Abstract:
An attenuator includes: a first circuit including a common collector or common drain amplifier formed of a first transistor having its control node connected to an input of the attenuator and its emitter or source connected to an intermediate node of the attenuator; and a second circuit including a common collector or common drain amplifier formed of a second transistor having its emitter or source connected to the intermediate node and its control node connected to an output of the attenuator.

Description:
[0001]    This application claims the priority benefit of FR Patent application number 14/62091, filed on Dec. 9, 2014, the contents of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to the field of high frequency attenuators, and also to the field of high frequency attenuators with variable attenuation for device testing. 
         [0004]    2. Description of the Related Art 
         [0005]    In certain applications, it may be desirable to provide an attenuator capable of attenuating high frequency signals, for example having a frequency higher than 120 GHz, and up to 175 GHz or more. 
         [0006]    For example, in the field of high frequency device characterization, a device under test may be driven with a high frequency input signal, and one or more output signals of the device under test are detected using a probe in order to determine characteristics of the device. In order to be able to accurately detect an output signal over a relatively broad voltage range, one or more attenuators are for example provided for reducing the voltage level of the output signal. 
         [0007]    There is however a difficulty in providing an attenuator capable of providing a relatively low level of attenuation, for example as low as −6 dB. 
         [0008]    Furthermore, there is a difficulty in providing an attenuator having a variable attenuation and/or that can operate over a relatively broad bandwidth, for example of 20 GHz or more. 
       BRIEF SUMMARY 
       [0009]    According to one aspect, there is provided an attenuator comprising: a first circuit including a common collector or common drain amplifier formed of a first transistor having its control node connected to an input of the attenuator and its emitter or source connected to an intermediate node of the attenuator; and a second circuit including a common collector or common drain amplifier formed of a second transistor having its emitter or source connected to the intermediate node and its control node connected to an output of the attenuator. 
         [0010]    According to an embodiment, the emitter or source of the first transistor is further connected to a first variable current source and the emitter or source of the second transistor is further connected to a second variable current source. 
         [0011]    According to an embodiment, the first variable current source is a third transistor receiving at its control node a biasing voltage and the second variable current source is a fourth transistor receiving at its control node the biasing voltage. 
         [0012]    According to an embodiment, the attenuator further comprises a control circuit for generating the biasing voltage based on a control signal. 
         [0013]    According to an embodiment, the control node of the first transistor is coupled to the input of the attenuator via the series connection of a first capacitor and a first waveguide, and the control node of the second transistor is coupled to the output node of the attenuator via the series connection of a second capacitor and a second waveguide. 
         [0014]    According to an embodiment, the emitter or source of the first transistor is coupled to the intermediate node via the series connection of a third capacitor and a third waveguide and the emitter or source of the second transistor is coupled to the intermediate node via the series connection of a fourth capacitor and a fourth waveguide. 
         [0015]    According to a further aspect, there is provided a probe comprising: an integrated circuit comprising the above attenuator connected to at least one input pin suitable for connecting an output pad of a device under test to the integrated circuit. 
         [0016]    According to an embodiment, the integrated circuit comprises a matching network connecting the attenuator to the at least one input pin. 
         [0017]    According to an embodiment, the integrated circuit further comprises: a first power detector, the attenuator and first power detector being both connected to the at least one input pin via a splitter; and a second power detector connected to the output of the attenuator. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]    The foregoing and other features and advantages will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  schematically illustrates an attenuator according to an embodiment of the present disclosure; 
           [0020]      FIG. 2  schematically illustrates a test system according to an embodiment of the present disclosure; and 
           [0021]      FIG. 3  schematically illustrates a detection and attenuation circuit of the test system of  FIG. 2  in more detail according to an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In the following description, an attenuator is described in relation to the particular application of device characterization. Such an attenuator can however be used in any of a broad range of applications where the attenuation of high frequency signals is desired. For example, possible alternative applications include wireless receivers, or variable gain amplifiers in wireless transmitters. 
         [0023]    The term “approximately” as used herein implies a tolerance of plus or minus  10  percent of the value in question. 
         [0024]      FIG. 1  illustrates an attenuator  100 , which is for example implemented on an integrated circuit, in other words as an “on-chip” solution. 
         [0025]    The attenuator  100  comprises a circuit portion  100 A on the left-hand side having elements referenced with the suffix “A”, and a circuit portion  100 B on the right-hand side having elements referenced with the suffix “B”. It will be noted that the circuit portions  100 A,  100 B are broadly symmetrical with each other around an intermediate node  101  of the attenuator. 
