Abstract:
The length of a transmission line delay circuit is halved by inserting a directional coupler in the signal path ahead of the transmission line and either open circuiting or short circuiting the other end of the transmission line. The signal traveling through the directional coupler and through the transmission line is reflected back at the end of the transmission line, and the reflected signal is carried to the coupled output of the directional coupler. Since the signal travels through the transmission line twice, the delay time is doubled. Therefore, for a given delay requirement, the length of transmission line required is only half of the length required for prior art transmission delay lines.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/599,746 filed Aug. 6, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to delay lines and, more particularly, to transmission line delay lines.  
         [0004]     2. Description of Related Art  
         [0005]     Transmission lines can be used as delay lines in many applications such as, for example, interference cancelling circuits used with radio transmitters and receivers, both fixed and mobile, and as delay lines in radar systems. In addition, such delay lines can also be used as signal storage media for sonar and electronic countermeasures equipment.  
         [0006]     Radio transmission towers that simultaneously transmit and receive radio signals create interference in the received signals when the transmitter is transmitting. The method usual used to attenuate this interference is to sample the transmitted signal, invert the signal and add the inverted signal to the received signal to cancel out the interference.  
         [0007]     While the interference arises from the coupling of the signal from the transmitting into the receiving antennas at the top of the tower, the circuitry to remove the interference is located at the base of the tower. Thus, the delay time for the signal to propagate from the transmitter up the tower to the transmitting antenna and back down again must be compensated for. Presently the signal delay is realized by passing the output signal from the transmitter through a length of transmission line that is approximately twice the height of the tower before the signal is inverted and added to the receiver input. The practical result is that a relatively large spool of costly cable sits at the base of the tower for each transmitter.  
         [0008]     It can be appreciated that a circuit and method that can essentially half the length of cable required for transmission delay lines is highly desirable.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In one aspect the present invention generally provides interference cancellation circuit that receives an interfering signal at an input terminal that is coupled to a port A of a directional coupler. A port B of the directional coupler is coupled to a first end of a transmission line, the other end of which is either open circuited or short circuited. A coupled output port C of the directional coupler passes a signal reflected from the open or short circuited end of the transmission line to a signal conditioning circuit. The signal conditioning circuit provides the delayed, conditioned compensating signal at an output terminal.  
         [0010]     Yet another aspect of this invention is to provide a delay circuit having a directional coupler having a port A for receiving an input signal, a port B for outputting the signal a port A to a one end of a transmission line, the other end of which is either open circuited or short circuited. A coupled port C output terminal captures the signal entering port B and provides the delayed input signal.  
         [0011]     A still further aspect of this invention is to provide a method for delaying a signal by passing the signal from port A through to port B of a directional coupler and into one end of a transmission line. The other end of the transmission line is either open circuited or short circuited causing the signal propagating from the directional coupler to be reflected back. An output port C captures the reflected signal that is essentially a delayed replica of the input signal.  
       OBJECTS OF THE INVENTION  
       [0012]     It is, therefore, one of the primary objects of the present invention to provide a transmission line delay circuit which uses only half of the transmission cable used in conventional transmission line delay circuits.  
         [0013]     Another object, of the present invention, is to provide a method of increasing the apparent directivity of the directional couplers used in the line-doubler delay line of the present invention.  
         [0014]     Yet another object, of the present invention, is to provide a simplified method for fine adjusting the delay of the delay line.  
         [0015]     In addition to the above-described objects and advantages of the present invention, various other objects and advantages will become more readily apparent to those persons who are skilled in the same and related arts from the following more detailed description on the invention, particularly, when such description is taken in conjunction with the attached drawing and appended claims.  
