Patent Publication Number: US-2005135520-A1

Title: Multi-branch radio frequency amplifying apparatus and method

Description:
FIELD OF THE INVENTION  
      This invention relates generally to radio frequency amplifiers, and more particularly, to multiple branched radio frequency level detectors having extended uniform dynamic range.  
     BACKGROUND  
      Logarithmic amplifiers can be divided into two basic classifications. These classifications are ‘true’ logarithmic amplifiers and demodulating logarithmic amplifiers. Generally speaking, demodulating logarithmic amplifiers provide the logarithm of the envelope of an input signal, and true logarithmic amplifiers provide the logarithm of the entire signal. For this reason, true logarithmic amplifiers are often referred to as ‘baseband’ logarithmic amplifiers, because they generally operate on ‘pulse’ type waveforms. Each type of logarithmic amplifier faces its own set of design challenges. For example, if a baseband log-amp is to resolve very short pulses or accurately track rapidly varying amplitude information, the dynamic range and the group delay as a function of input level are of prime concern. The dynamic range and group delay both relate to how accurately changes in ‘instantaneous’ power can be resolved (in timing and in log-magnitude), however large operational bandwidth is not required to accommodate an intermediate frequency (IF) or radio frequency (RF) carrier. In this case, the main design tradeoff is between the allowable input dynamic range and the maximum allowable group delay variation. In a situation where a demodulating logarithmic amplifier must provide the average power in an RF carrier without the aid of a down-conversion operation, bandwidth and input dynamic range are the chief concerns. Group delay variations are not important, because it is not necessary to resolve the fine detail of the envelope variations when computing a long-term ‘power’ average. Therefore the main design tradeoff is between the input dynamic range and the maximum allowable carrier frequency. Probably the most challenging applications for logarithmic amplifiers involve either the implementation of very wide bandwidth ‘true’ logarithmic amplifiers, or in performing fast video detection on a signal modulated by a carrier frequency. For the latter application, a logarithmic amplifier must be able to accommodate the desired carrier frequencies, and it must provide low group delay variation over the entire input dynamic range. In this case, maximum allowable carrier frequency, maximum allowable group delay variation, and allowable input dynamic range must be considered equally.  
      Some prior art solutions to this set of problems utilize a branched pair of power detectors. However these prior art solutions that fall under the classification of ‘extended dynamic range level-detectors’ generally require that the circuit adapt itself to the level of the input signal through the use of internal variable attenuators. The use of the variable attenuators and the implementation of all of the associated control logic adds an additional layer of complexity to the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with objects and advantages thereof, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:  
       FIGS. 1-3  are electrical schematic diagrams of branched radio frequency amplifiers consistent with certain embodiments of the present invention.  
       FIG. 4  is a graph of amplifier output versus radio frequency power input for a branched radio frequency amplifier consistent with certain embodiments of the present invention.  
    
    
     DETAILED DESCRIPTION  
      The invention is intended to extend the useful dynamic range of a demodulating logarithmic amplifier. A radio frequency level detector having extended uniform dynamic range contains a branching circuit that receives a radio frequency signal and sends it to two or more separate branches. One branch contains a fixed attenuator coupled to a rectifier, to create an attenuated rectified output that is proportional to the envelope of the radio frequency signal. The rectified signal is fed to a number of serially coupled limiting amplifier stages, and after each amplification stage the output is converted from a voltage signal to a current signal. All of the current signals are subsequently summed. This provides a current output signal that increases uniformly as a function of radio frequency power over a the first part of the dynamic range and remains constant as a function of radio frequency power over the second part of the range. The second of the two separate branches contains another fixed attenuator, which is larger than the previous fixed