LOW COST MULTIPLEXER FOR MULTIBAND RADIOS

Abstract: The invention is a radio which operates in multiple bands. A radio which operates in n frequency bands in accordance with the invention includes n radio modules (24, 26), each radio module operating in one of the n frequency bands and having an output with an output impedance; and a multiplexer (102) having n inputs and an output with each of the n inputs being coupled to an antenna; and wherein the output impedance of the radio modules is only matched to the impedance of the antenna in its operating band and is mismatched to the impedance of the antenna in the n-1 other frequency bands of the other radio modules and the multiplexer comprises n transmission lines, each transmission line being located in a different input, presenting a matched impedance to the antenna in the operating band of the radio module that it connects to and transforms the output impedance of the radio module in the n-1 other frequency bands to substantially an open circuit.

Description: LOW COST MULTIPLEXER FOR MULTIBA D RADIOS BACKGROUND OF THE INVENTION Technical Field The present invention relates to multiple band radios and, more particularly, to multiple band radios having improved antenna performance. Description of the Prior Art Fig. 1 illustrates a prior art dual band mobile telephone 10. The mobile phone 10 operates in a first band, such as 824-924 MHz and in a second band, such as 1850-1990 MHz. The multiple band mobile telephone 10 is comprised of an antenna 12, a commercially purchased diplexer 14 , which is surface mounted on a printed circuit board 16 in accordance with state of the art manufacturing techniques. The diplexer 14 has a pair of inputs 18 and 20 and an output 22 which is connected to antenna 12. The input 18 is coupled to a first radio duplexer 24 which operates in the first band and a second radio duplexer 26 which operates in the second band. The diplexer 14 has specifications set by the manufacturer to work when connected to radio duplexers having an output impedance such as 50 ohms. The output impedance of the multiple band radio duplexers 24 and 26 out-of-band does not have a coupling impedance which matches the impedance specified by the manufacturer of the diplexer 14 for the inputs thereof. As a result of the mismatch of the output impedance of duplexers 24 and 26 with the impedance specified for the inputs 18 and 20 of the diplexer, the warranty of the manufacturer regarding the operational specifications of the diplexer may not be met. The diplexer 14 has a cost of approximately $0.50 per unit. This cost appreciably contributes to the cost of a mobile telephone 10. Therefore, the prior art dual band mobile telephone of Fig. 1 has a two-fold disadvantage. First, the use of commercially purchased diplexers 14 does not permit out-of- band operation within the diplexer manufacturer ' s specifications of matching the output impedance of the radio duplexers 24 and 26 with the input impedance of the inputs at which the diplexer is specified to operate. Furthermore the cost of the diplexer, which also requires additional manufacturing operations of surface mounting, adds considerably to the overall cost of a mobile telephone. SUMMARY OF THE INVENTION The present invention is a radio operating in a plurality of frequency bands equaling n which eliminates the use of a multiplexer (diplexer when operated in two bands) surface mounted on a printed circuit board to provide coupling with the radio antenna. In accordance with the invention, the prior art surface mounted diplexer is replaced with a multiplexer which is formed with printed electrical conductors on a printed circuit board within the radio. A number of radio modules, also equal to n, are mounted on the printed circuit board each having a different operating frequency band of the n bands and having an output with an output impedance. The multiplexer has n inputs and an output. Each of the n inputs of the multiplexer is coupled to the output of a different radio module and the output of the multiplexer is coupled to the radio antenna. The output impedance of each radio module, which is coupled to a different input of the multiplexers, is only matched to the impedance of the antenna for an operating band and is mismatched to the impedance of the antenna for any other of the n-1 frequency bands. The printed circuit conductors of the multiplexer comprise n transmission lines. Each transmission line is located in a different input, transforms the output impedance of each radio module presented to the antenna and the transforming is such that a matched impedance is still presented to the antenna in the operating band of the radio module connected to the transmission line and that substantially an open circuit is still presented to the antenna in the other n-1 frequency bands of the other radio modules which enhances the efficiency of the antenna coupling when compared to the prior art of Fig. 1. The radio modules in a preferred embodiment of the invention comprise transceiver modules in a mobile telephone. In