     
    
     DESCRIPTION OF THE DRAWING  
       [0016]      FIG. 1  is a schematic diagram of a prior art transmission line delay circuit;  
         [0017]      FIG. 2  is a schematic diagram of a line-doubler delay circuit according to the present invention;  
         [0018]      FIG. 3  is a graph of the calculated group delay of the transmission line delay circuit of  FIG. 1 ;  
         [0019]      FIG. 4  is a graph of the calculated group delay of the line-doubler delay circuit of  FIG. 2 ;  
         [0020]      FIG. 5  is a graph of the measured insertion loss for the delay circuits of  FIGS. 1 and 2 ;  
         [0021]      FIG. 6  is a graph of the measured group delay for the delay circuits of  FIGS. 1 and 2 ;  
         [0022]      FIG. 7  is a cascaded arrangement of the line-doubler delay circuit of  FIG. 2 ;  
         [0023]      FIG. 8  is a graph of the measured group delay of the transmission line delay circuit of  FIG. 1  and a two stage cascaded arrangement of the line-doubler delay circuit of  FIG. 2 ;  
         [0024]      FIG. 9  is a block diagram of a radio tower with an accompanying shelter; and  
         [0025]      FIG. 10  is a block diagram of an interference cancellation circuit which includes the line-doubler delay of  FIG. 2 . 
     
    
       [0026]     It will be appreciated that for purposes of clarity and where deemed appropriate, reference numerals have been repeated in the figures to indicate corresponding features, and that the various elements in the drawings have not necessarily been drawn to scale in order to better show the features of the invention.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     Turning now to the drawings,  FIG. 1  is a schematic diagram of a prior art transmission delay circuit shown generally as  10 . A signal input at terminal  12  appears at output terminal  14  after the time required for the signal to propagate through the transmission line  16 .  
         [0028]      FIG. 2  is a schematic diagram of an alternative delay line using the line-doubler delay circuit of the present invention, shown generally as  20 . The line-doubler delay circuit is implemented using a directional coupler  22  connected to a transmission line  24 . Significantly, the length of the transmission line  24  of the line-doubler  20  is only half the length of the transmission line  16  of  FIG. 1  to achieve the same delay.  
         [0029]     A signal applied to the input port  26  of the directional coupler  22  passes through the directional coupler  22  to a second port  28  and into a first end  30  of transmission line  24  to second end  32 . Advantageously, the second end  32  of the transmission line  26  is open circuited or short circuited causing the signal to reflect back through the transmission line  24  and enter into the second port  28  of the directional coupler  22 . A sample of the reflected signal is coupled onto a third port  34  of directional coupler  22 .  
         [0030]     Since the signal appearing at third port  34  travels from the first end  30  to the second end  32  of the transmission line  24  and back again, the total delay is twice the delay of a of a signal traveling in one direction only as occurs in the transmission line delay circuit of  FIG. 1 .  
         [0031]      FIG. 3  is a graph of the calculated group delay for the transmission line  16  of  FIG. 1  having a length of one thousand feet. As shown in  FIG. 3 , the group delay is approximately 825 nsec from 150-175 MHz.  
         [0032]      FIG. 4  is the same type of graph as  FIG. 3  but for the line-doubler of  FIG. 2 . As shown in  FIG. 4 , the group delay is also approximately 825 nsec, but with a transmission line of only 500 feet.  
         [0033]      FIG. 5  is a graph showing the measured insertion loss using the delay circuits of  FIGS. 1 and 2 . Line  40  is the insertion loss for the transmission line delay circuit of  FIG. 1 , and line  42  is the insertion loss for the delay circuit of  FIG. 2 .  
         [0034]      FIG. 6  is the plot of the laboratory measurements of the group delay for a six hundred foot section of LMR-400 using the transmission line delay circuit of  FIG. 1 , shown as trace  44 , and the plot for a three hundred foot section of LMR-400 using the line-doubler delay circuit of  FIG. 2 , shown as trace  46 . The approximate group delay for both lines is 730 nanoseconds, and the variation in group delay between the two traces is at most 5 nanoseconds. The directional coupler used in the test was a model C6998-10 manufactured by Werlatone, Inc.  
         [0035]     While the delay using the delay circuit of  FIG. 2  is realized simply and effectively in half the physical length of the transmission line compared to the delay circuit of  FIG. 1 , some parameters must be considered prior to implementation. These parameters include insertion loss of the coupled port and adequate directivity to isolate the forward wave from the reflected wave of the delayed signal. This balancing of forward versus reflected waves is the primary concern in realization of the line-doubler delay. In most cases commercially available components possess adequate performance to satisfy the above requirements.  