attenuator. The attenuated signal is fed to a radio frequency level detector circuit to create a current output signal that is nearly constant as a function of radio frequency power over the first part of the range and increases uniformly as a function of radio frequency power over the second part of the range. This current output signal is summed along with the current signals from the first branch to provide a single output current signal that increases smoothly and uniformly as a function of radio frequency power over the entire dynamic range. While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail, specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding elements in the several views of the drawings. When designing an RF level detector the chief design tradeoff involves the minimum required input dynamic range and the maximum required carrier frequency. When applied to existing logarithmic RF level detectors, the architecture in this section provides greater flexibility in performing this tradeoff. Referring now to  FIG. 1 , a radio frequency level detector  100  is arranged to receive an RF signal  102  at an input node  104 . The RF signal  102  is split into two signals  112 ,  114  and sent to two parallel branches  110 ,  115  of the detector  100 . The first or upper branch  115  contains a means for attenuating a signal  140 , such as an attenuator, coupled to a rectifying means or rectifier  150  coupled to a plurality of serially coupled amplifying means such as limiting amplifier stages  160 - 163  and a plurality of voltage-to-current converters (g m )  171 - 173 . The fixed attenuator  140  receives the RF signal  102 , attenuates it at a predetermined value, and then passes the attenuated signal to the rectifier  150 , where it is rectified to create a rectified signal that is proportional to the envelope of the radio frequency signal  102 . The means for rectifying can be a half-wave rectifier, a full wave rectifier, or a squaring cell. The rectified signal is then passed to a series of N limiting amplifiers  160 ,  161 ,  162 ,  163  that are serially connected, output of one to the input of the next (i.e. head to tail). Each limiting amplifier, with the exception of the first, has associated with it a transconductance cell (voltage-to-current converter)  171 ,  172 ,  173  at the output of the amplifier. In practice, there can be any number of N amplifiers and N-1 voltage-to-current converters, where N is an integer equal to or greater than two (2). Each sequential amplification stage increases the level of the RF signal, and after each amplification stage, the signal is passed to the next amplifier in the chain, and (except for the first amplifier) it is also passed to an associated means for converting, such as a transconductance cell, where the voltage signal is converted to a current signal. Each of the transconductance cells passes their current signals to a common means for summing all of the current signals, such as a summing cell  190  that totals all the values to create a summed value for the amplified upper branch signal. The result of this manipulation of the original RF signal  102  is a current signal that increases monotonically as a function of radio frequency power over a first range of RF power, and at a certain point, stops increasing and remains substantially constant as a function of RF power over the remainder of the power range. This phenomena can be seen in graphical form in  FIG. 4 , where the curve  401  represents the output current as a function of RF input power for the upper branch  115  of the level detector  100 . Note that the first portion  402  of the curve increases smoothly and generally monotonically as the RF power increases, up to approximately −10 dBm, whereupon the slope of the curve decreases and it flattens out in the second portion  403  to remain essentially constant over further increases in RF power. The terms ‘monotonic’ and ‘monotonically’ are commonly understood to refer to functions between partially ordered sets. A mathematical function is said to be monotonically increasing if, whenever x≦y, then f(x)≦(y). An increasing function is also called order-preserving for obvious reasons. Likewise, a function is decreasing if, whenever x≦y, then f(x)≧f(y). A decreasing function is also called order-reversing. If the definitions hold with the inequalities (≦,≧) replaced by strict inequalities (&lt;, &gt;) then the functions are called strictly increasing or strictly decreasing. Those of ordinary skill in the art will appreciate that the transition between the first range and the second range is not abrupt, but the slope of the curve changes continually over a brief intermediate span of RF power.  
