Abstract:
A system for comparing, measuring, or providing a reference signal based on a oscillator having variable loop gain is described. Only when the oscillator loop gain is at least the value of one does the oscillator produce an AC output signal. The oscillator&#39;s ability to oscillate is controlled by the one or more sensor/transducer input signal levels. In some cases, negative feedback of the AC output signal is also used to control the loop gain of the oscillator circuit keeping the loop gain at or close to the value of one. The system&#39;s output signal indicates whether the oscillator is oscillating or not, or the AC signal level required to just maintain oscillation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The benefits of filing this invention as the following listed Provisional application for patents by Fred Mirow are claimed;
   “COMPARATOR MEASURING SYSTEM AND A/D USING LOOP GAIN SENSING”, U.S. PTO 60/904956 Filed Mar. 5, 2007,   “VOLTAGE REFERENCE SYSTEM USING CONSTANT LOOP GAIN”, U.S. PTO 60/874163 filed Dec. 11, 2006,   “SENSOR MEASURING SYSTEM WITH CONSTANT LOOP GAIN”, U.S. PTO 60/873398 filed Dec. 07, 2006,   “SENSOR MEASURING SYSTEM WITH CONSTANT LOOP GAIN”, U.S. PTO 60/874558 filed Dec. 13, 2006, and   “SENSOR MEASURING SYSTEM USING SENSOR TO DETERMINE LOOP GAIN”, U.S. PTO 60/792437 filed Apr. 17, 2006.   
 
     
     BACKGROUND OF THE INVENTION 
       [0007]    This invention relates to systems for comparing, measuring or providing a reference signal based on variable oscillator loop gain and is described. Only when the oscillator loop gain is at least one does the oscillator produces an AC signal. The oscillator ability to oscillate is controlled by the one or more sensor or transducer input signal levels. Also in some cases negative feedback of the oscillator&#39;s AC signal is used to control the loop gain of the oscillator circuit, keeping the loop gain close to or at the value of one. The system&#39;s output signal depends on whether the oscillator is oscillating or not, or the oscillator&#39;s AC signal level required to just maintain oscillation. 
         [0008]    Transducers and sensors have often been used to provide the input signals for many forms of prior art transducer instrumentation, for example, scales or balances, accelerometers, and pressure transducers or proximity gauges. In such systems, the precision at which measurements can be made is very much a function of the stability of the circuit interfacing the sensor portion and read-out-portion of the system. In general, the prior art systems have permitted relatively low precision measurements due to the sensitivity of the interface circuit to combinations of various factors such as drive signal waveshape, drive signal amplitude, drive signal frequency, and temperature dependent component parameter variation. 
         [0009]    More particularly, the class of system exemplified by U.S. Pat. No. 3,318,153 to Lode includes circuit interface which generates an output signal as derived from a rectification and summing of current signals whose amplitude is dependent on both the magnitude and frequency of an applied drive signal, thereby requiring a high voltage drive signal at relatively low frequencies and establishing a relatively large power requirement. Thus, that system has a relatively high sensitivity to both drive signal amplitude and frequency, and, as a result, means is provided by Lode to maintain the amplitude-frequency product for the drive signal to be constant. 
         [0010]    An other application of this invention is as a signal reference system. Stable references are required for circuits such as A/D converters, measurement devices, and voltage regulators to name only a few. 
         [0011]    A buried-Zener reference is one way to produce a reference voltage. Another way to produce a reference voltage is with a bandgap voltage circuit. Bandgap voltage circuits can operate with a lower supply voltage than buried-Zener references and can also require less power. Operation of a bandgap voltage reference is well-known and produces a reference voltage that corresponds to the bandgap voltage of the used substrate material. In the most common case of silicon, this bandgap voltage is approximately 1.2 volt. The conventional bandgap reference circuits can therefore not be used for generating the necessary reference voltage in systems with supply voltages of 1.5 volt and below. 
         [0012]    An objective of the present invention is to provide a system for comparing, measuring or providing a reference signal that has a high temperature, radiation, and voltage stability due to its reliance on passive component ratios to set circuit thresholds operating values. Passive components such as resistors and capacitors are more stable under these conditions. This invention increases system accuracy by making the accuracy dependent on passive component ratios instead of transistor or signal frequency stability. 
         [0013]    An other objective of the present invention to provide a system for providing an output signal level dependant on the impedance or gain associated with a transducer or sensor using a system being relatively independent of the system signal amplitude, frequency, and waveshape. Additionally the system is relatively simple and inexpensive to implement. 
         [0014]    An other objective of the invention is to provide a reference voltage or current which is insensitive to temperature and to variations in the main power supply and yet can produce a voltage at less than the bandgap voltage. 
