Abstract:
A current modulation filter for mitigating fluctuations in current on a power supply line due to time dependent current demands of a load circuit includes a biasing circuit for providing a source voltage reference and a quiescent current reference and a load voltage sensing circuit for providing a voltage measurement of the operating voltage of the load circuit. A current sensing circuit is electrically interposed between the power source and the load circuit. A voltage drop across the current sensing circuit is transmitted as a voltage difference to a current controller which subsequently supplies or sinks current to the power supply line so as to maintain a constant current level thereon.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention described herein is directed to supply line filtering for power supplies. More specifically, the present invention compensates for fluctuations in a power supply current caused by time-dependent fluctuation and load current demand.  
         [0003]     2. Description of the Prior Art  
         [0004]     Although line filtering of supply lines in power distribution systems is well known, it continues to be a field in which beneficial improvements are made. The widespread use of the switching power supplies and electronic circuitry, especially in personal computers, have allowed smaller packaging of systems, but often must be accompanied by supply line filtering to ensure proper operation of the load circuitry.  
         [0005]     Of course, not all supply line noise is caused by the supply side circuitry. Time-dependent load demand also introduces a noise component on the supply line, which is typically overcome by adequate regulation of the power supply. Generally, the power regulation is implemented at the power supply using a separate voltage sensing circuit coupled to the load.  
         [0006]     In many applications, such as in laser light modulation of optical computing and optical communication, fluctuations in supply current due to changing load conditions present not only power regulation problems, but also considerable security concerns. This is because the modulation of laser light, which forms the transmitted data, may be introduced onto the power supply line as a modulated current. The current modulation may easily be read by an inductive probe coupled to the supply line. This allows a third party to monitor communications which would otherwise be secure.  
         [0007]     Prior art line filters which are designed to mitigate amplitude modulation of voltages fail to prevent current modulation of the supply line in systems where the power source has a low internal impedance and where the frequency distribution of the modulation is low. In such systems, current in the supply line can be modulated to a significant degree without a correspondingly significant degree of voltage modulation.  
         [0008]     In light of the foregoing discussion of the prior art, there is an apparent need for a current modulation filter for mitigating current fluctuations in a power distribution system.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention is a current modulation filter to be electrically interposed between a power source and a load circuit. The current modulation filter of the present invention includes a biasing circuit having a first voltage reference node and a current reference node and is electrically coupled in parallel connection to the power source. The current modulation filter also includes a load voltage measurement circuit having a second voltage reference node and being electrically coupled in parallel connection to the load circuit. A current sensing circuit is series connected between the power source and the load circuit and a current control circuit is coupled to the first voltage reference node, the second voltage reference node, the current reference node and to the load circuit. The current control circuit supplies electrical current to, or sinks electrical current from, the load circuit in response to a voltage difference between the first voltage reference node and the second voltage reference node such that current through the current sensing circuit remains constant to within a predetermined value over a predetermined range of load current demand.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a block diagram illustrating the basic architecture of the current modulation filter of the present invention; and  
         [0011]      FIG. 2  is a schematic diagram of an exemplary embodiment of the current modulation filter of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     Referring to  FIG. 1 , there is shown a block diagram illustrating the interconnection of the primary components of the present invention. As illustrated in the Figure, current modulation filter  10  is electrically interposed between a power source  70  and a load circuit  80 . Whereas the current modulation filter of  FIG. 1  is shown as a separate component, it should be obvious to the ordinarily skilled artisan that the current modulation filter of the present invention may be incorporated into other elements within an electronic system.  
         [0013]     Current modulation filter  10  is coupled to power source  70  via supply line  2  and ground reference line  4  and is coupled to load circuit  80  via supply line  6  and ground reference line  8 . In certain embodiments of the present invention, as is shown in  FIG. 1 , ground reference line  4  and ground reference line  8  are connected together at a common reference node  19 , which is generally electrically coupled to a system ground terminal.  
         [0014]     As shown in  FIG. 1 , current modulation filter  10  includes a biasing circuit  30  for providing a power source voltage reference at terminal  32  and for setting a quiescent current of current controller  20  at terminal  34 . Current modulation filter  10  further includes a load voltage sensing circuit  40  which provides a voltage level corresponding to that of supply line  6  at terminal  44 .  
         [0015]     Supply line  2  on the side of the power source is separated from supply line  6  on the side of the load circuit by a current sensing circuit  50 . Current sensing circuit  50  provides a voltage difference at terminal  54  with respect to terminal  52  when current passes therethrough. This difference in voltage is detected and reproduced as a voltage difference between the source reference voltage at terminal  32  and the load reference voltage at terminal  44 . The voltage difference is transmitted to current controller  20  at terminals  24  and  22 . Current controller  20  will add or subtract current at terminal  26  so that the current in supply line  6  remains constant. Stated another way, the current controller supplies electrical current to, or sinks electrical current from, supply line  6  so that the voltage difference across current sensing circuit  50  remains constant. When the voltage difference in current sensing circuit  50  is constant, so too is the current flowing therethrough, which thereby extinguishes any current modulation to within acceptable levels.  
         [0016]     Referring now to  FIG. 2 , there is shown a schematic diagram of an exemplary embodiment of the current modulation filter of the present invention. Note that like reference indicators in  FIG. 2  with those of  FIG. 1  refer to the same element in both figures.  
         [0017]     As is shown in  FIG. 2 , current modulation filter  10  is electrically interposed between power source  70  and load circuit  80 . Power source  70  is represented by a DC voltage source producing a voltage VS. This voltage is produced at supply terminal  72  with reference to ground terminal  74 . Power source  70  is coupled to the current modulation filter at terminals  12  and  14  by supply line  2  and reference line  4 , respectively.  
         [0018]     Load circuit  80  is coupled to current modulation filter  10  via supply line  6  and reference line  8  at delivery terminal  82  and ground terminal  84 , respectively. The load circuit is modeled as a load resistance RL and parallel with a modulation source circuit  90  represented as the series connection of AC voltage source VM, capacitor CM and resistor RM. As previously stated, the actual modulation of current on supply line  6  may be produced by the driver circuitry of a laser diode in a laser communication system. For simplicity, this modulation is modeled by the current modulation source  90  for producing the current modulation on supply line  6 .  
         [0019]     Biasing circuit  30  of  FIG. 1  is constructed in  FIG. 2  from series-connected resistors R 2 , R 4  and R 6  and bypass capacitor C 3 . The series-connected resistors R 2 , R 3 , R 6  are interconnected at interposing nodes  32  and  34 , which function as the source voltage reference node and the quiescent current reference node, respectively. The biasing network  30  is electrically coupled to the supply line  2  at terminal  17  and is coupled to reference line  4  at node  19 .  
         [0020]     Load voltage sensing circuit  40  is constructed in  FIG. 2  from series-connected resistors R 3  and R 5 . The series-connected resistors R 3  and R 5  are interconnected at interposing node  44 , which serves as the load voltage reference node. Load voltage sensing circuit  40  is coupled to supply line  6  at terminal  42  and to reference line  8  at terminal  19 . As is shown in  FIG. 2 , and was discussed with reference to  FIG. 1 , reference lines  4  and  8  are interconnected at terminal  19  which is further connected to a system ground terminal.  
         [0021]     Current sensing circuit  50  is constructed from a small resistor R 1 , as shown in  FIG. 2 . Resistor R 1  produces a voltage drop corresponding to the current flowing therethrough. A corresponding voltage difference is produced at load voltage reference node  44  with respect to source voltage reference node  32 . The voltage difference is provided to current controller  20  at terminals  22  with respect to terminal  24 .  
         [0022]     In the exemplary embodiment of  FIG. 2 , current controller  20  is an active circuit consisting of a high gain operational amplifier A 1  and transistor Q 1 . Whereas transistor Q 1  is shown as a field effect transistor, it should be understood that other transistor architectures, such as bipolar junction transistors, may be used. Transistor Q 1  supplies, or sinks, the necessary current to maintain the constant current level on supply line  6 , as discussed hereinabove. Similarly, where amplifier A 1  is shown in  FIG. 2  as an operational amplifier, other differential amplifiers may be used, provided the amplifier has sufficient gain to produce an amplified voltage signal responsive to very small voltage differences presented at its inputs.  
         [0023]     The gate of transistor Q 1  (or the base of a bipolar junction transistor) is electrically coupled to the output terminal of amplifier A 1  to receive a control signal therefrom. The control signal is the aforementioned voltage swing responsive to the voltage difference presented at its input terminals. The source of transistor Q 1  (or the collector in the bipolar junction transistor) is connected to the current reference node  34  via terminal  28  of current control circuit  20 . As previously stated, current reference node  34  biases transistor Q 1  at a specific, predetermined quiescent current level by methods well known in the art. Bypass capacitor C 3  removes any modulation from the positive input of amplifier A 1 , i.e., assures that voltage reference node  32  is maintained at a constant reference voltage.  
         [0024]     The drain of transistor Q 1  (or the emitter of a bipolar junction transistor) is coupled to the load circuit  80  via terminal  26  of current controller  20 . As is well known, transistor Q 1  will produce a current in the drain thereof responsive to a voltage at its gate in an approximately linear manner. Thus, the voltage produced by amplifier A 1  subsequently produces a corresponding current in the drain of transistor Q 1  so as to maintain a constant current level on supply line  6 . Capacitor C 2  is provided to add stability to the high gain amplifier A 1 , which is a well known stabilization technique.  
         [0025]     As shown in  FIG. 2 , amplifier A 1  receives its operating power from power source  70  via terminals  21  and  23 . Thus, amplifier A 1  should be constructed and properly bypassed, so as to prevent modulation of current in supply line  2  corresponding to the modulation signal produced at its output. Without this precaution, current flowing through R 1  would simultaneously be modulated by the current modulation on supply line  6  caused by the load circuit modulation source and by the current modulation on supply line  2  caused by the fluctuating rail voltage of amplifier A 1 . It should then be apparent that such an arrangement would minimize the effectiveness of current modulation filter  10 .  
         [0026]     The effectiveness of the current modulation filter of the present invention may be demonstrated by way of a specific example by setting the circuit elements of  FIG. 2  to the values shown in Table 1.  
                                                                               TABLE 1                           Exemplary Circuit Parameters                CIRCUIT ELEMENT   VALUE                            VS   9   VDC                VM   17 V p—p, 50 Ω                CM   100   μF           RM   120   Ω           RL   17   Ω           R1   1   Ω           R2   1.8   kΩ           R3   1   kΩ           R4   3.6   kΩ           R5   2.7   kΩ           R6   40   Ω           C2   5   pF           C3   22   μF                      
 
         [0027]     Resistor R 6  is used to set the quiescent bias current of Q 1 , which is set to 50 mA DC in the exemplary embodiment. Additionally, the biasing circuit  30  sets the source voltage reference node  32  at 6.7 VDC. When the circuit of  FIG. 2  is constructed with the values of Table 1 and a current probe is coupled to supply line  2  and an additional current probe is coupled to supply line  6 , a 70 dB reduction and current modulation in supply line  2  as compared to supply line  6  has been measured.  
         [0028]     The above description is illustrative and not restrictive. Any variations, alternatives and modifications will become apparent to those of skill in the art upon review of this disclosure. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.