      Returning now to  FIG. 1 , the second or lower branch  110  of the radio frequency level detector  100  contains a fixed attenuator  120  coupled to a logarithmic RF level detector  130 . This fixed attenuator  120  receives the raw RF signal  112  and has an attenuation factor that is larger than the fixed attenuator  140  in the upper or first branch. The reason for this is to produce an output that is essentially flat over the first portion or range of the power curve. The attenuator  120  clips the RF signal so severely that the output is very small compared to the respective output of the first or upper branch of the circuit over the same range of RF power input. The output level is close to zero over much of this range, although it does increase slightly near the higher end of the region. The output does not have to be zero, but it is important that it be very small in relation to the output value of the upper branch in this same RF power region, so as to not contribute substantially to the overall summed current output. The fixed attenuator  120  then passes the attenuated signal to the logarithmic RF level detector  130 , such as a successive compression detector, where it is amplified and converted into a current signal. A successive compression detector is but one type of circuit that may be employed as a logarithmic RF level detector  130 . This amplified second signal is then passed to the summing cell  190  where it is summed along with the current values from the upper or first branch  115  to produce a combined current output  195  for the radio frequency level detector  100 . Referring again to  FIG. 4  where the curve  405  depicts the output current as a function of RF input power for the lower branch  110  of the level detector  100 , note that the first portion  406  of the curve is essentially flat and remains generally constant as the RF power increases, up to approximately −10 dBm, whereupon the curve increases smoothly and generally monotonically in the second portion  407  over further increases in RF power. Note that the current output (y axis values) of the first portion  406  of the lower branch  110  are very small compared to the respective values in the first portion  402  of the upper branch  115  output. At the left end of the curve, the values approximate zero, and increase only marginally until about −10 dBm. Generally, the current output values of the lower branch are less than one tenth of the value of respective portions of those of the upper branch over the first range. When the current signals  401 ,  405  for both the upper and lower branches are summed at the summing cell  190  to produce a combined output  195 , the two curves become superimposed to create the result depicted in the topmost curve  410  in  FIG. 4 . Note that the combined current output signal  410  increases smoothly and monotonically as a function of radio frequency power over the entire range of RF power (the first range and the second range and the intermediate transition region).  FIG. 4  demonstrates that the instant invention achieves nearly ideal log-linear performance over an input RF power range of −38 dBm to 12 dBm. Through proper choice of the amplifier gains and transconductance values in the upper branch, the transition between the upper and lower branches is essentially unseen. The upper branch output  401  demonstrates the effect of omitting the transconductance cell at the input and output of the first limiting amplifier  160 . For values of RF IN &gt;˜−7 dBm, the upper branch forms an output current ‘pedestal’  403  on which the output current  407  from the lower branch is superimposed. It should be noted that both branches in  FIG. 1  are operational at all input power levels. Therefore, no variable attenuators (and no associated control logic) are required in order to select the appropriate branch as a function of input power level.  
      Although the embodiment depicted here contains two branches, the structure is easily expanded to incorporate additional branches. Each of the additional branches must be of the same form as the upper branch in  FIG. 1 . This means that each additional branch can have an arbitrary number of limiting amplifier stages, however the transconductance cells cannot be placed at the input or output of the first amplifier in the chain. This restriction is necessary so that the additional branch outputs will saturate at a given output current (i.e. form a ‘pedestal’ for another branch).  
      The structure depicted in  FIG. 1  can also be modified by performing linear full wave or half wave rectification in a distributed manner between the individual limiting amplifier stages. For example, rectification can be performed at the input to each limiting amplifier. In essence, the rectified output of each limiting amplifier is further rectified at the input to the following limiting amplifier.  
       FIG. 2  describes an alternate embodiment of the invention previously described. A radio frequency level detector  200  is arranged to receive an RF signal  102  at an input node  104 . The RF signal  102  is split into two signals  112 ,  114  and sent to two parallel branches  210 ,  215  of the detector  200 . The first or upper branch  215  contains a fixed attenuator  240  coupled to a plurality of serially coupled limiting amplifier stages  260 ,  261 ,  262 ,  263  and a plurality of voltage-to-current converters  271 ,  272 ,  273 . The fixed attenuator  240  receives the RF signal  102 , attenuates it at a predetermined value, and then passes the attenuated signal to a series of N (where N is an integer greater than 2) limiting amplifiers that are serially connected, output of one to the input of the next. Each limiting amplifier, with the exception of the first, has associated with it a transconductance cell (voltage-to-current converter)  271 ,  272 ,  273  at the output of the amplifier. Each sequential amplification stage increases the level of the RF signal, and after each amplification stage, the signal is passed to the next amplifier in the chain, and it is also passed to an associated transconductance cell, where the voltage signal is converted to a current signal. Each of the transconductance cells in turn passes their current signals to an associated linear rectifier  281 ,  282 ,  283 , and each rectified signal is then passed to a common summing cell  290  that sums up all the values to create a summed value for the amplified upper branch signal. The result of this manipulation of the original RF signal  102  is a current signal that increases monotonically as a function of radio frequency power over a first range of RF power, and at a certain point, stops increasing and remains substantially constant as a function of radio frequency power over the remainder of the power range.  FIG. 4  depicts this in graphic form, where the curve  401  represents the output current as a function of RF input power for the upper branch  215  of the level detector  200 . Note that the first portion  402  of the curve increases smoothly and generally monotonically as the RF power increases, up to a transition region between −10 dBm and 0 dBm, whereupon the curve flattens out in the second portion  403  to remain essentially constant over further increases in RF power.  
