A hot standby transmitter switching system

Abstract: In a hot standby transmitter system, each identical transmitters, one of which is a primary and another one is a standby, comprises an image-cancel type mixer (41, 51), which outputs upper or lower sideband waves of a local frequency, according to its internal connection controlled by a switch controller (3). The switch controller (3) controls the internal switches (416, 516) and an RF switch (6), so that the primary transmitter outputs a predetermined primary one of the sidebands and the standby transmitter outputs another sideband, and the RF switch (6) selectively connects the output of the primary transmitter to outside. The standby transmitter undesirably outputs a small level of the primary sideband though it should be theorectically zero. This low level primary wave component in the standby transmitter's output allows a reduction in the isolation amount of the RF switch (6), compared with high isolation amount required in a prior art method where the standby transmitter outputs the primary wave in as high as the primary transmitter level, in order to keep the ratio of power level from the primary transmitter to the power level from the standby transmitter higher than a predetermined requirement level. The circuit to detect a fall in the output power is simple compared with a complex circuit required in another prior art method where the standby transmitter outputs the less primary wave than the primary transmitter.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a system to switch a hot standby auxiliary transmitter in a radio transmitting apparatus. Description of the Related Art A radio transmitting system is generally provided with an auxiliary transmitter in order to improve the operation reliability of the system. For this auxiliary system, there are two types, i.e. an auxiliary route system and a standby transmitter system. In the latter system, there are further two types. The first one is a hot standby transmitter where, while a primary transmitter is outputting its power, an auxiliary transmitter stands-by with its power source voltage applied thereto so as to keep the auxiliary transmitter operating in order to avoid a drift in its operation characteristics taking place on application of its operating voltage to the standby transmitter. The second one is a cold standby system where the auxiliary transmitter stands-by without being applied with its power source voltage thereto. A hot standby transmitter system is shown in FIG. 1. In the figure, it is assumed now that a #1 transmitter 1 is outputting its power to antenna, and a #2 transmitter 25 is standing by, as an operating auxiliary transmitter, outputting an equal frequency and an equal power output level to those of #1 transmitter 1. A radio frequency switch 6 for selectively connecting one of the two transmitters to antenna is then connected to #1 transmitter 1. In this type of hot standby transmitter system, in the output of the radio frequency switch 6 a ratio D/U (Demand/Unnecessary) of the output power D from the primary transmitter 1 to the power U leaked through the radio frequency switch 6 from the auxiliary transmitter 25 must be more than a predetermined value. If this D/U ratio is small, the power leaked from the auxiliary transmitter deteriorates the characteristics of the output power from the primary transmitter, because the frequency and its phase of the output of the two transmitters 1 and 25 can not perfectly conform to each other. Details of FIG. 1 structure are hereinafter described. #1 transmitter 1 and #2 transmitter 25 are identically constituted and identically operate. Therefore, explanation of detail of the transmitter is made on #1 transmitter only. A baseband signal is input to #1 transmitter 1 and #2 transmitter 25 via a splitter circuit 22. In a modulator 11, an internal frequency of, for example, 70 MHz, is modulated by the input baseband signal. In a mixer 12, the modulated signal from modulator 11 is mixed with a transmitter local oscillation signal, which is generated in a transmitter local oscillator 14, and a radio frequency wave of the transmitter frequency f₁, for example, 6 GHz, is output therefrom. This radio frequency signal is amplified by a high power amplifier 13 up to a predetermined level, such as 1 watt, and is input to a radio frequency switch 6. An automatic gain control (AGC) circuit 15 keeps the output level of the amplifier 13 at a predetermined level, and outputs an alarm signal when the output level of the amplifier 13 becomes lower than a predetermined threshold level. Radio frequency switch 6 is composed of, for example, a first and second groups, D₁ and D₂, of PIN diodes. When radio frequency switch 6 is to connect #1 transmitter to antenna, a DC (direct current) current through the first group PIN diodes D₁ are cut off, and a DC current is applied to the the second group PIN diodes, each controlled by a control signal from a switch control