Patent Publication Number: US-9841445-B2

Title: Power detector

Description:
TECHNICAL FIELD 
     The present invention discloses a novel power detector. 
     BACKGROUND 
     Power detectors are circuits which are useful in a wide variety of applications, e.g. in receivers in wireless communications systems, in radar systems or in TV sets. 
     With the increase in data speeds, the demands for lower response times for power detectors also increase. Conventional power detectors which usually comprise a transistor or a diode, together with a low pass filter, are usually unable to provide the desired fast response times, especially with a stable output signal. 
     Another kind of power detector which has been proposed in order to deliver fast response times is a differential or mixer based power detector. However, such power detectors require the use of a balun which usually comprises reactive components, such as, for example, coupled transmission lines. Such reactive components may degrade the response times, and may also cause distortions of the output signal and limit the power detector&#39;s frequency bandwidth. 
     SUMMARY 
     It is an object to provide a power detector which obviates at least some of the disadvantages of previously available power detectors. 
     This object is obtained by means of a power detector which comprises a first and a second bipolar junction/FET transistor. The first transistor is arranged as a common base/gate transistor with its base/gate being biased by a bias voltage and the second transistor is arranged as a common emitter/source transistor with its emitter/source being grounded. 
     The power detector also comprises a diode or a current source connected to ground from the emitter/source of the first transistor and in the power detector an input port is connected to the emitter/source of the first transistor and to the base/gate of the second transistor. An output port is connected to the collectors/drains of the first and second transistor, with the collectors/drains also being connected to a DC supply via a first resistor. 
     In some embodiments which use a diode, the diode comprises a third transistor connected so as to function as a diode, by means of the base/gate of the third transistor being connected to the collector/drain of the third transistor. In addition, the third transistor&#39;s emitter/source is connected to ground, its collector/drain is connected to the emitter/source of the first transistor, and the base/gate of the third transistor is connected to the base/gate of the second transistor. 
     In embodiments, the power detector additionally comprises a fourth bipolar junction/FET transistor, arranged with said output port and said first resistor connected to its collector/drain and its emitter/source to the collectors/drains of the first and second transistors, with its base/gate subjected to a bias voltage via a second resistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail in the following, with reference to the appended drawings, in which 
         FIG. 1  shows a first embodiment of a power detector, and 
         FIG. 2  shows a second embodiment of a power detector, and 
         FIG. 3  shows output harmonics of the power detector of  FIG. 2  as a function of un-modulated RF-signals, and 
         FIG. 4  shows the DC output as a function of RF input power for the power detector of  FIG. 2 , and 
         FIG. 5 a    shows a modulated RF input signal at a data rate of 20 Gbps at a carrier frequency of 150 GHZ, and  FIG. 5 b    shows the corresponding output signal for the power detector of  FIG. 2 , and 
         FIG. 6  shows various characteristics of the power detector of  FIG. 2  for an input RF signal with a data rate of 20 Gbps and a carrier frequency of 150 GHz, and 
         FIGS. 7 a  and 7 b    show the output signal from the power detector of  FIG. 2  for an input RF signal with a data rate of 10 Gbps and 40 Gpbs, respectively, and 
         FIG. 8  shows a third embodiment of a power detector, and 
         FIG. 9  shows a fourth embodiment of a power detector, and 
         FIG. 10  shows output harmonics of the power detector of  FIG. 9  as a function of un-modulated RF-signals, and 
         FIG. 11  shows the DC output as a function of RF input power for the power detector of  FIG. 9 , and 
         FIG. 12 a    shows a modulated RF input signal at a data rate of 20 Gbps at a carrier frequency of 150 GHZ, and  FIG. 12 b    shows the corresponding output signal for the power detector of  FIG. 9 , and 
         FIG. 13  shows various characteristics of the power detector of  FIG. 9  for an input RF signal with a data rate of 20 Gbps and a carrier frequency of 150 GHz, and 
         FIGS. 14 a  and 14 b    show the output waveform for the power detector of  FIG. 9  for an input signal with a data rate of 10 Gbps and 40 Gbps, respectively, at a carrier frequency of 150 GHz. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like numbers in the drawings refer to like elements throughout. 