      Returning back to  FIG. 2 , the second or lower branch  210  of the radio frequency level detector  200  contains a fixed attenuator  220  coupled to a logarithmic RF level detector  230 . This fixed attenuator  220  receives the raw RF signal and has an attenuation factor that is larger than the fixed attenuator  240  in the upper or first branch. The reason for this is to produce an output that is essentially flat over the first portion or range of the power curve. The attenuator  220  alters the RF signal such that the output is very small compared to the respective output of the first or upper branch of the circuit over the same range of RF power input. The fixed attenuator  220  then passes the attenuated signal to the logarithmic RF level detector  230 , such as a successive compression detector, where it is amplified and converted into a current signal. A successive compression detector is but one type of circuit that may be employed as a logarithmic RF level detector  230 , and those of ordinary skill in the art are aware of other detectors that may be substituted with equal efficacy. This amplified second signal is then passed to the summing cell  290  where it is summed along with the current values from the upper or first branch  215  to produce a combined current output  295  for the radio frequency level detector  200 . Referring again to  FIG. 4  where the curve  405  depicts the output current as a function of RF input power for the lower branch  210  of the level detector  200 , note that the first portion  406  of the curve is essentially flat and remains generally constant as the RF power increases, up to approximately −10 dB, whereupon the slope of the curve continually changes through a transition region until about 0 dBm where the slope becomes constant and the log of the amp output increases smoothly and generally monotonically in the second portion  407  over further increases in RF power. Note that the current output values (y axis values) of the first portion  406  of the lower branch  210  are very small compared to the respective values in the first portion  402  of the upper branch  215  output. At the left end of the curve, the values approximate zero, and increase only marginally until about −10 dBm. Generally, the current output values of the lower branch are less than one tenth of the value of respective portions of those of the upper branch over the first range. When the current signals for both the upper and lower branch are summed at the summing cell  290  to produce a combined output  295 , the two curves are superimposed to create the result depicted in the topmost curve  410  in  FIG. 4 . Note that the combined current output signal  410  that increases smoothly and monotonically as a function of radio frequency power over the entire range of RF power.  
      This embodiment functions in a manner similar to that depicted by the structure of  FIG. 1 , except signal rectification in the low and intermediate range input power branches is performed after the signal is sampled at the output of each limiting amplifier. This leads to the additional constraint that each of the rectifiers in the upper branch of  FIG. 2  must be linear (i.e. no squaring cells). Each of the limiting amplifier cells in the embodiment depicted in  FIG. 2  operates at the RF carrier frequency, whereas in  FIG. 1  they do not. As in the previous embodiment, this embodiment can be expanded to include additional branches. Each of the additional branches must be of the same type as the upper branch, and transconductance cells can be placed at any point along the chain except at the input or output of the first limiting amplifier.  
       FIG. 3  illustrates yet another embodiment implemented with three limiting amplifier stages in the upper branch, and a 3-stage successive detection logarithmic level detector in the lower branch. A radio frequency level detector  300  is arranged to receive an RF signal  102  at an input node  104 . The RF signal  102  is split into two signals  112 ,  114  and sent to two parallel branches  310 ,  315  of the detector  300 . The first or upper branch  315  contains a fixed attenuator  340  coupled to a full wave rectifier  350  coupled to three serially coupled limiting amplifier stages  360 ,  361 ,  362  and two voltage-to-current converters  371 ,  372 . The fixed attenuator  340  receives the RF signal  102 , attenuates it at a predetermined value, and then passes the attenuated signal to the rectifier  350 , where it is rectified to create a rectified signal that is proportional to the envelope of the radio frequency signal  102 . The rectified signal is then passed to the three limiting amplifiers  360 ,  361 ,  362  that are serially connected, the output of one to the input of the next. Each sequential amplification stage increases the level of the RF signal, and after each amplification stage, the signal is passed to the next amplifier in the chain. The final two limiting amplifiers have associated with them a transconductance cell (voltage-to-current converter)  371 ,  372  at the output of the amplifier, where the voltage signal is converted to a current signal. All the current signals are passed to a common summing cell  390  that sums up all the values to create a summed value for the amplified upper branch signal. The result of this manipulation of the original RF signal  102  is a current signal that increases monotonically as a function of radio frequency power over a first range of RF power, and at a certain point, stops increasing and remains substantially constant as a function of radio frequency power over the remainder of the power range.  