circuit 21. An unnecessary signal components in the output signal from the radio frequency switch 6 are rejected by a band-pass filter 24. Output of the band-pass filter 24 is delivered to an antenna which is not shown in the figure. On the other hand, output signal from #2 transmitter 25 is reflected by the second group PIN diodes D₂ in the switch 6 and is absorbed in a dummy load, which is not shown in the figure. When AGC circuit 15 detects a fall in output level of amplifier 13, an alarm signal output therefrom triggers switch controller 21. Then, the switch controller 21 outputs the switch control signal to the radio frequency switch 6 so that the diode groups D₁ and D₂ are switched. That is, the first group diodes D₁ is applied with a DC current and the DC current in the second group diodes D₂ are cut off, so that the output signal of #2 transmitter passes through the transmitter switch 6, while the output signal of #1 transmitter is choked by the first group PIN diodes D₁ in the radio frequency switch 23. Output signal levels from radio frequency switch 6 is determined by its isolation characteristics, because the output levels of #1 transmitter and #2 transmitter are identical. On the other hand, D/U ratio of the output signal from the radio frequency switch 6 is required to be, for example, more than 40 dB. In order to satisfy this requirement, many of expensive PIN diodes, for example, three PIN diodes in the single group, must be used, which causes an increase in the cost of the system. Furthermore, in order to reduce the cost of the radio frequency switch 6, there may be employed another method that the output signal level from the standby transmitter is reduced by a predetermined amount so as to decrease the output level of the radio frequency switch 6. This method is functionally equivalent to an increase in the isolation of the radio frequency switch 6, without increasing the number of the PIN diodes. In this method, a threshold level of the AGC circuit 15 for detecting whether the transmitter is outputting normal power level must be adjusted when the standby transmitter signal level is reduced. This method requires a complicated circuit configuration of an output level detection circuit in the AGC circuit 15. In other words, there is a problem in that either of the radio frequency switch becomes expensive or the circuit configuration of the output level detection circuit becomes complicated. SUMMARY OF THE INVENTION It is an object of the present invention to provide a hot standby transmitter system which employs less expensive RF switch or less expensive circuit for detecting a fall in output power. According to the present invention each of identical transmitters, one of which is a primary transmitter and another one of which is a standby transmitter, comprises an image-cancel type mixers. The image-cancel type mixer output either one of upper or lower sideband waves of a local frequency signal, according to a change of internal connection of the image-cancel type mixer. A transmitter switch controller controls the internal connection and an RF switch which selectively connects outputs of the transmitters, so that the primary transmitter outputs a predetermined primary one of the two sideband waves, the standby transmitter outputs another sideband, i.e. an image of the primary wave, and the RF switch selectively connects the output of the primary transmitter to outside. The standby transmitter, which has been switched so as to output the image wave, outputs a theoretically zero, however, actually low level of the primary wave, too. The low level primary wave output from the standby transmitter allows a reduction in the isolation amount of the RF switch, while keeping the ratio of the primary sideband level from the primary transmitter to the primary sideband component level from the standby transmitter higher than a predetermined requirement level. Moreover, the circuit to detect a fall in the transmitter output power level can be simple because of the identical output power levels of the two transmitters. The above-mentioned features and advantages of the present invention, together with other objects and advantages, which will become apparent, will be more fully described hereinafter, with reference being made to the accompanying drawings which form a part hereof, wherein like numerals refer to like parts throughout. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a block diagram of a standby transmitter system; FIG. 2 shows a block diagram for explaining the principle of the present invention; FIG. 3 shows a preferred embodiment of the present invention; FIGs. 4 show a block diagram for explaining the function of the image-cancel type mixer having a signal switch therein; and FIGs. 5 show levels of the signal measured at the input terminals and at the output terminal of the radio frequency switch. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to a block diagram of FIG. 2, the principle of the present invention is explained. The numerals 41 and 51 respectively denote first and second image-cancel type mixers, which are substantially identical to each other. An input signal of an internal frequency f IF (referred to hereinafter as IF) of, for example, 70 MHz, is input to the first image-cancel type mixer 41. A local frequency signal of frequency f LO , for example, 6000 MHz, is also input to the image-cancel type mixer 41. The image-cancel type mixer 41 mixes its input signals of frequency f IF and local frequency f LO , so as to output an upper sideband wave, i.e. an added frequency (f LO + f IF ) = 6000 MHz + 70 MHz = 6070 MHz. or a lower sideband wave, i.e. a subtracted frequency (f LO - f IF ) = 6000 MHz - 70 MHz = 5930 MHz. A signal switch 416 in the image-cancel type mixer 41 selects which of the upper or lower sideband waves the image-cancel type mixer 41 outputs. Configuration and operation of the image-cancel type mixer is described later in detail. A transmitter switch controller 3 outputs control signals to signal switch 416 and a signal switch 516 in the second image-cancel type mixer 51, and to a radio frequency switch (referred to hereinafter as RF switch) 6. Input terminals 1 ○ and 2 ○ of RF switch 6 are connected to outputs of the first and second image-cancel type mixers 41 and 51, respectively. An output terminal 3 ○ of RF switch 6 is connected to an antenna, which is not shown in FIG. 2. Control signals output from the transmitter switch controller 3 determine that: primary one of the image-cancel type mixers 41 (or 51) out of the two image-cancel type mixers outputs a predetermined primary one of the upper or lower side band waves, and another image-cancel type mixer 51 (or 41) , as an auxiliary, i.e a standby, outputs another sideband wave than that the primary image-cancel type mixer, and RF switch 6 selectively connects the output of the primary image-cancel type mixer to the antenna. In other words, the primary wave is output from a primary transmitter to antenna, and an image of the primary wave is output from the standby transmitter, and then, the transmitter switch controller 3 determines which the primary transmitter is. Referring to FIGs. 5, signal levels output from the first and second image-cancel type mixers are hereinafter explained, where it is assumed that the primary first image-cancel type mixer 41 is selected to output the lower sideband wave LSB through RF switch 6 while the standby second image-cancel type mixer 51 is selected to output the higher sideband wave USB, which is the image frequency of the lower sideband LSB. FIG. 5 1 ○ shows signal levels measured at an output terminal 1 ○ of the first image-cancel type mixer 41. FIG. 5 2 ○ shows signal levels measured at an output terminal 2 ○ of the second image-cancel type mixer 51. FIG. 5 3 ○ shows signal levels measured at an output terminal 3 ○ of RF switch 6. In FIGs. 5, dotted lines indicate reference signal levels which do not actually appear at each measuring point, for easy comprehension. In FIGs. 5 1 ○ and 5 2 ○ , level L T1 of LSB and level L i2 of USB are 3 dB higher than the output level L s of local mixers 412 and 413 in the image-cancel type mixer because the level L T1 and L i2 are respectively sum of the outputs of first and second mixers 412 and 413 as described later in formulas (3) and (6). Though the residual amount of the image frequency wave USB of the primary image-cancel type mixer should be theorectically zero as described later, there still is some residual image level in the practical circuit structure, as denoted with level L i1 in FIG. 5 1 ○ . That is, level L i1 of USB is suppressed by the image canceling amount L c , for example, 20 db, down below the mixer output level L s . Because of the same reason, the output of the standby image-cancel type mixer 51 includes its image frequency wave LSB having a level L i2 . Thus, as seen in FIG. 5 2 ○ , at the input terminals of the RF switch 6, there appear a desirable wave (D) , i.e. LSB in this case, of level L T1 from the primary image-cancel type mixer 41 and an unnecessary LSB wave (U) of level L T2 from the standby image-cancel type mixer 51. Consequently, the D/U ratio measured at the input terminals of the RF switch becomes (L c + 3) db. The D/U ratio in the prior art configuration shown in FIG. 1 is 0 dB, because in prior art FIG. 1 configuration the signal levels of the desired frequency waves measured at the input terminals of the RF switch are equal. This means that the present invention virtually increases the isolation of the RF switch by (L c + 3) dB. Therefore, amount of the isolation of an RF switch 6 can be less than that of the prior art RF switch 23 by the amount of the virtual isolation increase, (L c + 3) dB. FIG. 5 3 ○ shows an output spectrum of the RF switch 6, where the levels of both the output spectrums LSB and USB from the standby image-cancel type mixer are respectively lowered, below the dotted lines levels L i2 