         [0026]    The circuit  100 A comprises a common-collector amplifier formed of an npn bipolar transistor  102 A having its base coupled to an input  103  of the attenuator. This input  103  receives an input signal RF IN . The emitter of the bipolar transistor  102 A is connected to a variable current source  104 A. In the example of  FIG. 1 , the variable current source  104 A is implemented by a MOS transistor having its source connected to ground and receiving, at its gate, a control voltage V BIAS . The emitter of the bipolar transistor  102 A is also coupled to the intermediate node  101  of the attenuator. 
         [0027]    Similarly, the circuit  100 B comprises a common-collector amplifier formed of an npn bipolar transistor  102 B having its base coupled to an output  105  of the attenuator. This output  105  provides an output signal RF OUT . The emitter of the bipolar transistor  102 B is connected to a variable current source  104 B. In the example of  FIG. 1 , the variable current source  104 B is implemented by a MOS transistor having its source connected to ground and receiving, at its gate, the control voltage V BIAS . The emitter of the bipolar transistor  102 B is also coupled to the intermediate node  101  of the attenuator. 
         [0028]    In alternative embodiments, the bipolar transistors  102 A,  102 B could be replaced by MOS transistors, such that they form common drain amplifiers rather than common collector amplifiers. Furthermore, in some embodiments, the variable current sources  104 A,  104 B could be implemented by other types of devices, such as bipolar transistors. 
         [0029]    The circuits  100 A,  100 B of  FIG. 1  for example further comprise other elements adapted to improve the circuit characteristics at high frequencies. 
         [0030]    For example, the circuit  100 A comprises a waveguide  106 A connected between the collector of the bipolar transistor  102 A and a supply voltage rail V cc . A capacitor  108 A is for example connected between the supply voltage rail V cc  and ground for RF and DC decoupling. Furthermore, the base of the transistor  102 A is for example connected to a supply voltage rail V bb  via a resistor  110 A, and to one node of a capacitor  112 A. The capacitor  112 A for example provides low frequency isolation of the base of the transistor  102 A from the input RF signal as well as RF and DC decoupling, and for example has a capacitance in a range 30 to 150 fF, for example approximately 50 fF. The other node of capacitor  112 A is for example connected via a waveguide  114 A and a further waveguide  116 A to the input node  103 . A ground stub, in the form of a further waveguide  120 A, for example connects an intermediate node  122 A between the waveguides  114 A and  116 A to ground. The emitter of transistor  102 A is for example connected to the intermediate node  101  via the series connection of a capacitor  126 A and a waveguide  128 A. The capacitor  126 A for example has a capacitance in the range 50 to 150 fF, and for example of approximately 50 fF. Similarly, the circuit  100 B for example comprises a waveguide  106 B connected between the collector of the bipolar transistor  102 B and a supply voltage rail V cc . A capacitor  108 B is for example connected between the supply voltage rail V cc  and ground. Furthermore, the base of the transistor  102 B is for example connected to a supply voltage rail V bb  via a resistor  110 B, and to one node of a capacitor  112 B. The capacitor  112 B for example has a capacitance equal to that of the capacitor  112 A. The other node of capacitor  112 B is for example connected via a waveguide  114 B and a further waveguide  116 B to the output node  105 . A ground stub, in the form of a further waveguide  120 B, for example connects an intermediate node  122 B between the waveguides  114 B and  116 B to ground. The emitter of transistor  102 B is for example connected to the intermediate node  101  via the series connection of a capacitor  126 B and a waveguide  128 B. The capacitor  126 B for example has the same capacitance as capacitor  126 A. 
         [0031]    The intermediate node  101  between the two circuits  101 A,  101 B is for example connected to ground via a further waveguide  132 . 
         [0032]    A control block (CTRL)  134  for example generates the biasing voltage V BIAS  provided to the gates of transistors  104 A,  104 B based on a control signal G indicating a desired attenuation of the attenuator. In some embodiments, the value of the control signal G is a digital value programmed by a user. In other embodiments, the control signal G is for example a voltage signal, and could be generated by other circuits not represented in  FIG. 1 , for example in the case that the attenuation is automatically adapted based on a feedback loop. 
         [0033]    The present inventors have found that, by providing an attenuator having circuit portions each comprising an amplifier connected in a symmetrical fashion with respect to an intermediate node, the attenuation provided by the attenuator can be relatively constant over a large frequency bandwidth of over 20 GHz, and for example for a frequency bandwidth of up to 40 GHz or more. For example, the inventors have found that the circuit of  FIG. 1  is able to provide a relatively uniform attenuation at approximately −6 dB over the frequency band of 135 to 175 GHz. Furthermore, the input and output impedances of the attenuator can be precisely controlled, and well matched with each other. 