         [0015]    A further objective of the invention to provide circuits that are less susceptible to process variances by relying on impedance ratios thereby providing a more consistently manufacturable circuit. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    According to this invention, one or more transducers, this term also includes sensors, having a variable impedance or gain indicative of a physical or electrical parameter of interest, such as voltage, current, displacement, vibration, spacing, size, pressure and the like, are used along with other elements to form a oscillator. 
         [0017]    A oscillator consist of a feedback network section combined with an amplifier section. The feedback network is connected to the input and output of the amplifier so as to provide positive feedback and be capable of oscillating. The amplifier has a phase shift of about 0 degrees, the feedback network supplies the remaining phase shift necessary to make the total phase shift at the frequency of oscillation 0 degrees. If the amplifier is phase inverting having a phase shift of about −180 degrees, than the feedback network would provide the required phase shift amount to have 0 degrees total. There are many well known phase shift networks that can be used as part of feedback network such as the twin T and the Wien bridge. 
         [0018]    This oscillator will only oscillate when the loop gain of the amplifier and feedback network is at least one and the phase shift is zero. The oscillator state can readily be detected. When oscillating a AC signal is present and when not oscillating, no AC signal is present. This AC signal may also directly be used as a output signal indicating the loop gain level or preferably a detector connected to the oscillator provides the desired type of output signal, indicating if the AC signal is present or not and in some cases the level of the AC signal. 
         [0019]    The amplifier section has a high temperature and voltage stability due to its reliance on component ratios to set circuit thresholds operating values. Resistor networks can be used in the amplifier section to increase phase and gain stability against the effects of temperature and voltage. One well know approach is to use a high gain amplifier which uses negative feedback to accurately set the amplifier section&#39;s gain. The negative feedback is optimally obtained by using a resistor divider network in which the resistor temperature and voltage characteristics are matched. The effects of temperature on the resistor network are then decreased. This also decreases the effects on the amplifier section since the gain of it is primarily determined by the resistor divider network. 
         [0020]    The feedback network can also use circuits that depend on the ratio of circuit components, including capacitors, inductors, and resistors, to provide stable gain over temperature. 
         [0021]    There are many well known methods for implementing oscillator gain varying elements depending on the selected transducers type. One method is to use signal controlled variable impedance devices. Some examples for use with a voltage input signal are variable resistors obtained by using a FET with it&#39;s gate connected to input. An other approach is a variable capacitor type using a reverse biased semiconductor junction such as a diode. As the DC voltage across the diode is varied it&#39;s capacitance also varies. Also a current input signal can be applied by using as one example a variable inductor obtained by using a inductor with a ferrite core which uses the current through it to vary it&#39;s inductance. Physical input signals can be applied by using as one example a variable capacitor such as a MEMS capacitor in which the physical spacing of the capacitor plates and resulting capacitance value are varied in response to the input signal level. 
         [0022]    Likewise there are many well known methods for obtaining signal controlled variable gain devices. An example of this are transistors. The transistor&#39;s gain is varied by varying it&#39;s DC operating current level in response to input thus obtaining a signal controlled variable gain device. Also, the inductive coupling (mutual inductance) between inductors may be changed in response to an input signal. 
         [0023]    A comparator is formed by using two or more transducers that receive input signals. The input signals levels are compared by using the transducer&#39;s output signals to vary the oscillator&#39;s loop gain. As the ratio of transducer&#39;s impedance or gain level varies, the loop gain also varies. When the transducers change the loop gain to less than one, the oscillation stops and the output indicates that no AC signal is present. The AC signal level being zero or not zero therefore indicates the comparison of the two transducer&#39;s input signal levels. To form a sensor measuring system from the comparator system one of the transducers is replaced with a fixed known impedance or gain level so that there is only one input signal. 
         [0024]    It is also possible to provide an analog output signal level representing a sensor measuring system signal input level. By using the input signal level and negative feedback of the oscillator&#39;s AC signal level to both control the oscillator&#39;s loop gain. As the input signal level changes the loop gain, the AC signal level also changes so as to just maintain oscillation, keeping the loop gain value at or close to one. The oscillator&#39;s AC signal level therefore indicates the transducer input signal level. 
         [0025]    An output signal level indicating the comparator&#39;s two input signal ratio level or the sensor measuring system input signal level can also be obtained by changing the oscillator loop gain in response to the input signal levels and a independent gain control signal and monitoring whether the oscillator is oscillating or not. The value of the independent gain control signal represents the comparator&#39;s two inputs signals ratio level or the sensor&#39;s input signal level when the oscillator&#39;s AC signal level changes from not zero to zero or vice a versa. The independent gain control signal maybe a digital signal or can be converted to a digital signal to provide a digital output signal. Also a detector may be connected to the oscillator output indicating if the AC signal is present or not with the preferred type of signal, such as a digital high/low. 