and L T2 corresponding to the input levels shown in FIG. 5(2) , down to the levels L i21 and L T21 denoted with solid lines, by the isolation amount L ISO . Consequently, the final D/U ratio measured at the output of the RF switch becomes (L c + 3 + L ISO ) dB. Increase in the virtual isolation amount is same even when the image-cancel type mixer 41 and 51 and the RF switch 6 are switched with each other so that first image-cancel type mixer 41 selects the upper sideband USB output and the second image-cancel type mixer 51 selects the lower sideband LSB, because the first image-cancel type mixer 41 and the second image-cancel type mixer 51 are identical and the RF switch is symmetric. Referring to a block diagram of FIG. 3, a preferred embodiment of the present invention is hereinafter described in detail. As described above, the second transmitter 5 is identical to the first transmitter 4; therefore, description of the first transmitter 4 represent the description of the second transmitter 5. The first transmitter 4 comprises: a pre-amplifier 47; an image cancel type mixer 41; a band rejection filter 43; a power amplifier 44; an AGC (automatic gain control) circuit 45; and a local frequency oscillator 46. The image-cancel type mixer 41 includes a first signal splitter 411; a signal switch 416; a second signal splitter 414; a first local mixer 412; a second local mixer 413; and a hybrid circuit 415. An IF signal, for example, 70 MHz, modulated with a base band signal (simply referred to hereinafter as an IF signal) is input to pre-amplifier 47. Functions of the above-described components in the first transmitter 4 are described later in detail. Outputs of the first transmitter 4 and the second transmitter 5 are input to an RF switch 6 via a circulator 47 and 48, respectively. RF switch 6 is controlled by a transmitter switch controller 3 so as to selectively output proper one of the outputs from the two transmitters as described above to a third circulator 51. An output of third circulator 51 is input to a band pass filter 7. An output of band pass filter 7 is transmitted typically to an antenna which is not shown in the figure. Referring to FIGs. 4, the constitution and operation of image-cancel type mixer 41 including signal switch 416 are hereinafter explained. An output signal of pre-amplifier 47 is input to a first 90°-splitter 411. It is now assumed that the input signal to first 90°-splitter 411 is sinω IF t, and a local frequency signal input to a second 90°-splitter 414 from the local oscillator is cos LO t. The input signal sinω IF t is divided by first 90°-splitter 411 into sinω IF t, a same phase signal B, and cosω IF t, which is 90°-delayed signal A after the input sinω IF t. 90°-splitter 411 is of widely known and commercially available one. Signal switch 416 is a double-pole double-throw switch, which may be composed of semiconductor elements, such as diodes or transistors, according to a widely known technique. Now, switch 416 is at the position shown in FIG. 4(a), where signal A is connected via terminal A′ to the first mixer 412, and signal B is connected via terminal B′ to the second mixer 413. Second splitter 414, which is a 90°-splitter is of widely known and commercially available one, and divides the 6000 MHz input local frequency signal and outputs a same-phase local frequency signal, cosω LO t, to first mixer 412 and a 90°-delayed local frequency signal, sinω LO t, to second mixer 413. When the first and second mixers 412 and 413 are respectively input with the above-described signals, first mixer 412 outputs: Sm₁ =- 1 2 [cos(ω IF t + ω LO t) + cos(ω LO t - ω IF t)] and second mixer 413 outputs: Sm₂ =- 1 2 [cos(ω IF t + ω LO t) - cos(ω LO t - ω IF t)] These two signals are summed in a co-phase hybrid circuit 415, which then outputs: -cos(ω LO t + ω IF t) This is an upper side band wave USB of the local frequency f₀ and includes none of the lower sideband wave LSB, cos(ω LO t - ω IF t), which is an image frequency of the first transmitter 4. Therefore, this type of mixer circuit is called an image-cancel type mixer. The co-phase hybrid circuit 415 is of well-known and commercially available one. Next, when the connection of signal switch 416 is switched as shown in FIG. 4(b), where signal A is connected to terminal B′, and signal B to terminal A′, first mixer 412 outputs: Sm 1′ = 1 2 [sin(ω IF t + ω LO t) - sin(ω LO t - ω IF t)] and second mixer 413 outputs: Sm 2′ = 1 2 [sin(ω IF t + ω LO t) + [sin(ω LO t - ω IF t)] Then, co-phase hybrid circuit 416 outputs: sin(ω LO t - ω IF t) This is a lower sideband wave LSB of the local frequency f₀ and includes none of the upper sideband USB, sin(ω LO t + ω IF t). Thus, according to the status of signal switch 416 the output frequency of the image-cancel type mixer, i.e. the output of the transmitter, is switched to proper one of the upper or lower sideband. A bandreject filter 43 rejects local frequency component f₀ undesirably included in the output of the image-cancel type mixer 41. Power amplifier 44 amplifies the output of the bandreject filter 43, including the primary lower sideband and some undesirably remaining upper sideband USB, up to, for example, 1 watt. RF switch 6 typically comprises first and second PIN diodes 6-1 and 6-2, connected to shunt the respective transmission lines coming in from first transmitter 4 and second transmitter 5. When a reverse DC voltage ia applied to first PIN diode 6-1, the PIN diode does not disturb the signal from first transmitter 4 to transmit through RF switch 6. Then, a DC current is applied to second PIN diode 6-2 so that the signal from second transmitter 5 is reflected by the second PIN diode 6-2. The reflected signal power is absorbed by a dummy load 50 connected to a second circulator 50, which is connected between the second transmitter 5 and RF switch 6 and is of widely known and commercially available one. When RF switch 6 is switched so as to reflect the power from first transmitter 4, the reflected power is absorbed by a dummy load 49 connected to a first circulator 47 connected between first transmitter 4 and RF switch 6. RF switch 6 having only a single PIN diode in each input line can provide an isolation amount L ISO of typically 20 dB, at 6 GHz. Therefore, the D/U ratio (L c + 3 + L ISO ) db measured in the output terminal of RF switch 6 achieves typically 20 + 3 + 20 = 43 dB, which is adequate to satisfy the requirement, 40 db. RF switch 6 using only a single PIN diodes in each input line thus replaces expensive prior art RF switch 23 using as many as two PIN diodes in each input line, and contributes to reduction of the cost of a hot standby transmitter system. AGC circuit 45 is of a widely known circuit to control the output level of amplifier 45 at a predetermined level, 1 watt, and also detects a fall in the output power level caused typically from a fault of the transmitter. When the AGC circuit 45 detects that the output level of the primary transmitter, i.e. of the first transmitter 4 in this case, becomes lower than a predetermined threshold level, a signal, as an alarm, is output therefrom to transmitter switch controller 3. Then, transmitter switch controller 3 confirms that no alarm is received from an AGC circuit (not shown in the figure) in the standby transmitter, i.e. the second transmitter 5 in this case, and transmits switch control signals to the first transmitter 4 so as to select upper sideband wave USB, to the second transmitter 5 so as to select the lower sideband wave LSB, and to RF switch 6 so as to allow the signal output from the second transmitter 5 to pass therethrough and reflect the signal output from the first transmitter 4. Then, the lower and upper sideband waves, LSB and USB, respectively transmitted from the second transmitter and the first transmitter are substantially identical to those of the previous switch state. Though in the above-described preferred embodiment a local frequency oscillator is provided in each transmitter, the local frequency signal may be supplied from an oscillator located outside the transmitter. Though in the above-described preferred embodiment a single PIN diode is employed for each input line of the RF switch 6, it is apparent that the quantity of the PIN diodes may be increased according to the isolation requirement. When a plurality of PIN diodes are employed, the PIN diodes are preferably located in a quarter wavelength pitch. Though in the above-described preferred embodiment the switching of the transmitters are carried out according to a signal detection a fall in the output power of the amplifier, it is apparent that the transmitter switching may be carried out according to a manual instruction. Though in the above-described preferred embodiment the RF switch is composed of PIN diodes, it is apparent that the present invention may be embodied in a system employing other type of RF switch than that employing PIN diodes. The advantageous effects of the present invention are summarized as follows: Compared with high isolation amount required in a method in Fig. 1 where the standby transmitter outputs the primary wave in as high as the primary transmitter level, the low level primary wave component in the standby transmitter's output allows a reduction in the isolation amount of the RF switch, while keeping the ratio of the signal level from the primary transmitter to the signal level from the standby transmitter higher than a predetermined requirement level. Though a complex circuit to detect a fall in the output power level is required in a method where the standby transmitter outputs the primary wave in a less signal level than the primary transmitter, the circuit can be simple in the present invention. The many features and advantages of the invention are apparent from the detailed specification and thus, it is intended by the appended claims to cover all such features and advantages of the system which fall within the scope of the invention.