a preferred embodiment, the number of bands is equal to two with one of the bands being 824- 894 MHz and another of the bands being 850-990 MHz. Each radio module has a reflection coefficient of at least 0.90 in the operating band of other radio modules. With the aforementioned high reflection coefficient, the substantially mismatched output impedance of each of the n radio modules for the other n-1 frequency bands is transformed by the transmission line having a length less than a full wavelength of the midrange of the operating frequency band to an open circuit which prevents the loading of the inactive radio modules with the output from the active radio module. This mode of antenna coupling results in a substantial impedance match between the output of the radio module for the operating frequency to the antenna without the transfer of substantial RF power to the n-1 other radio modules. A radio which operates in n frequency bands in accordance with the invention includes n radio modules each having an output, an antenna, and a multiplexer having n inputs and an output with each of the n inputs being coupled to the output of a different radio module and the output of the multiplexer being coupled to the antenna; and wherein the multiplexer consists of printed electrical conductors on a circuit board. The printed electrical conductors form n transmission lines, each transmission line being located in a different input, presenting a matched impedance to the antenna in the operating band of the radio module connected thereto and substantially an open circuit to the antenna for n-1 other frequency bands. Each radio module has a reflection coefficient of at least 0.9 in magnitude in the operating frequency band of the n-1 other radio modules. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a block diagram of a prior art dual band mobile telephone having a discrete diplexer mounted on a circuit board. Fig. 2 illustrates a radio operating in n frequency bands in accordance with the present invention. Fig. 3 illustrates an example of the present invention used in a dual band mobile telephone. Like reference numerals identify like parts throughout the drawings. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 illustrates a radio 100 in accordance with the present invention which operates in n frequency bands. While a preferred embodiment of the present invention is a dual band mobile telephone, the present invention is not limited to two band operation and further may be utilized in diverse radio applications besides mobile telephones. The description of the like parts in Fig. 1 is incorporated herein by reference and below with reference to Fig. 3. The present invention differs from the prior art of Fig. 1 by providing a mutliplexer 102 which is formed without the surface mounted diplexer 14 of the prior art of Fig. 1 and comprises n transmission lines 104 which are each formed by electrical conductors printed on printed circuit board 16 by conventional circuit board manufacturing techniques (which are not part of the present invention) on a surface of the printed circuit board 16. As a consequence of direct deposition of the multiplexer by conventional printed circuit board manu acturing techniques on a surface of the printed board 16 from electrical conductors, the cost of the discrete diplexer 14 of the prior art is eliminated which substantially reduces the manufacturing cost of a multiband mobile telephone by approximately $0.50 per unit. Each of the radio duplexers (modules) 24 and 26 has an output impedance which is only matched to the impedance of the antenna 12, e.g. 50 ohms, for the operating band of the particular module and is mismatched to the impedance of the antenna for the n-1 other bands of the other radio modules. Each of the n transmission lines 104 is located in a different input 18 and 20, transforms the output impedance of each radio module presented to the antenna 12 and the transforming is such that a matched impedance is still presented to the antenna in the operating band of each radio module connected to the transmission line and that substantially an open circuit is presented to the antenna 12 in the other n-1 frequency bands of the other radio modules. The presenting of a substantially open circuit impedance to the antenna 12 for the n-1 operating bands of the other radio modules minimizes loading of the active duplexer consequent from at least one other of the n-1 duplexers (only one is illustrated) . As a result, substantially all of the RF power for radio operation is coupled between the antenna 12 and the operating radio duplexer and substantially no RF power is transferred to each of the other n-1 duplexers or RF modules which is not operating. The duplexers or RF modules are designed to have an operating band with the output impedance thereof matching the impedance of the antenna. In order to effectively transform the output impedance for each of the radio duplexers 24 and 26 for the operating bands of the other duplexers to substantially an open circuit, it is desirable