         [0034]    An application of the attenuator  100  of  FIG. 1  in a test system for a device under test will now be described with reference to  FIGS. 2 and 3 . 
         [0035]      FIG. 2  illustrates a test system  200  comprising an integrated circuit  201  comprising a device under test (DUT)  202 . The DUT  202  for example has connection pads, there being six in the example of  FIG. 2 , three of which are input RF pads  203 , and three of which are output RF pads  204 . 
         [0036]    The three input pads  203  are connected to a probe  206  via which input power is applied to the DUT in the form of one or more test signals. The probe  206  for example comprises output pins  210  for contacting the pads  203 , and a circuit  208  for generating the test signals applied to the pads  203  via the output pins  210 . 
         [0037]    A further probe  212  is for example in contact with the three output pads  204  of the DUT  202 , and comprises pins  216  for respectively contacting the three pads  204 , and a test circuit  214  providing attenuation and detection. The test circuit  214  is for example adapted to measure parameters of the DUT, such as noise figure, optimum power, etc. The test circuit  214  is for example implemented by an integrated circuit positioned in the probe  212 , the pins  216  forming input pins of the integrated circuit. Thus, whereas prior art solutions generally connect the test circuit to the probe via a cable that can be tens of centimeters long, in the system  200 , the test circuit  214  is advantageously integrated within the probe. The output pads  204  of the DUT and the test circuit  214  can therefore be separated by a relatively short distance, for example in the order of several millimeters. 
         [0038]      FIG. 3  schematically illustrates the test circuit  214  of  FIG. 2  in more detail according to an example embodiment. As indicated above, this circuit is for example implemented by an integrated circuit. 
         [0039]    The circuit  214  for example comprises an input  302  connected to one of the pins  216  of the probe  212  (not illustrated in  FIG. 3 ). In the test circuit  214 , the input  302  is for example connected to the input of a matching network  304 , which for example has an input impedance of Z 1 , for example of approximately 50 ohms. The output impedance of the matching network  304  is for example equal to an impedance Z 2 , which may be the same as or different from the impedance Z 1 . In some embodiments, the output impedance Z 2  is equal to approximately 25 ohms. 
         [0040]    The output of matching network  304  is connected to a power splitter  306 , which splits the signal into two parts, for example of approximately equal power. One of the outputs of the splitter  306  is connected to a power detector  308 , which detects the power of the signal. The other output of the splitter  306  is for example provided to an attenuator  310 , which is for example implemented by the circuit  100  of  FIG. 1 . The input impedance of both the power detector  308  and of the attenuator  310  are for example chosen to be equal to the impedance Z 1 , and the output impedance of attenuator  310  is for example chosen to be equal to the impedance Z 2 . For example, in the case that the input and output impedances of the attenuator are different from each other, the attenuator may comprise, in addition to the circuit of  FIG. 1 , a matching network at its output to bring the output impedance to the appropriate value. 
         [0041]      FIG. 3  also illustrates a subsequent stage of power detection and attenuation, comprising a further splitter  312 , a further power detector  314 , and a further attenuator  316 . These elements are for example the same as the elements  304 ,  308  and  310  respectively, and will not be described in detail. By providing several stages of attenuation and power detection, the circuit  214  is capable of detecting the power of the output signal of the DUT at various levels of attenuation, and thus for a broad range of the input power levels of the DUT. 
         [0042]    An advantage of the attenuator described herein is that it is capable of providing a relatively low level of attenuation, for example as low as −6 dB. Furthermore, it is capable of providing a variable level of attenuation, by adjustment of the control value G. Furthermore, the attenuator is capable of operating over a relatively broad bandwidth, for example of 20 GHz or more. 
         [0043]    Having thus described at least one illustrative embodiment, various alterations, modifications and improvements will readily occur to those skilled in the art. 
         [0044]    For example, it will be apparent to those skilled in the art that the particular circuitry illustrated in  FIG. 1  provides just one example implementation, and that different arrangements of waveguides, resistors and capacitors would be possible, and one or more of these components could be omitted, depending on the particular application. 
         [0045]    Furthermore, it will be apparent that while a control circuit  134  is provided allowing the attenuation of the attenuator of  FIG. 1  to be controlled, in some embodiments this control circuit could be omitted, the attenuator being adapted to provide a relatively constant attenuation. Furthermore, the variable current sources  104 A,  104 B could be replaced by elements of fixed impedance, such as by resistors or waveguides. 
         [0046]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.