Claims: A hot standby transmitter switching system of a radio transmitting apparatus, comprising: a first transmitter (4) and a second transmitter (5), each including an image-cancel type mixer (41, 51) which mixes a local frequency (f LO ) and a signal frequency (f IF ) and outputs a radio frequency wave selected from an upper sideband wave (f LO + f IF ; USB) and lower sideband (f LO - f IF ; LSB) wave of said local frequency (f LO ), each transmitter (4, 5) outputting an output wave of said image-cancel type mixer (41, 51) respectively installed therein, said image-cancel type mixer (41, 51) having a signal switch (416, 516) therein for determining said wave to be output from said image-cancel type mixer (41, 51); a transmitter switch controller (3) for outputting control signals to each of said signal switches (416, 516), so that said second transmitter (5) selectively outputs the other sideband wave (LSB or USB) than that which said first transmitter (4) outputs; and a radio frequency switch (6), controlled by said switch controller (3), for selectively outputting an output wave from a primary transmitter (4 or 5) which outputs a primary wave (USB or LSB) which is a predetermined one of said upper or lower sidebands (USB, LSB), wherein, while one of said transmitters (4 or 5) is outputting said primary wave (USB or LSB), the other transmitter (5 or 4) as a standby transmitter is kept operating to output the other sideband wave (LSB or USB) than said primary wave (USB or LSB). A hot standby transmitter switching system according to claim 1, characterized in that said image-cancel type mixer (41, 51) further comprises: a first 90° signal-splitter (411) for being supplied with a signal on an internal frequency carrier (IF=sinω IF t), and outputting a first signal (-cosω IF t) which is 90° delayed from said input signal (sinω IF t) and a second signal (sinω IF t) of the same phase to said input signal (sinω IF t); a second 90° signal-splitter (414) for being supplied with a local frequency signal (f IF =cosω LO t), and outputting a first local frequency signal (cosω LO t) of the same phase as said input local frequency signal and a second local frequency signal (sinω LO t) which is 90° delayed from said input local frequency signal; a first mixer (412) for being supplied with said first or second signal (-cosω IF t or sinω IF t) and said first local frequency signal (cosω LO t), and outputting a first mixed signal (Sm1, Sm1′); a second mixer (413) for being supplied with said second or first signal (sinω IF t or -cosω IF t) and said second local frequency signal (sinω LO t), and outputting a second mixed signal (sm2, Sm2′); and a hybrid circuit (415) for combining said first mixed signal (Sm1, Sm1′) and said second mixed signal (Sm2, Sm2′), and outputting a lower sideband (LSB=sin(ω LO t-ω IF t )) or a higher sideband (USB=-cos(ω IF t +ω LO t) of said local frequency signal (f IF ) according to a connection of said first and second signals (-cosω IF t, sinω IF t) to said first and second mixers (412, 413). A hot standby transmitter switching system according to claim 1, characterized in that each of said transmitters (4, 5) further comprises: an AGC circuit (45) for detecting a fall in output power level and outputting a signal to switch said switch controller (3). A hot standby transmitter switching system according to claim 3, characterized in that said fall in output power is detected by comparing with a predetermined threshold level. A hot standby transmitter switching system according to claim 4, characterized in that said fall in output power is caused from a fault in said respective transmitter (4, 5). A hot standby transmitter switching system according to claim 1, characterized in that each of said transmitters (4, 5) further comprises a local frequency signal generator (46) for outputting said local frequency signal (f LO ). A hot standby transmitter switching system according to claim 1, characterized in that each of said transmitters (4, 5) is of a multi-channel radio transmission type. A hot standby transmitter switching system according to claim 1, characterized in that said radio frequency switch (6) is composed of PIN diodes (6-1, 6-2).

Inventors: NISHIMURA AKIRA, NISHIMURA, AKIRA
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H04B1
IPC codes: H03C1, H04B1