for the reflection coefficient of each of the radio modules to have a magnitude of at least 0.90 in the operating band of the other radio modules. It is well known to those persons skilled in the art than the reflection coefficient is comprised of a magnitude and phase angle and is equal to the ratio of the reflected voltage or current divided by the incident voltage or current. When there is a substantial mismatch between the output impedance of each of the radio duplexers, the operating band for the other duplexers and the impedance of the antenna 12, the length of the transmission line located in the input is calculated, as is described below, to have a length equal to a fractional wavelength at the midrange operating frequency for the operating band(s) of the other radio module (s) which transforms the substantially mismatched output impedance of the radio duplexer for the operating band of the other radio module (s) to substantially an open circuit when coupled to the antenna 12. In a perfect situation, where the reflection coefficient is equal to 1, the impedance of each radio duplexer is located on the outside diameter of a Smith chart which is a well-known mechanism used for determining transmission line terminations and impedances at different positions on a transmission line. See Chapter 3 of Microwave Devices and Circuits , Liao, Prentice Hall Inc. ©1980. However, as stated below, a preferred are of operation on a Smith chart is with a reflection coefficient of 0.90 or greater. A reflection coefficient of 0.90 or greater for each radio duplexer facilitates the achieving of a substantially open circuit impedance presented to the antenna for the operating band of the other radio duplexers. When the number of radio modules or duplexers is greater than two, each of the other non-operating radio modules or duplexers must be considered in the presenting of a substantially open circuit for all of the other non- operating radio modules or duplexers to the antenna 12 for the operating bands of the other modules. In this circumstance, optional frequency selective circuits 106 may be located in one or more of the inputs of the multiplexer 102, which may be low or high pass filters comprised of capacitive and inductive elements, to add impedance to one or more of the inputs to the antenna 12 in order to present a sum of the individual output impedances for the operating band of the other radio modules which is a substantially open circuit to the antenna from the operating band of all of the other radio modules or duplexers. Example The following example illustrates calculation of the length of the transmission lines 104 which are required to couple the radio modules illustrated in Fig. 3 to the antenna 12 to produce a substantial open circuit for out-of-band operation. Freq. (MHz) [1 [3 wherein | ~ is the reflection coefficient 824-894 0.97/50° 0.18/310° 1850-1990 0.2/ 170° 0.92/140° a) 824-894 MHz Operation In order to transform fl to a near open circuit, the electrical length of ^ is 50° or 0.14λ at 860 MHz. Assuming a microstrip formed by printed circuit board manufacturing techniques on a known FR4 substrate (effective dielectric constant 3.5), the physical length of <_ η is given by = 26.1mm wherein Eeff is the effective dielectric constant of the icrostrip and = 0.97Z0 0 . The impedance presented to the antenna 12 by the 1850-1990 MHz. arm (duplexer 26 + βj) at 824-894 MHz is Z 2 = 3.3 kΩ Power loss due to Z 2 is calculated to be 0.07dB whereas the loss of the diplexer 14, is about 0.5dB. Meanwhile, the 824-894 MHz. arm (duplexer 24 + Z 2 ) will remain matched because [3 has low magnitude (0.18) and fo has Zo=50Ω. b) 1850-1990 MHz Operation To transform [3 to near open (| ~ 4 = 0.92Z0 0 ), the electrical length of 1.2 is 140° or 0.39λ at 1920 MHz. using microstrip on the aforementioned FR4 substrate, the physical length is given by 3 10 8 1 = 32.6mm and [4 = 0.92Z0". The impedance presented to the antenna 12 by the 824-894 MHz. arm (duplexer 24 + h) at 1850-1990 MHz. is Z 4 = 1.2 KΩ Power loss due to Z„ is calculated to be ~ 0.2dB versus 0.5dB loss produced by the diplexer 14 of the prior art. Likewise, the matched condition of the 1850-1990 MHz arm (duplexer 26 + Ci) is unaffected. The above calculation has assumed that it and are lossless. In practice, the power loss of the transmission lines will slightly decrease the loading impedance and thus increase the power loss of the active arm. However, the losses of such short lines are very low (<0.2dB) and the actual power loss will be at least comparable to the diplexer 14. In order to improve the performance of the multiplexer 102 relative to the prior art diplexer 14, the magnitude of reflection coefficient for the operating bands of the n-1 other radio modules is preferable chosen to be 0.90 or above. While the invention has been described in terms of its preferred embodiments, it is understood that numerous modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. It is intended that all such modifications fall within the scope of the appended claims.

Claims: Claims 1. A radio which operates in n frequency bands comprising: n radio modules, each radio module operating in one of the n frequency bands and having an output with an output impedance; an antenna having an impedance; and a multiplexer having n inputs and an output with each of the n inputs being coupled to the output of a different radio module and the output of the multiplexer being coupled to the antenna; and wherein the output impedance of each radio module is only matched to the impedance of the antenna for an operating frequency band and mismatched to the impedance of the antenna for any other n-1 frequency bands of other radio modules and the multiplexer comprises n transmission lines, each transmission line being located in a different input, transforming the output impedance of each radio module presented to the antenna and the transforming is such that a matched impedance is still presented to the antenna in the operating band of the each radio module connected to the transmission line and that substantially an open circuit is presented to the antenna in the other n-1 frequency bands of the other radio modules. 2. A radio in accordance with claim 1 wherein: the radio modules comprise transceiver modules. 3. A radio in accordance with claim 1 wherein: n is equal to 2 ; and one of the bands is 824 to 894 MHz and another one of the bands is 1850 to 1990 MHz. 4. A radio in accordance with claim 2 wherein: n is equal to 2; and one of the bands is 824 to 894 MHz and another one of the bands is 1850 to 1990 MHz. 5. A radio in accordance with claim 1 wherein: each radio module has a reflection coefficient of at least 0.90 in the operating frequency band of the other radio modules. 6. A radio in accordance with claim 2 wherein: each radio module has a reflection coefficient of at least 0.90 in the operating frequency band of the other radio modules. 7. A radio in accordance with claim 3 wherein: each radio module has a reflection coefficient of at least 0.90 in the operating frequency band of the other radio modules. 8. A radio in accordance with claim 4 wherein: each radio module has a reflection coefficient of at least 0.90 in the operating frequency band of the other radio modules. 9. A radio in accordance with claim 1 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 10. A radio in accordance with claim 2 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 11. A radio in accordance with claim 3 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 12. A radio in accordance with claim 4 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 13. A radio in accordance with claim 5 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 14. A radio in accordance with claim 6 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 15. A radio in accordance with claim 7 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 16. A radio in accordance with claim 8 wherein: the multiplexer including n transmission lines comprises printed electric conductors on a printed circuit board within the radio. 17. A radio in accordance with claim 5 wherein the multiplexer further comprises: a frequency selective circuit. 18. A radio in accordance with claim 17 wherein: the frequency selective circuit comprises a low pass filter. 19. A radio in accordance with claim 17 wherein: the frequency selective circuit comprises a high pass filter. 20. A radio in accordance with claim 6 wherein the multiplexer further comprises: a frequency selective circuit. 21. A radio in accordance with claim 20 wherein: the frequency selective circuit comprises a low pass filter. 22. A radio in accordance with claim 20 wherein: the frequency selective circuit comprises a high pass filter. 23. A radio in accordance with claim 7 wherein the multiplexer further comprises: a frequency selective circuit. 24. A radio in accordance with claim 23 wherein: the frequency selective circuit comprises a low pass filter. 25. A radio in accordance with claim 23 wherein: the frequency selective circuit comprises a high pass filter. 26. A radio in accordance with claim 8 wherein the multiplexer further comprises: a frequency selective circuit. 27. A radio in accordance with claim 26 wherein: the frequency selective circuit comprises a low pass filter. 28. A radio in accordance with claim 26 wherein: the frequency selective circuit comprises a high pass filter. 29. A radio which operates in n frequency bands comprising: n radio modules each having an output; an antenna; and a multiplexer having n inputs and an output with each of the n inputs being coupled to the output of a different radio module and the output of the multiplexer being coupled to the antenna; and wherein the multiplexer consists of printed electrical conductors on a circuit board. 30. A radio in accordance with claim 29 wherein: the printed electrical conductors form n transmission lines, each transmission line being located in a different input, presenting a matched impedance to the antenna in the operating band of the radio module connected thereto and substantially an open circuit to the antenna for n-1 other frequency bands. 31. A radio in accordance with claim 30 wherein: each radio module has a reflection coefficient of at least 0.9 in magnitude in the operating frequency band of other radio modules.

Inventors: FU TAOLING, FU, TAOLING
Applicants: FU TAOLING, NOKIA MOBILE PHONES LTD, FU, TAOLING, NOKIA MOBILE PHONES LIMITED
Application year: 2000
Main classification: H04B1
IPC codes: H04B1