Abstract:
An apparatus and method for providing an electrically adjustable RF delay in which a splitter splits an input signal into two signal paths, one signal path providing a delay fixed at an integral number of wavelengths of a desired center frequency and both signal paths providing electrically adjustable attenuation. A combiner combines the signals passing through the signal paths, such that the sum of the electrically-adjustable attenuation provided by the signal paths adds to unity, whereby the input signal is delayed by an adjustable time depending upon the attenuations provided by the signal paths.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to adjustable radio-frequency delay elements. More specifically, the present invention relates to apparatus for electrically adjusting delay in a radio-frequency systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     Various radio-frequency systems, notably feed forward amplifiers and predistorters, require the adjustment of delay through signal paths. This is typically done by varying lengths of transmission line, tuning filters for group delay, adding fixed delay increments, or other methods. All of these approaches require mechanical adjustment, circuit modifications, or other operations that greatly complicate the manufacturing process and increase cost. There is a need for a method to electrically adjust the delay in these applications without introducing complications or impairments that render the approach unworkable.  
       SUMMARY OF THE INVENTION  
       [0003]     According to one aspect of the invention, there is provided an electrically adjustable RF delay element, including a splitter to split an input signal into two signal paths. One signal path is delayed by a fixed integral number of wavelengths of a desired center frequency and both signal paths are attenuated using electrically adjustable attenuation such that the sum of the electrically adjustable attenuation provided by the signal paths adds to unity. A combiner then combines the signals passing through the signal paths. The input signal is delayed by an adjustable time depending upon the attenuations provided by the signal paths.  
         [0004]     Preferably, a balanced variable attenuator provides the splitter and signal paths and an RF delay element having a delay fixed at an integral number of wavelengths of a desired center frequency. The fixed delay element is connected between port  3  of the balanced variable attenuator and a first input of the combiner and a second input of the combiner is connected to port  2  of the balanced variable attenuator, whereby an RF signal applied to port  1  of the balanced variable attenuator is delayed by an adjustable time depending upon the degree of attenuation provided by the balanced variable attenuator and is provided at the output of the combiner.  
         [0005]     Preferably, the balanced variable attenuator is formed from identical shunt-mounted reflective attenuators are connected between a pair of quadrature hybrid couplers or by identical series-mounted reflective attenuators are connected between a pair of quadrature hybrid couplers. Preferably, the reflective attenuators are PIN diodes.  
         [0006]     According to another aspect of the invention, there is provided a method for providing an electrically adjustable delay of an RF signal. The method includes the steps of splitting the RF signal into two intermediate signals; delaying one intermediate signal by a fixed integral number of wavelengths of a desired center frequency; attenuating both intermediate signals by electrically-adjustable attenuation factors such that the sum of the attenuation factors is unity; and combining the intermediate signals into an output signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:  
         [0008]      FIG. 1  is a block diagram of an electrically adjustable delay element in accordance with an embodiment of the present invention;  
         [0009]      FIG. 2  is a block diagram of a conventional balanced attenuator using PIN diodes in a series configuration;  
         [0010]      FIG. 3  is a block diagram of a conventional balanced attenuator using PIN diodes in a shunt configuration;  
         [0011]      FIG. 4  is a block diagram of an electrically adjustable delay element in accordance with an embodiment of the present invention using PIN diodes in a series configuration;  
         [0012]      FIG. 5  is a block diagram of an electrically adjustable delay element in accordance with an embodiment of the present invention using PIN diodes in a shunt configuration; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Delay is equal to the negative of the first derivative of phase with respect to time. As such, it should be realized that any approach used to adjust delay must not just adjust phase, but actual delay.  
         [0014]     In general, each embodiment of the invention uses two signal paths, one that has a fixed delay greater than the other by N/F 0  where N is an integer and F 0  is the center frequency of the intended system. The non-delayed path is multiplied by a scalar A that can have a value from 0 to 1, and the delayed path is multiplied by a scalar equal to (1−A). The two products are then summed, and since the delay difference between them is equal to N wavelengths, they add in phase. The resulting delay of the combined signals is equal to zero when A=1, and is equal to N/F 0  when A=0. Between A=0 and A=1, the delay scales linearly. The useful bandwidth over which this relationship holds true is inversely proportional to N. In practical application, the limit of N is about 4 for reasonable bandwidth.  
         [0015]     Referring now specifically to  FIG. 1 , an electrically adjustable RF delay element in accordance with an embodiment of the invention is indicated generally by reference numeral  10 . The delay element  10  has an RF input port  12  and an RF output port  14 . Two signal paths run from the RF input port  12  to the RF output port  14 . An input signal received at the RF input port  12  is split by a splitter  16  into two paths which are recombined at a combiner  18  and feed to the RF output port  14 . The first of the two paths includes a first attenuator  20  and the second path includes a second attenuator  22  and a fixed delay element  24 . As mentioned above the two paths have delays that differ by N/F 0  seconds and the attenuators  20 ,  22  are adjustable so that the sum of their attenuations factors is constant.  