         [0026]    A reference signal system can be formed by further eliminating all input signals and just using negative feedback of the oscillator&#39;s AC signal level to control the oscillator&#39;s loop gain. The oscillator&#39;s AC signal level controls a gain varying element of the oscillator. As the oscillator&#39;s AC signal level varies the loop gain varies. The oscillator&#39;s AC signal level is of a value that just maintains oscillation by keeping the loop gain value at or close to one. Since in the reference signal system all gain levels are constant except that controlled by the oscillator&#39;s AC signal level, the oscillator&#39;s AC signal level also remains a constant signal level that can be used as a reference. In addition, a detector having it&#39;s output signal level controlled by the AC signal level can be used to provide at it&#39;s output the preferred type of output signal, such as for example a voltage or current. 
         [0027]    It is also understood that varying the phase of the oscillator&#39;s amplifier and feedback network loop will also control the oscillator&#39;s ability to oscillate. Alternatively in the above applications the transducers may be used to control the oscillator&#39;s loop phase instead of gain. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments. 
           [0029]      FIG. 1  shows a block diagram of comparator system  1 ; 
           [0030]      FIG. 2  shows a block diagram of comparator system  1 A; 
           [0031]      FIG. 3  shows a diagram of oscillator system  10 B; 
           [0032]      FIG. 4  shows a block diagram of comparator system  1 B; 
           [0033]      FIG. 5  shows a diagram of the amplifier system  8 A; 
           [0034]      FIG. 6  shows a block diagram of sensor measurement system  11 ; 
           [0035]      FIG. 7  shows a block diagram of sensor measurement system  11 A; 
           [0036]      FIG. 8  shows a block diagram of sensor measurement system  11 B; 
           [0037]      FIG. 9  shows a diagram of the amplifier system  8 B; 
           [0038]      FIG. 10  shows a diagram of oscillator system  10 E; 
           [0039]      FIG. 11  shows a diagram of feedback network  12 B; 
           [0040]      FIG. 12  shows a diagram of feedback network  2 C; 
           [0041]      FIG. 13  shows a diagram of feedback network  2 D; 
           [0042]      FIG. 14  shows a block diagram of reference system  14 ; 
           [0043]      FIG. 15  shows a block diagram of reference system  14 A; 
           [0044]      FIG. 16  shows a block diagram of reference system  14 B; 
           [0045]      FIG. 17  shows a diagram of oscillator system  15 C; 
           [0046]      FIG. 18  shows a block diagram of analog to digital comparator measurement systems  300 ; 
           [0047]      FIG. 19  shows a block diagram of analog to digital comparator measurement systems  300 A; 
           [0048]      FIG. 20  shows a block diagram of analog to digital sensor measurement systems  400 ; and 
           [0049]      FIG. 21  shows a block diagram of analog to digital sensor measurement systems  400 A; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    An embodiment of the invention is shown in  FIG. 1 . The comparator system  1  comprising oscillator  10 , transducer  91 , and transducer  93 . Oscillator  10  receives signals to it&#39;s gain control inputs from transducer  91  and transducer  93 . The output of oscillator  10  is on line  6  which is connected to terminal  86 . Transducer  91  receives input at input  5  and transducer  93  receives input at input  92 . It is also understood that additional transducers with their respective inputs can also be used. 
         [0051]    As the signal level of input  5  varies in comparison to input  92 , the ratio of the transducer&#39;s impedance or gain level also varies which in turn varies the loop gain. When the loop gain of oscillator  10  is reduced below the value of one, the oscillation stops, and the AC signal level on line  6  becomes zero. 
         [0052]    Another embodiment of the invention is shown in  FIG. 2 . The comparator system  1 A comprising transducer  91 , transducer  93 , detector  82 , and oscillator  10 A comprising feedback network  2  and amplifier  3 . Feedback network  2  is connected to the input and output of amplifier  3  by lines  7  and  6  so as to be capable of oscillating. 
         [0053]    Feedback network  2  also receives gain control input signals from transducer  91 , and transducer  93 . The output of amplifier  3  is also connected to detector  82  input. The output signal of detector  82  is at terminal  86 . An AC signal is present on line  6  only when the oscillator  10 A is oscillating and detector  82  indicates whether or not an AC signal is present on line  6 . Detector  82  converts the signal on line  6  to the type of output signal desired at terminal  86 , which for example can be a digital signal. 