     The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the invention. 
       FIG. 1  shows a first embodiment of a power detector  100 . 1. As shown in  FIG. 1 , the power detector  100  comprises a first  110  and a second  115  bipolar junction transistor. The first transistor  110  is arranged as a common base transistor biased by a bias voltage V b1  at its base, and the second transistor  115  is arranged as a common emitter transistor, with its emitter being grounded. 
     The notions of common base and common emitter transistors are well defined in the art, but can be described as follows: In a common base transistor, the emitter of the transistor serves as the input terminal, the collector serves as the output terminal and the base is connected to ground, directly or indirectly. 
     In a common emitter transistor, the base of the transistor serves as the input terminal, the collector is the output terminal, and the emitter is common, e.g. by virtue of being connected to ground, directly or indirectly. 
     The power detector  100  comprises a, diode  120 , which serves as a DC path to ground from the emitter of the first transistor  110  by means of being connected as follows: the emitter of the first transistor  110  is connected to the anode of the diode  120 , and the cathode of the diode  120  should be grounded. In addition, the base of the second transistor  115  is connected to the anode of the diode. 
     The power detector  100  has an input port  105  connected to the emitter of the first transistor  110  and to the base of the second transistor  115 , and thereby also to the anode of the diode  120 . 
     The power detector  100  has an output port  125  which is connected to the collectors of the first  110  and second  115  transistor, with these collectors in this embodiment being connected to each other as well. These collectors are also connected to a DC supply V c  via a first resistor  130 . 
     In a second embodiment  200 , shown in  FIG. 2 , a third transistor  220  is used instead of the diode  120 , with the third transistor being connected so as to have the function of a diode, which is accomplished by means of the base of the third transistor  220  being connected to the collector of the third transistor  220 . The diode which is formed in this way is then connected as follows in order to obtain the function of the diode of  FIG. 1 : the third transistor&#39;s emitter is connected to ground and its collector is connected to the emitter of the first transistor  110 . As shown in  FIG. 2 , in the embodiment  200 , the bases of the third and second transistors  120  and  115  are also connected to each other, which equals the connection of the base of the second transistor  115  to the anode of the diode  110 , as shown in  FIG. 1 . 
     As shown in  FIG. 2 , in this embodiment, the input port  105  is connected to the anode of the diode formed by the third transistor  220  by means of being connected to the collector of the transistor  220 , and thereby also to its base. The other connections exhibited by the input port  105  in the embodiment  100  of  FIG. 1  are present in the embodiment  200  as well: in the embodiment  200 , in similarity to the embodiment  100  of  FIG. 1 , the power detector  200  has the input port  105  connected to the emitter of the first transistor  110  and to the base of the second transistor  115 . 
     In the power detector  200 , the output port  125  is connected to the collectors of the first  110  and second  115  transistor, with these collectors in this embodiment being connected to each other as well. These collectors are also connected to a control voltage V c  via the first resistor  130 . 
     The function of the embodiments  100  and  200  is as follows: Input signals are applied at the input port  105 , and thereby to the base of the second transistor  115  and the emitter of the first  110  transistor simultaneously. Due to the inherent nonlinearity of a transistor as a component, the output signals of the second  115  and first  110  transistors will contain DC components as well as harmonics of the input signal, i.e. signals at frequencies which are multiples of the frequency of the input signal. 
     The 1 st  harmonics in the output signals of the second  115  and first  110  transistors have a 180 phase difference between them, since the second transistor  115  is configured as a common emitter transistor, and the first transistor  110  is configured as a common base transistor. Thus, by connecting the output terminals (i.e. the collectors) of the second  115  and first  110  transistors as is done in the embodiments  100  and  200 , the 1 st  harmonics generated by Q 2  and Q 3  will be cancelled, given that they have the same amplitude; otherwise, the 1 st  harmonic will “merely” be suppressed. This cancellation/suppression of the 1 st  harmonic is a desired feature for a power detector, since all harmonics create a ripple in the output signal of a power detector, and the 1 st  harmonic has the “heaviest” contribution to this output ripple. 