         [0016]     A difficulty arises in attempting to multiply the delayed and non-delayed signals by A and (1−A). While it is possible to split an input signal, multiply the output signals with separate variable attenuators by A and (1−A), delay the second signal, and then recombine, there are problems with this approach. First, A and (1−A) are linear parameters which are difficult to accurately produce using conventional attenuators, which are typically linear in dB. It is doubly difficult to produce A and a perfectly matching (1−A). Second, splitting the input signal in two and then later recombining yields a minimum insertion loss of 6 dB. It is desirable to keep the insertion loss as low as possible. Hence, while it is believed that an embodiment of the invention shown in  FIG. 1  using conventional attenuators can provide an electrically variable delay, the other embodiments described below in relation to  FIGS. 4 and 5 , which utilize modifications of the conventional balanced variable attenuators depicted in FIGS.  2  and  3 , are preferable.  
         [0017]      FIG. 2  depicts a conventional balanced variable attenuator, generally indicated by reference numeral  30 , in which identical series-mounted reflective attenuators  32 ,  34  are connected between an appropriately connected pair of quadrature hybrid couplers  36 ,  38 . Reflective attenuators  32 ,  34  are typically PIN diodes. The conventional balanced variable attenuator  30  has four ports, usually referred to as ports  1  to  4 . Ports  1  and  3  are connected to one quadrature hybrid coupler  36 , whereas ports  2  and  4  are connected to the other hybrid coupler  38 . Port  1  is connected to an RF input  40 , while port  2  is connected to an RF output  42 . The port  3  and  4  are terminated with terminators  44 ,  46 . If PIN diodes are used as attenuators  32 ,  34 , then the attenuation between the RF input  40  and the RF output  42  may be controlled by varying the bias on the attenuators  32 ,  34  in a manner understood by those skilled in the art.  
         [0018]     Similarly,  FIG. 3  depicts an alternative form of conventional balanced variable attenuator, generally indicated by reference numeral  50 , in which identical shunt-mounted reflective attenuators  52 ,  54  are connected between an appropriately connected pair of quadrature hybrid couplers  56 ,  58 . Reflective attenuators  52 ,  54  are typically PIN diodes. The conventional balanced variable attenuator  50  has four ports, usually referred to as ports  1  to  4 . Ports  1  and  3  are connected to one quadrature hybrid coupler  56 , whereas ports  2  and  4  are connected to the other hybrid coupler  58 . Port  1  is connected to an RF input  60 , while port  2  is connected to an RF output  62 . The port  3  and  4  are terminated with terminators  64 ,  66 . If PIN diodes are used as reflective attenuators  52 ,  54 , then the attenuation between the RF input  60  and the RF output  62  may be controlled by varying the bias on the reflective attenuators  52 ,  54  in a manner understood by those skilled in the art.  
         [0019]     The embodiments of the invention shown in  FIGS. 4 and 5 , are each a modification of one of the conventional balanced variable attenuators  30 ,  50  shown in  FIGS. 2 and 3 . As such, the same reference numerals are used where appropriate.  
         [0020]     In the electrically-adjustable RF delay elements indicated generally by reference numeral  70  in  FIG. 4  and reference numeral  80  in  FIG. 5 , the termination  46 ,  66  connected to port  3  of the balanced variable attenuator  30 ,  50  has been replaced by a delay element  72  connecting port  3  of the balanced variable attenuator  30 ,  50  to an input of a combiner  74 . The delay element  72  provides a fixed delay of N/F 0 , as discussed above, which means that the phase of a signal passing through the delay element  72  is unaffected. Port  2  of the balanced attenuator  30 ,  50  is now connected to the other input of the combiner  74  and the output of the combiner  74  becomes the RF output  76  of the electrically-adjustable RF delay element  70 ,  80 .  
         [0021]     The delay provided by either form of electrically-adjustable RF delay element  70 ,  80  is varied by adjusting the bias applied to the reflective attenuators  32 ,  34 ,  52 ,  54 . If the voltage transfer characteristic from port  1  to port  2  of either form of electrically-adjustable RF delay element  70 ,  80  is S 21  and the voltage transfer characteristic from port  1  to port  3  is S 31 , then |S 31 |+|S 21 |=1. Therefore |S 31 |=1−|S 21 |, which is the characteristic needed to feed the non-delayed and delayed paths. Stated equivalently, if the attenuation from node  1  to node  2  is A, then the attenuation from node  1  to node  3  must be (1−A), which is exactly what is needed. Further, the phases of the signals at the inputs of the combiner  74  are the same at −90 degrees. The combiner  74  is shown as an in-phase combiner although it is possible to use a quadrature combiner as long as the 90-degree phase difference is accounted for in the delay element  72 .  
         [0022]     This approach has two distinct advantages. First, scalars A and (1−A) are accurately created by the action of the modified balanced attenuator. This would be very difficult to accomplish with a pair of attenuators. Second, the insertion loss of this approach is 3 dB, which is 3 dB less than the approach using two attenuators.  
         [0023]     The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto by those of skill in the art without departing from the scope of the invention that is defined solely by the claims appended hereto.