         [0036]     In cases where commercial components do not supply adequate directivity (typically &lt;30 dB), an alternative approach is possible as shown in the schematic of  FIG. 7 . The use of multiple directional couplers, in the fashion detailed below, assists in reducing the directivity requirement and actually enabling the realization of extremely long delays with much shorter physical cable lengths.  
         [0037]     As shown in  FIG. 7 , three line-doublers  50 ,  52 , and  54  are cascaded or placed in series to provide a single delay. The use of multiple directional couplers assists in reducing the directivity requirements and actually enabling the realization of extremely long delays with much shorter physical cable lengths. The use of multiple line-doublers, with the shorter transmission lines reduces the loss in signal strength between the signal entering the directional coupler  22  at the input port  26  relative to the signal entering the second port  28  to thereby effectively enhance the overall directivity of the combined directional couplers  22 .  
         [0038]     For example, instead of using a single six hundred foot length of coax, three one hundred foot lengths would be required using the circuit of  FIG. 7 .  
         [0039]      FIG. 8 , similar to  FIG. 6 , is a graph of the group delay for a fifty foot section of RG-223 using the circuit of  FIG. 1 , shown as trace  48 . Trace  50  is the group delay for two line-doublers each with twelve and one-half foot sections of RG-223. The variation in group delay between the two traces is at most 5 nanoseconds.  
         [0040]      FIG. 9  is a pictorial schematic of a radio transmission tower  60  having a base station shelter  62  and shown generally as  64 . The tower  60  has a transmitting antenna  66  and a receiving antenna  68  at the top of the tower  60 . A transmitter cable  70  and a receiver cable  72  couple the antennas to six radios  74  in the shelter  62 . Coupled between the transmission cables  70  and the six radios  74  is an interference cancelling circuit  76  and a signal combiner  78 . The interference cancelling circuit  76  couples the receiver cable  72  to the six radios  74 . The six transmission cables from the six radios are coupled through the interference cancelling circuit  76  to the signal combiner  78  that combines the six transmission signals and places the combined signal onto the transmitter cable  70 . Alternatively, the signal combiner  78  can be located in the interference cancelling circuit  76 .  
         [0041]     The interference canceling circuit  76  cancels the interference due to the coupling of the signal from the transmitting antenna  66  into the receiving antenna  68 .  
         [0042]      FIG. 10  is a block diagram of the interference cancelling circuit  76  of  FIG. 9 . As shown in  FIG. 10  six transmitter input signals at input terminals  80  pass through six directional couplers  82  before passing to the six transmitter antennas  66 . Each of the signals coupled from the six directional couplers  82  are passed to one of six line-doubler delay lines  84 . The outputs of the six line doubler delay lines  84  are input to six weighting circuits  86 .  
         [0043]     The six signals at input terminals  88  from the six receiving cables  72  are each passed through a first directional coupler  90  and a second directional coupler  92  before passing through the six output terminals  94 . An output  96  of each of the weighting circuits  86  is connected to the directional couplers  90  to inject the inverse of the six transmitter signals to thereby remove the transmitter interference. The sample output from the directional couplers  92  are input to the weighting circuits  86  and are used to adjust the amplitude and phase of outputs of the weighting circuits by techniques known in the art.  
         [0044]     An additional benefit of the line doubler approach is the single-ended nature of the delay line. This means the delay is achieved from the input of the directional coupler to the coupled port of the directional coupler. The single-ended delay line can be stored or placed in a remote location with no requirement to return the cable to get the sample from the far end of the cable.  
         [0045]     Lastly, the delay line can be trimmed at the far end by simply cutting off the end of the cable until the proper delay length is achieved. The far end, the open-circuited end, can be left unterminated, however, physically sealed or electrically capped if needed.  
         [0046]     Although the invention has been described in part by making detailed reference to a certain specific embodiment, such detail is intended to be, and will be understood to be, instructional rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made on the structure and mode of operation without departing from the spirit and scope of the invention as disclosed in the teachings contained herein.