         [0054]    As the ratio of signal level of input  5  to input  92  varies and changes the loop gain of feedback network  2  and amplifier  3  lower than the value of one, oscillator  10 A stops oscillating. The AC signal level on line  6  at the detector  82  input becomes zero and the digital signal at terminal  86  changes state, for example from a high to a low level. Thus the digital signal at terminal  86  is controlled by the ratio of signal level of input  5  to input  92 . 
         [0055]    Referring now to oscillator system  10 B in  FIG. 3 . Oscillator  10 B consist of feedback network  2 A and amplifier  3 A. Oscillator  10 B is a variation of the well known by those skilled in the art, Wien bridge oscillator. Amplifier  3 A is shown implemented using an OP-AMP with gain setting resistors. The output of OP-AMP  30  is connected to line  6  and it&#39;s positive input is connected to line  7 . It&#39;s negative input is connected to one end of resistors  31  and resistor  33 . The other end of resistor  31  is connected to line  6  and the other end of resistor  33  is connected to ground. The gain of amplifier  3 A is a function of the resistance ratio of resistor  31  and resistor  33  since the gain of OP-AMP  30  is much greater than the gain set by the resistors  31  and resistor  33 . 
         [0056]    Feedback network  2 A provides phase shift dependant on frequency. The input signal to feedback network  2 A is connected by line  6  and the output signal is applied to line  7 . Transducer  91 B is implemented by transducer capacitor  74 . Transducer  93 B is implemented by transducer capacitor  75 . Transducer capacitor  75  is connected between line  6  and resistor  72 . The other side of resistor  72  is connected to line  7 , resistor  73  and transducer capacitor  74 . The other side of resistor  73  and transducer capacitor  74  are connected to ground. 
         [0057]    The capacitance of transducer capacitor  74  is varied by the signal at input  5 . The capacitance of transducer capacitor  75  is varied by the signal at input  92 . By changing the capacitance of transducer capacitor  74  or  75 , it&#39; impedance is changed. Thus transducer capacitor  74  and  75  both function as variable impedance transducers. The usual practice is to set resistors  73  and  72  equal value and that of transducer capacitors  75  and  74  nearly equal, but other combinations can also be used. As the impedance ratio of transducer capacitor  74  to transducer capacitor  75  varies the gain of feedback network  2 A also varies. 
         [0058]    Alternatively in comparator system  1 B shown in  FIG. 4 , the transducer  91  and transducer  93 , outputs are connected to the amplifier instead of feedback network  2 . Amplifier  8  is a variable gain amplifier whose gain is controlled by one or more input signals. In this case amplifier  8  receives gain control signals from the outputs of transducer  91  and transducer  93 . 
         [0059]    A version of Amplifier  8  is shown in  FIG. 5 . Amplifier  8 A is implemented using OP-AMP  30 . Transducer  91 A is implemented by transducer resistor  27  which has it&#39;s resistance controlled by the signal at input  5 . Transducer  93 A is implemented by transducer resistor  38  which has it&#39;s resistance controlled by the signal at input  92 . The output of OP-AMP  30  is connected to line  6  and it&#39;s positive input is connected to line  7 . It&#39;s negative input is connected to one end of and transducer resistor  38  and transducer resistor  27 . The other end of transducer resistor  38  is connected to line  6  and the other end of transducer resistor  27  is connected to ground. 
         [0060]    The gain of amplifier  8 A is a function of the resistance ratio of transducer resistor  38  and transducer resistor  27  since the gain of OP-AMP  30  is much greater than the gain set by the transducer resistors  38  and transducer resistor  27 . Using this approach, amplifier  8 A has stable phase shift and gain verses temperature and voltage, but the gain varies only as a function of the ratio of input  5  to input  92 . 
         [0061]    A sensor measuring system embodiment of the invention is shown in  FIG. 6 . The sensor measuring system  11  comprising oscillator  10 , transducer  91 , transducer  93  and detector  83 . Oscillator  10  receives signals to it&#39;s gain control inputs from transducer  91  and transducer  93 . The output of oscillator  10  is on line  6  which is connected to the detector  83  input. Detector  83  converts the signal on line  6  to the type of output signal desired on line  87  and at terminal  86 . The output signal of detector  83  on line  87  is applied to the input of transducer  93  and also terminal  86 . Alternatively if an AC signal is compatible with transducer  93  input, the signal on line  6  can be directly connected to line  87  and terminal  86  without detector  83  being used, or using detector  83  only to convert the signal on line  6  to the type of output signal desired at terminal  86 . Transducer  91  receives input at input  5 . It is understood that multiple transducers can also be used. 
         [0062]    The signals at input  5  and detector  83  output are both used to control the oscillator&#39;s loop gain. As the signal level of input  5  varies the transducer  91  impedance or gain also varies which in turn varies the loop gain. The detector  83  output also controls the loop gain through transducer  93 . The output signal of detector  83  is a signal proportional to the AC signal level on line  6 . As the AC signal level on line  6  increases the loop gain of oscillator  10  is reduced. As the transducer  91  output changes the loop gain, the AC signal level on line  6  also changes so as to just maintain oscillation with a loop gain at or close to one. Thus the signal level at terminal  86  is controlled by the signal level at input  5 . 
         [0063]    Another embodiment of the sensor measuring system is shown in  FIG. 7 . The sensor measuring system  11 A comprising oscillator  10 D, transducer  91 , transducer  93  and detector  83 . Oscillator  10 D contains feedback network  12  and amplifier  8 . Feedback network  12  is connected to the input and output of amplifier  8  by lines  6  and  7  so as to be capable of oscillating. Feedback network  12  also receives a gain control input signal from transducer  91 . Transducer  91  receives input signal at input  5 . Detector  83  is also connected to line  6 . An AC signal is present on line  6  only when the system is oscillating. The output signal of detector  83  on line  87  is applied to the input of transducer  93  and also terminal  86 . The output of transducer  93  is connected to the gain control input of amplifier  8 . Alternatively the transducer  91  output can be connected to amplifier  8  instead of feedback network  12  and the output signal of transducer  93  applied to a gain control input of feedback network  12 . 
         [0064]    The signals at input  5  and detector  83  output are both used to control the oscillator&#39;s loop gain. As the signal level of input  5  varies, the transducer&#39;s impedance or gain also varies, which in turn varies the loop gain. The detector  83  output also controls the loop gain through transducer  93 . The output signal of detector  83  is a signal proportional to the AC signal level at it&#39;s input. As the output signal level of detector  83  increases, the loop gain of oscillator  10 D is reduced. As the transducer  91  output changes the loop gain the detector  83  output also changes so as to just maintain oscillation with a loop gain at or close to one. Thus the signal level at terminal  86  is controlled by the signal level at input  5 . 
         [0065]    Another embodiment of the invention is shown in  FIG. 8 . The sensor measuring system  11 B comprising transducer  91 , transducer  93 , detector  83 , and oscillator  10 A comprising feedback network  2  and amplifier  3 . Feedback network  2  is connected to the input and output of amplifier  3  by lines  6  and  7  so as to be capable of oscillating. Feedback network  2  also receives gain control input signals from transducer  91 , and transducer  93 . The output of amplifier  3  is also connected to detector  83 . An AC signal is present on line  6  only when the system is oscillating. The output signal of detector  83  is on line  87  and also at terminal  86 . The output signal of detector  83  on line  87  is applied to the input of transducer  93  and also terminal  86 . Transducer  91  receives input at input  5 . It is understood that multiple transducers can also be used. 
         [0066]    The output signal of detector  83  is a signal proportional to the AC signal level at it&#39;s input. As the output signal level of detector  83  increases the gain of feedback network  2  is decreased. The gain of feedback network  2  is also controlled by the output signal level of transducer  91 . As the signal level of input  5  varies and changes the loop gain of feedback network  2  and amplifier  3 , the signal level on line  87  varies to keep the loop gain at or close to the value of one such that the system just maintains oscillation. Thus the signal level at terminal  86  is controlled by the signal level at input  5 . 
         [0067]    Amplifier  8 B shown in  FIG. 9  is implemented using an OP-AMP  30  with resistor  33  and transducer  93 A. Transducer  93 A is implemented by transducer resistor  38 . The output of OP-AMP  30  connected to line  6  and it&#39;s positive input is connected to line  7 . It&#39;s negative input is connected to one end of resistor  33  and transducer resistor  38 . The other end of transducer resistor  38  is connected to line  6  and the other end of resistor  33  is connected to ground. The resistance ratio of resistor  33  to transducer resistor  38  determines the gain of amplifier  8 B since the gain of OP-AMP  30  is much greater than the gain set by the resistors  33  and transducer resistor  38 . Using this approach, amplifier  8 B has stable phase shift and gain verses temperature and voltage, but the gain varies only in accordance with the input signal of transducer  93 A. 
         [0068]    It is also understood that resistor  33  can be replaced by a transducer resistor. The gain of amplifier  8 B is now determined by the resistance ratio of two transducer resistors. When the same input signal is applied to both of them such that the impedance of one transducer resistor increases while the other decreases, the sensitivity of amplifier  8 B to the input signal is increased while further reducing temperature dependence. 
         [0069]    Referring now to oscillator system  10 E in  FIG. 10 . Oscillator  10 E consist of feedback network  12 A and amplifier  8 B. Oscillator  10 E is a variation of the well known by those skilled in the art, Wien bridge oscillator. 
         [0070]    Feedback network  12 A provides phase shift dependant on frequency. The input signal to feedback network  12 A is connected by line  6  and the output signal is applied to line  7 . Transducer  91 B is implemented by transducer capacitor  74 . Capacitor  71  is connected between line  6  and resistor  72 . The other side of resistor  72  is connected to line  7 , resistor  73  and transducer capacitor  74 . The other sides of resistor  73  and transducer capacitor  74  are connected to ground. 
         [0071]    The capacitance of transducer capacitor  74  is varied by the signal level at input  5 . By changing the capacitance of transducer capacitor  74  it&#39; impedance is changed. Thus transducer capacitor  74  functions as variable impedance transducer. The usual practice is to set resistors  73  and  72  equal value and that of capacitors  71  and  74  close in value, but other combinations can also be used. As the value of transducer capacitor  74  varies the gain of feedback network  12 A also varies. 
         [0072]    Referring now to  FIG. 11  is feedback network  12 B which is the same as feedback network  12 A except that capacitor  71  is replaced by transducer  96  which is implemented by transducer capacitor  75 . Transducer capacitor  75  and transducer capacitor  74  both function as variable impedance transducers. The same signal at input  5 , is applied to both of them so that the capacitance of one transducer capacitor increases while the other decreases thus increasing the sensitivity of feedback network  12 B to input  5  while further reducing temperature dependence. 
         [0073]    Referring now to feedback network  12 C in  FIG. 12  which is the well known twin T network that is used with a phase inverter amplifier to provide the phase shift needed for oscillation to be possible. Transducer  91 B is implemented by transducer capacitor  74 . Capacitor  25  is connected between line  6  and resistor  26  and capacitor  24 . The other side of capacitor  24  is connected to line  7 . Resistor  22  is connected between line  6  and transducer capacitor  74  and resistor  21 . The other side of resistor  21  is connected to line  7 . The other sides of resistor  26  and transducer capacitor  74  are connected to ground. 
         [0074]    The usual practice is to use resistors  21  and  22  of equal value and also equal value capacitors  24  and  25 . Resistor  26  is set to ½ the value of resistor  21 . As the capacitance of transducer capacitor  74  approaches twice the value of capacitor  24  the gain of feedback network  12 C approaches zero. 
         [0075]    Referring now to  FIG. 13  is feedback network  12 D which is the same a feedback network  12 C except that transducer capacitor  74  is replaced by capacitor  28  and resistor  26  is replaced by transducer  91 A. Transducer  91 A is implemented by transducer resistor  27 . Capacitor  28  is set to twice the capacitance of capacitor  24 . The resistance of transducer resistor  27  is varied by the input signal of transducer  91 A. Thus transducer resistor  27  functions as variable impedance transducer. As the resistance of transducer resistor  27  approaches ½ the value of resistor  21  the gain of feedback network  12 D approaches zero. 
         [0076]    A reference system embodiment of the invention is shown in  FIG. 14 . The reference system  14  comprising oscillator  15 , transducer  93 , and detector  83 . Oscillator  15  receives signals to it&#39;s gain control input from transducer  93 . The output of oscillator  15  is on line  6  which is connected to the detector  83  input. Detector  83  converts the signal on line  6  to the type of output signal desired on line  87  and at terminal  86 . The output signal of detector  83  on line  87  is applied to the input of transducer  93  and also terminal  86 . In the case of a reference current system the output signal would be the current level through line  87 , or in a voltage reference the voltage is on line  87 . Alternatively if a AC signal is compatible with transducer  93  input, the signal on line  6  can be directly connected to line  87  and terminal  86  without detector  83  being used, or using detector  83  only to convert the signal on line  6  to the type of output signal desired at terminal  86 . 
         [0077]    Detector  83  is used to control the oscillator&#39;s loop gain. The detector  83  output signal applied to transducer  93  controls a gain varying element of the loop. It is understood that detector  83  may be connected to addition transducers controlling gain varying elements of the loop. The output signal level of detector  83 , of which one form may be a DC voltage or current, is a signal proportional to the AC signal level at it&#39;s input. As the output signal level of detector  83  increases the loop gain of oscillator  15  is reduced. The detector output signal level is of a value that just maintains oscillation. In effect detector  83  provides negative feedback to maintain a low AC signal level on line  6  which occurs at a loop gain value at or close to one. The detectors output signal level therefore remains a constant value that can be used as a reference since the loop gain is constant except for the variation caused by the variation in the output of detector  83 . 
         [0078]    Another embodiment of the invention is shown in  FIG. 15 . The reference system  14 A comprising oscillator  15 A, transducer  93 , and detector  83 . Oscillator  15 A contains feedback network  9  and amplifier  8 . Feedback network  9  is connected to the input and output of amplifier  8  by lines  6  and  7  so as to be capable of oscillating. The output of amplifier  8  is also connected to detector  83  input. An AC signal is present on line  6  only when the system is oscillating. The output signal of detector  83  on line  87  is applied through transducer  93  to the gain control input of amplifier  8  and also to terminal  86 . The gain of amplifier  8  is varied in response to detector  83  output signal level. 
         [0079]    The output signal level of detector  83  is proportional to the AC signal level at it&#39;s input. As the output signal level of detector  83  increases, the loop gain of feedback network  9  and amplifier  8  is reduced. The detector  83  output signal level is of a value that just maintains oscillation by keeping the loop gain value at or close to one. The detector  83  output signal level therefore remains a constant value. 
         [0080]    Another embodiment of the invention is shown in  FIG. 16 . The reference system  14 B comprising transducer  91 , detector  83 , and oscillator  15 B comprising feedback network  12  and amplifier  3 . Feedback network  12  is connected to the input and output of amplifier  3  by lines  6  and  7  so as to be capable of oscillating. Feedback network  12  also receives input signals from transducer  91 . The output of amplifier  3  is also connected to detector  83 . The output signal of detector  83  on line  87  is applied to transducer  91  and also terminal  86 . An AC signal is present on line  6  only when the system is oscillating. 
         [0081]    The output signal level of detector  83  is proportional to the AC signal level at it&#39;s input. As the output signal level of detector  83  increases the gain of feedback network  12  is decreased. The detector output signal level is of a value that just maintains oscillation by keeping the loop gain value at or close to one. The detector  83  output signal level therefore remains a constant value. 
         [0082]    Referring now to oscillator system  15 C in  FIG. 17 . Oscillator  15 C consist of feedback network  12 A and amplifier  3 A. Oscillator  15 C is a variation of the well known by those skilled in the art, Wien bridge oscillator. 
         [0083]    Amplifier  3 A is shown implemented using OP-AMP  30  with resistors  31  and  33 . The output of OP-AMP  30  is connected to line  6  and it&#39;s positive input is connected to line  7 . It&#39;s negative input is connected to one end of resistors  31  and resistor  33 . The other end of resistor  31  is connected to line  6  and the other end of resistor  33  is connected to ground. The resistance ratio of resistor  31  and resistor  33  determines the gain of amplifier  3 A since the gain of OP-AMP  30  is much greater than the gain set by the resistors  31  and resistor  33 . 
         [0084]    Feedback network  12 A provides phase shift dependant on frequency. A input signal to feedback network  12 A is on line  6  and the output signal is applied to line  7 . Transducer  91 B is implemented by transducer capacitor  74 . Capacitor  71  is connected between line  6  and resistor  72 . The other side of resistor  72  is connected to line  7 , resistor  73  and transducer capacitor  74 . The other sides of resistor  73  and transducer capacitor  74  are connected to ground. The capacitance of transducer capacitor  74  is varied by the signal level on line  87 . By changing the capacitance of transducer capacitor  74  it&#39; impedance is changed. The usual practice is to set resistors  73  and  72  equal value and that of capacitors  71  and  74  nearly equal, but other combinations can also be used. As the signal level on line  87  varies, the value of transducer capacitor  74  varies, causing the gain of feedback network  12 A to also vary. 
         [0085]    Referring now to  FIG. 18 , analog to digital comparator measurement systems  300  provides a output, which maybe digital, representing the value of the signal level ratio of the inputs applied to input  5  and input  92 . 
         [0086]    Transducers  91 , and  93  outputs are connected to gain control inputs of oscillator  20  along with the output of signal generator  99 . The oscillator  20  output on line  6  connected to terminal  86  is the presence or absence of an AC signal. The signal generator  99  output is a time varying signal that is connected to a gain control input of oscillator  20  and terminal  89 . The signal generator  99  output signal maybe a digital signal or can be converted to a digital signal to provide a digital output signal. Signal generator  99  continually recycles through it&#39;s full output range. 
         [0087]    The output signal level at terminal  86  changes when the oscillator  20  loop gain level increase above or decreases below the minimum loop gain value for oscillation to occur and be present on line  6 . The level of oscillator  20  loop gain is a function of the signal level ratio of input  5  to input  92  and also the signal generator  99  output signal value. Since the signal generator  99  output also controls the level of oscillator  20  loop gain, the signal generator  99  output value at terminal  89  indicates the signal level ratio of input  5  to input  92  when the change from presence to absence or vice a versa of an AC signal occurs at terminal  86 . 
         [0088]    Referring now to  FIG. 19 , is analog to digital comparator measurement systems  300 A which provides a digital output representing the level of the ratio of input  5  to input  92  signal levels. Oscillator  20 A consist of feedback network  2  and amplifier  18 . Feedback network  2  is connected to the input and output of amplifier  18  by lines  6  and  7  so as to be capable of oscillating. Feedback network  2  also receives input signal from transducer  91  and transducer  93 . Transducer  91  receives input signal from input  5  and transducer  93  receives input signal from input  92 . The output of amplifier  18  is also connected to detector  82  input. The output signal of detector  82  is connected to terminal  86 . This output signal is a digital signal indicating the presence or absence of an AC signal on line  6 . Signal generator  99 A consist of oscillator  85  and counter  84 . Oscillator  85  is an independent oscillator. The frequency of oscillator  85  is lower than that of the AC signal when present on line  6 . Counter  84  counts the number of oscillator  85  output cycles. The output of counter  84  is a digital signal which is connected to the gain control input of amplifier  18  and terminal  89 . The counter  84  output signal value determines the gain level of amplifier  18 . Counter  84  after reaching it&#39;s maximum count resets to zero and starts counting again. 
         [0089]    The detector  82  output signal at terminal  86  changes when the loop gain level increase above or decreases below the minimum loop gain value for oscillation to occur and be present on line  6 . The level of amplifier  18  gain required for oscillation is a function of the signal level ratio at input  5  to input  92 . Since the counter  84  output signal value controls the level of amplifier  18  gain, counter  84  output signal value at terminal  89  also represents the level of the signal level ratio of input  5  to input  92  when the digital signal at terminal  86  changes state. 
         [0090]    The preferred sequence of operation is for the counter to start with the gain of amplifier  18  much higher then required for oscillation to start, and then to reduce amplifier  18  gain. By starting at a high gain value the oscillation starts faster and more reliably. 
         [0091]    Referring now to  FIG. 20 , is analog to digital sensor measuring system  400  which provides a output, which maybe digital, representing the value of the signal level at input  5 . 
         [0092]    Transducers  91  output along with the output of signal generator  99 , are connected to the gain control inputs of oscillator  30 . The oscillator  30  output on line  6  connected to terminal  86  is the presence or absence of an AC signal. The signal generator  99  output is a time varying signal that is connected to a gain control input of oscillator  30  and terminal  89 . The signal generator  99  output signal maybe a digital signal or can be converted to a digital signal to provide a digital output signal. Signal generator  99  continually recycles through it&#39;s full output range. 
         [0093]    The output signal at terminal  86  changes when the oscillator  30  loop gain level increase above or decreases below the minimum loop gain value for oscillation to occur and be present on line  6 . The level of oscillator  30  loop gain is a function of input  5  and also the signal generator  99  output signal value. Since the signal generator  99  output signal also controls the level of oscillator  30  loop gain, the signal generator  99  output signal value at terminal  89  indicates the signal level of input  5  when the change from presence to absence or vice a versa of an AC signal occurs at terminal  86 . 
         [0094]    Referring now to analog to digital sensor measuring system  400 A in  FIG. 21 . Oscillator  30 A consist of feedback network  12  and amplifier  18 . Feedback network  12  is connected to the input and output of amplifier  18  by lines  6  and  7  so as to be capable of oscillating. Feedback network  12  also receives input signal from transducer  91  which receives input signal from input  5 . The output of Amplifier  18  is also connected to detector  82  input. The output signal of detector  82  is connected to terminal  86 . This output signal indicates the presence or absence of an AC signal on line  6 . Signal generator  99 A consist of oscillator  85  and counter  84 . Oscillator  85  is an independent oscillator. The frequency of oscillator  85  is lower than that of the AC signal when present on line  6 . Counter  84  counts the number of oscillator  85  output cycles. The output of counter  84  is a digital signal which is connected to the gain control input of amplifier  18  and terminal  89 . The counter  84  output signal value determines the gain level of amplifier  18 . Counter  84  after reaching it&#39;s maximum count resets to zero and starts counting again. 
         [0095]    The detector  82  output signal at terminal  86  changes when the loop gain level increase above or decreases below the minimum loop gain value for oscillation to occur and be present on line  6 . Since the counter  84  output signal controls the level of amplifier  18  gain, counter  84  output signal value at terminal  89  also represents the signal level of input  5  when the digital signal at terminal  86  changes state. 
         [0096]    Although the above description has been directed to preferred embodiments of the invention, it will be understood and appreciated by those skilled in the art that other variations and modifications may be made without departing from the spirit and scope of the invention, and therefore the invention includes the full range of equivalents of the features and aspects set forth in the appended claims.