      Optionally, an additional transconductance cell  370  may be added at the output of the first limiting amplifier  360 , and tied to the summing cell  390 , as shown by the dashed lines in  FIG. 3 . As described above, this transconductance cell is not normally present in this embodiment, but may be added as the circuit designer desires.  
      The second or lower branch  310  is similar to the upper branch  315  of the radio frequency level detector  300  in that it also contains a fixed attenuator  320  coupled to a full wave rectifier  352  coupled to three serially coupled limiting amplifier stages  365 ,  366 ,  367  which are coupled to four voltage-to-current converters  374 ,  375 ,  376 ,  377 . This fixed attenuator  320  receives the raw RF signal  102  and has an attenuation factor that is larger than the fixed attenuator  340  in the upper or first branch  315 . The reason for this is to produce an output that is essentially flat over the first portion or range of the power curve. The attenuator  320  clips the RF signal so that the output is very small compared to the respective output of the first or upper branch  315  of the circuit over the same range of RF power input. The fixed attenuator  320  then passes the attenuated signal to the rectifier  352 , where it is rectified to create a rectified signal that is proportional to the envelope of the radio frequency signal  102 . The rectified signal is then passed to a series of three serially connected limiting amplifiers  365 ,  366 ,  367 . Each limiting amplifier has associated with it a transconductance cell  375 ,  376 ,  377  at the output of the amplifier, and the first limiting amplifier  365  has a transconductance cell  374  tied to a common node between the output of the rectifier  352  and the input of the amplifier  365 . Each sequential amplification stage increases the level of the RF signal, and after each amplification stage, the signal is passed to the next amplifier in the chain, and it is also passed to an associated transconductance cell, where the voltage signal is converted to a current signal. Each of the transconductance cells passes their current signals to a common summing cell  390  where they are summed along with the current values from the upper or first branch  315  to produce a combined current output  395  for the radio frequency level detector  300 . The curve  405   FIG. 4  depicts the output current as a function of RF input power for the lower branch  310  of the level detector  300 . Note that the first portion  406  of the curve is essentially flat and remains generally constant as the RF power increases, up to approximately −10 dBm, whereupon the curve increases smoothly and generally monotonically in the second portion  407  over further increases in RF power. Note that the current output (y axis values) of the first portion  406  of the lower branch  310  are very small compared to the respective values in the first portion  402  of the upper branch  315  output. Generally, the current output values of the lower branch are less than one tenth of the value of respective portions of those of the upper branch over the first range. When the current signals  401 ,  405  for both the upper and lower branch are summed at the summing cell  390  to produce the combined output  395 , the two curves are superimposed to create the result depicted in the topmost curve  410  in  FIG. 4 . Note that the combined current output signal  410  increases smoothly and monotonically as a function of radio frequency power over the entire range of RF power.  FIG. 4  demonstrates that the instant invention achieves nearly ideal log-linear performance over an input RF power range of −38 dBm to 12 dBm. Through proper choice of the amplifier gains and transconductance values in the upper and lower branches, the transition between the upper and lower branches is essentially unseen. The upper branch output  401  demonstrates the effect of omitting the transconductance cell at the input and output of the first limiting amplifier  360 . For values of RF IN &gt;˜−7 dBm, the upper branch forms an output current ‘pedestal’ on which the output current from the lower branch is superimposed. It should be noted that both branches in  FIG. 3  are operational at all input power levels. Therefore, no variable attenuators (and no associated control logic) are required in order to select the appropriate branch as a function of input power level. Although the embodiment depicted here contains two branches, the structure is easily expanded to incorporate additional branches, but each of the additional branches must be of the same form as the upper branch  315 .  
      In summary, without intending to limit the scope of the invention, this architecture allows the fabrication of a wide dynamic range RF power detector using relatively inexpensive semiconductor fabrication processes. For example, the RF components in  FIG. 3  would only have to cover approximately 28 dB of dynamic range, instead of the full 50 dB range [−38 dBm, 12 dBm]. Because the lower branch has a larger attenuator preceding it, while the upper branch rectifier is in compression, the lower branch rectifier will be entering its linearity ‘sweet spot’. Those skilled in the art will recognize that while the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.