     An example of the amplitude of the harmonics in the output signal from the power detector  200 , i.e. the signal at the output port  125  when a signal is applied to the input port  105 , is shown in  FIG. 3 : in this example, the input signal is a non-modulated sine wave with a power level of 0 dBm and a frequency which is swept from 80 GHz to 450 GHz.  FIG. 3  shows the amplitude of the 1 st , the 2 nd , the 3 rd , and the 4 th  harmonics as a function of frequency, and as seen, even though the 1 st  harmonic is the major harmonic, its amplitude is less than −17 dBm, while the amplitude of the 2 nd  harmonic is less than −38 dBm, and the 3 rd  and 4 th  harmonics are even lower. 
     In  FIG. 4 , the output signal (DC component only) of the power detector  200  as a function of input power is shown for an un-modulated input signal, i.e. a sine signal, at a frequency of 150 GHz. We can see that as the input power increases from 30 dBm to −2 dBm, the output voltage drops approximately 0.94V. 
     The transient response of the power detector  200  of  FIG. 2  to an input signal which is a modulated RF signal will now be discussed, with reference to  FIGS. 5 a  and 5 b   . The input RF signal which is used as an example here is on-off modulated, with a carrier which has a frequency of 150 GHz and a power level of 0 dBm. The on-off data is a period pulse series with a 50% duty cycle, and the pulse duration, T d , is 50 ps, and the data rate (1/T d ) is 20 Gbps. The time-domain of the input signal is shown in  FIG. 4 a   . The peak-to-peak voltage, V p-p , of the input signal is 334 mV. The time-domain output which the power detector  200  of  FIG. 2  generates for this input signal is shown in  FIG. 5 b   . The peak-to-peak voltage of the output is 190 mV, and a ripple exists in the output signal, with an amplitude of 79 mV. The amplitude of the ripple in the output signal is 41% of the peak-to-peak voltage V p-p  of the output signal. 
     As we can see in  FIGS. 5 a  and 5 b   , in the “on” state of the input signal, e.g., from 600 ps to 650 ps, the power detector  200  has a low-level output, since the output of the power detector  200  is inversely propositional to the input power, as was shown in  FIG. 4 . As the amplitude of the modulated input signal increases, the amplitude of the output voltage decreases. In the “off” state, e.g. from 650 ps to 700 ps, the power detector  200  has a high-level output signal. 
     For the case in which the input signal to the power detector  200  has a carrier frequency of 150 GHz and a data rate of 20 Gbps,  FIG. 6  shows the peak-to-peak voltage, V pp  of the output signal, the amplitude of the output ripple, as well as the ratio of the amplitude of the ripple and output V pp  as a function of the V pp  of the input signal. It can be seen that the V pp  of the output signal increases almost linearly with V pp  of the input signal. Consequently, we can see that the peak-to-peak voltage of the output signal is proportional to the peak-to-peak voltage of the input signal. 
       FIGS. 7 a  and 7 b    show the output signal for an input signal with a date rate of 10 Gbps ( FIG. 7 a   ) and 40 Gbps ( FIG. 7 b   ), for a fixed carrier frequency of 150 GHz and a power level of 0 dBm. It can be seen that the output signal is somewhat distorted as the data rate is increased up to 40 Gbps, but that the V pp  and the amplitude of the ripple of the output signal remain almost unchanged as the data rate is increased from 10 Gbps to 40 Gbps. 
     As has been pointed out previously in this text, the DC path from the first transistor  110  to ground can comprise either an ordinary diode or a bipolar junction transistor connected so as to constitute a diode from the first transistor  110  to ground. In a further embodiment  300 , which is shown in  FIG. 8 , the DC path can instead comprise a current source  320 , with the emitter of the first transistor  110  and the base of the second transistor  115  being connected to one end of the current source and the other end of the current source being connected to ground. The current source  320  is thus connected so as to form a DC path from the first transistor  110  to ground. 
