesian loop feedback may be employed.
• the transmitter incorporates the loop phase error correction technique described in WO 00/10247.
• the antenna is commonly mounted on a single housing with the amplifier and network portion - i.e. it is not a remote antenna.
• 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.
• 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.
• 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.
• the impedance transforming means preferably comprises an LC filter circuit
• the means for limiting the minimum impedance presented to the amplifier preferably comprises a resistor.
• 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.
• 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) .
• the present invention is particularly applicable to radio transmitters for use in mobile communications systems, such as the TETRA system.
• the present invention also extends to a mobile communications terminal that includes a transmitter arranged in accordance with the present invention.
• a terminal could be a mobile or base station, as is known in the art .
• 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.
• Figure 6 is a diagram identical to Figure 4 except that the matched impedance situation is simulated.
• FIG. 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
• 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.
• FIG. 3 shows a schematic diagram of an exemplary impedance transformation part of a network portion 8 in accordance with the invention.
• 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.
• pF pico-Farad
• nH nano-Henry
• 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.
• the series equivalent of the network shown in Figure 3 was a resistance of 3.08 Ohms and an inductance of 3.87 nH.
• 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 .
• 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.
• 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 4.
• 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.
• 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.
• a 10 Ohm resistor Rl is provided between the first inductor L2 and the first capacitor C4 in series with the latter.
• 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.
• 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.
• 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.
• 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.
• 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.

Applications Claiming Priority (2)

Publications (1)

Family

ID=27636520

Family Applications (1)

Country Status (4)

Families Citing this family (6)

Family Cites Families (10)

• 2003
  • 2003-06-13 GB GB0313781A patent/GB0313781D0/en not_active Ceased
• 2004
  • 2004-06-11 GB GB0413118A patent/GB2404088B/en not_active Expired - Fee Related
  • 2004-06-11 EP EP04736648A patent/EP1634369A1/de not_active Withdrawn
  • 2004-06-11 CN CN 200480019605 patent/CN1820408A/zh active Pending
  • 2004-06-11 WO PCT/GB2004/002469 patent/WO2004112237A1/en active Application Filing

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events