       FIG. 9  shows a further embodiment  400  of a power detector. In  FIG. 9 , components which have been shown in  FIGS. 1, 2 and 8  have retained their reference numbers. A difference between the embodiment  400  and the embodiments  100 ,  200  and  300  is that the embodiment  400  comprises a fourth bipolar junction transistor  405 . As shown in  FIG. 9 , the base of the fourth transistor  405  is biased with a bias voltage V b  through a second resistor  430 . As is also shown in  FIG. 9 , the collector of the fourth transistor  405  is connected to the output port  125  and to the DC supply V c  through the resistor  130 , and the emitter of the fourth transistor  405  is connected to the collectors of the first and second transistors  110 ,  115 . In other words, it could be said that the fourth transistor  405  is connected between the collectors of the first and second transistors  110 ,  115  and the output port  125 , as compared to the embodiments  100 ,  200  and  300 . 
       FIG. 10  shows the amplitudes of the 1 st , the 2 nd , the 3 rd , and the 4 th  order harmonics in the output signal of the power detector  400  of  FIG. 9  for a un-modulated input RF signal with power of 0 dBm. The amplitude of the 1 st  harmonic is around −20 dBm, which is 2˜8 dB lower than that of the power detectors  100  and  200  in the frequency range from 80 GHz to 450 GHz; consequently, the amplitude of the ripple in the output signal will be reduced. 
       FIG. 11  shows the output DC signal of the power detector  400  for a un-modulated input RF signal at a frequency of 150 GHz. As seen in  FIG. 11 , as the input power increases from −30 dBm to −2 dBm, the output DC voltage changes approximately 1.5V. 
       FIG. 12 a    shows an input signal to the power detector  400  of  FIG. 9 , said input signal being a modulated RF signal at a data rate of 20 Gbps over a carrier frequency of 150 GHz, with a power of 0 dBm, and  FIG. 12 b    shows the corresponding output signal. It can be seen that the peak-to-peak voltage of the input and the output signals are 323 mV and 190 mV, respectively. The amplitude of the ripple is 43 mV, which is about a half of that of the power detector  200 . 
       FIG. 13  shows the corresponding characteristics for the power detector  400  as  FIG. 6  did for the power detector  200 , i.e.  FIG. 13  shows how, the input peak-to-peak voltage is changed, the output peak-to-peak voltage, and the amplitude of the ripple, as well as the ratio of the amplitude of the ripple and output peak-to-peak voltage vary. As can be seen, the output peak-to-peak voltage is linear proportional to the input peak-to-peak voltage, and it can also be seen that the amplitude of the ripple is reduced compared with the power detector  200 . In the power detector  400 , the ratio of the amplitude of the ripple and the output peak-to-peak voltage is about 20%-30%. 
       FIGS. 14 a  and 14 b    show the output signals for an input data rate of 10 Gbps ( FIG. 14 a   ) and 40 Gbp ( FIG. 14 b   ) with a fixed carrier frequency of 150 GHz and a power level of 0 dBm. As seen in  FIG. 14 b   , even though the output waveform is distorted slightly when the date rate is 40 Gbps, the peak-to-peak voltage and the amplitude of the ripple do not change, compared with the output waveform at an input data rate of 10 Gbps. This indicates that the power detector  400  of  FIG. 9  is able to process high date rate signals with results that are even better than those of the power detector  200 . 
     Throughout in the text above as well as in the drawings, the transistors in the power detectors have been shown and described as bipolar junction transistors. As those skilled in the art will realize, other kinds of transistors can also be used, for example Field Effect Transistors, FETs. If such a substitution is made, the transistor terminals in the description above, as well as in the drawings, should be substituted as follows: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Bipolar junction transistor 
                 FET 
               
               
                   
                   
               
             
            
               
                   
                 Base 
                 Gate 
               
               
                   
                 Collector 
                 Drain 
               
               
                   
                 Emitter 
                 Source 
               
               
                   
                 Common base configuration 
                 Common gate configuration 
               
               
                   
                 Common emitter configuration 
                 Common source configuration 
               
               
                   
                   
               
            
           
         
       
     
     In the drawings and specification, there have been disclosed exemplary embodiments of the invention. However, many variations and modifications can be made to these embodiments without substantially departing from the principles of the present invention. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation. 
     The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims.