Abstract:
Multiple stage ranging circuitry for a measuring circuit includes a first ranging circuit that selectively amplifies an input signal by respective amounts according to the magnitude of the input signal thereto, and a second ranging circuit coupled to the output of the first ranging circuit selectively amplifies the output signal from the first ranging circuit by a respective amount according to the magnitude of that output signal to provide a measured output. The multiple stage ranging circuitry increases dynamic range and provides for high speed measurements over that dynamic range. 
     A method of ranging for high speed signal measurements over a wide dynamic range includes directing an input signal to a first ranging stage that responds at one speed to set ranging over a relatively wide variation in input signal magnitudes, and directing the output signal from the first ranging stage to a second ranging stage responsive at a second speed faster than the one speed to set ranging over a relatively narrower range of signal magnitudes to provide for signal measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/277,348, filed Mar. 19, 2001. 

   TECHNICAL FIELD 
   This invention relates generally, as indicated, to multiple stage ranging for measurement devices, and, more particularly, to apparatus and method employing multiple stage ranging for obtaining high speed, accurate signal measurements over a wide range of magnitudes. The invention also relates to bifurcated range stages to increase speed or dynamic range of measuring instruments and associated methods. 
   BACKGROUND 
   As the use of optical signals continues to increase and the speeds of equipment employing optical signals also increase, improved fast and accurate measurement techniques are needed. Optical signal carriers, such as fiber optic cable, and various connecting, switching, amplifying, and detecting devices require testing. As a present example of measuring speed, in some test instruments measurements are made at the rate of ten thousand or more per second. 
   An example of an optical test instrument, namely, an optical power meter for detecting loss factors in fiber optic communications, is described in U.S. Pat. No. 5,825,516. 
   An example of a device to be tested is that known as the Telecom DWDM system, that relies on deep optical filter components, which often reject at 25 dB to 50 dB and more, clearly a rather wide range. In other optical instruments signal strengths vary over an even wider range, e.g., between approximately −7 dB and approximately −45 dB. Validating performance of such devices may be performed using a tunable laser source and an optical power meter. However, conventional test systems, which include an optical power meter and associated measurement instrument, usually are generally linear-ranging instruments, and the range typically changes over three (3) to five (5) ranges, e.g., decades, as signals between 25 dB and 40 dB, for example, are measured. Usually it is desirable for a signal to be measured in the proper range setting of a measurement instrument to obtain the most accurate measurement. For example, usually it is undesirable to measure a small magnitude signal at an upper range of a multiple range measurement instrument because accuracy and resolution would be rather poor; similarly a large magnitude signal would not be measured at a low rage because the likely would be off-scale, as is well known. In conventional linear-ranging test systems although accurate measurements can be obtained, the time it takes to determine the correct range and to switch to that range can be a major component of the time required to perform the test and, thus, slows down the overall measurement procedure. 
   In conventional linear ranging measurement circuitry and in software ranging measurement circuitry, usually time is wasted determining the range in which the measurement circuitry should operate to measure signal strength of a given input signal. In a typical case the measurement circuitry measures the input signal using one range, usually that one range is whatever range was used for the last measurement made. The circuitry determines whether the signal is over range, i.e., larger than the measurement circuitry is able to measure while in the present range, and in such case, the ranging mechanism must select a higher range. Similarly, if the measurement demonstrates that the measured signal is below a prescribed level or percent of the signals typically measured in the present range of the measurement circuitry, e.g., below 10% of the largest signals measured at the present range, then a lower range must be selected. After selecting the new range, another measurement is made. If that measurement is in range, the measurement result is acceptable; but if that measurement shows the measured signal is above or below range, as was mentioned before, the range selection step must be repeated until an acceptable range is identified. To make a measurement in a new range, the measurement circuitry must “settle” so that the measurement circuitry is set with the newly selected range of measurement capability. Thus, total measurement time is the settling time times the number of range changes required to reach the proper range, plus the measurement time times the number of range changes. Accordingly, it is desirable to expedite the adjustments in measurement circuitry to reduce the time required for selecting range. 
   Measurements made using linear range measurement instruments are relatively accurate; but, as ranges must be changed, depending on the magnitude (or signal strength) of the measured signal, operation of such instruments may be relatively slow. Accordingly, there is a need to increase the speed of making such measurements. 
   In making measurements of signals that vary in magnitude over a relatively wide range, non-linear measurement systems, which include nonlinear amplifiers, sometimes have been used. An example of a non-linear amplifier is a logarithmic amplifier. A logarithmic amplifier based measurement instrument may be relatively faster than the linear ranging instruments mentioned above, because ranges do not have to be switched, or at least the number of switched ranges is fewer than for a linear ranging measurement instrument. However, logarithmic amplifiers compress the results of the measurement, which leads to a reduction in accuracy of the measurement, especially when measuring relatively smaller signals in the large range over which a signal magnitude or signal strength may vary. Accordingly, there is a need to improve the accuracy of such measurements. 
   Linear ranging measurement devices tend to be more sensitive and/or accurate than non-linear measurement devices; and non-linear measurement devices tend to be faster than linear ranging measurement devices. There is a need for increasing the speed of making measurement while maintaining a high level of sensitivity and accuracy. 
   The dynamic range of a measuring instrument having multiple ranges depends on the ranges. As the number of ranges increase and or the span of respective ranges increases, a reduction in speed and/or accuracy may occur due to the time constants in the ranging and/or measurement circuits. For example, the signal input or source has characteristic resistance and capacitance, which provides an “input time constant;” and the respective parts, e.g., amplifiers, of the ranging circuitry have respective characteristic resistance and capacitance, which also provides a time constant. The different ranging subcircuits in the ranging circuitry may have different respective time constants, which makes compensation for those differences as ranges change difficult, if not impossible. Absent compensation for the time constants, measurements may be taken of signals before a signal has stabilized, for example. Thus, there is a need to increase the speed of measurements made over relatively wide ranges. Likewise, there is a need to increase the dynamic range over which measurements can be made relatively fast. 
   Stated another way, a speed and accuracy limiting factor on high speed measurements is the time constants due to the source or input resistance and capacitance and the resistance and capacitance of respective amplifiers in the measuring and/or ranging circuitry. Due to such time constants, signals may not be adequately stable or formed for accurate high speed measurement thereof as the signals vary and/or as ranges are changed. Thus, there is a need to maintain accuracy and speed of such measurements. 
   SUMMARY 
   Briefly, according to an aspect of the invention, multiple stage ranging circuitry for measurement instruments has a stage to handle a relatively wide variations in input signals and another stage to handle a relatively narrow variation in input signals, the speed of operation of the latter stage being faster than the speed of operation of the former stage. 
   Briefly, according to an aspect of the invention, multiple stage ranging circuitry for measurement instruments has a stage to handle a relatively wide variations in input signals and another stage to handle a relatively narrow variation in input signals to increase dynamic range of the measurement instrument. 
   According to another aspect, a measuring circuit includes a first ranging circuit that selectively amplifies an input signal by a respective amount according to a prescribed input, and a second ranging circuit that selectively amplifies an input signal by a respective amount according to a prescribed input. 
   According to another aspect, a method of ranging for signals includes directing an input signal to a first ranging stage which is responsive at one speed to set ranging over a relatively wide variation in input sign magnitudes, and directing a signal representative of an output from the first ranging stage to a second ranging stage that is responsive at a second speed faster than the one speed to set ranging over a relatively narrower range of signal magnitudes. 
   According to another aspect, a method of increasing the dynamic range of meter ranging circuitry includes directing an input signal to a first ranging stage which selects respective ranges over a relatively wide variation in input signal magnitudes, and directing a signal representative of an output from the first ranging stage to a further ranging stage to set ranging over a relatively narrower range of signal magnitudes. 
   According to another aspect of the present invention a non-linear amplifier drives linear ranging circuitry in a measurement instrument. 
   According to another aspect a non-linear ranging device is used to control a linear ranging device. 
   According to another aspect, one stage of a multiple stage ranging circuit is controlled by a comparator circuit. 
   According to another aspect, a non-linear amplifier circuit and a linear optical power meter are cooperative in response to an input signal such that the non-linear amplifier circuit drives linear ranging circuitry of the optical power meter to set the linear measuring range thereof. 
   Another aspect relates to quickly and accurately measuring signals of the type wherein the signal strength varies over a wide range. 
   Another aspect is to provide a wide dynamic range for measurement circuitry. 
   Another aspect is to measure signals of the type wherein the signal strength may vary over a wide range of on the order of 50 dB, wherein accurate measurements can be obtained for small and large signals, and wherein the measurements can be made relatively fast. 
   Another aspect relates to apparatus for measuring signals over a large range, including measurement circuitry to measure signals, the measurement circuitry having multiple rage stages, each including a number of respective range determinative amplifiers and a range selector. 
   Another aspect relates to an optical power meter, including measurement circuitry for measuring electrical signals representative of detected light over a wide range of optical power, including ranging circuitry arranged in a number of stages to determine respective ranges of the optical power measured, and respective selectors in parallel with the ranging or measurement circuitry to control operation of the ranging circuitry to select the measuring range of the measurement circuitry in response to magnitude of signals provided the respective stages. 
   Another aspect ret to a range selector for measurement circuitry useful to measure inputs having a wide range of variation over a number of orders of magnitude, including a non-linear amplifier providing an output in response to an input representative of a signal to be measured, and range section circuits responsive to the non-linear amplifier to provide range selection signals for use to select operating range of such measurement circuitry. 
   Another aspect relates to a high speed range selector for optical power measuring apparatus including measurement circuitry having ranging circuitry and operable to measure signals over a number of ranges, characterized in that the ranging circuitry includes a number of stages, one having a larger range than the other, and the other being faster than the one. 
   A number of features are described herein with respect to embodiments of the invention; it will be appreciated that features described with respect to a given embodiment also may be employed in connection with other embodiments. 
   The invention comprises the features described herein, including the description, the annexed drawings, and, if appended, the claims which set forth in detail certain illustrative embodiments. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. 
   Although the invention is shown and described with respect to illustrative embodiments, it is evident that equivalents and modifications will occur to those persons skilled in the art upon the reading and understanding hereof. The present invention includes all such equivalents and modifications and is limited only by the scope of the claims if appended hereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the annexed drawings: 
       FIG. 1  is a schematic illustration of measurement circuitry according to an embodiment of the invention; 
       FIG. 2  is a schematic circuit diagram that provides non-linear ranging to control a linear ranging measurement device according to the invention illustrated in  FIG. 1 , for example; 
       FIG. 3  is a schematic illustration of an optical testing instrument including a number of optical power meters according to an embodiment of the invention; 
       FIG. 4  is a schematic circuit diagram, similar to  FIG. 2 , showing a generic range signal decoder to provide range signals to the measurement circuitry in response to the output from a non-linear amplifier; 
       FIG. 5  is a schematic block diagram illustration of an embodiment of the invention using multiple stage ranging circuitry; 
       FIG. 6  is a schematic circuit diagram and block diagram illustrating more details of the multiple stage ranging circuitry of  FIG. 5 ; 
       FIG. 7  is a schematic circuit diagram and block diagram illustrating the multiple stage ranging circuit of  FIGS. 5 and 6  showing details of exemplary range selectors; 
       FIG. 8  is a schematic circuit diagram in block form showing a processor responsive to range information to control setting or coordination of an analog to digital converter (A/D) output device; and 
       FIG. 9  is a computer program flow chart or logic diagram illustrating exemplary operation of the circuit of  FIG. 8 . 
   

   DESCRIPTION 
   Referring in detail to the drawings, wherein like reference numerals designate like parts in the several figures, and initially to  FIG. 1 , an optical power meter and measurement circuitry according to the invention is generally indicated at  10 . The optical power meter and measurement circuitry  10  includes a light detector  11 , measurement circuitry  12  and range selection circuit  13 . The optical power meter and measurement circuitry  10  measures light  14  received from a device under test  15  (DUT). 
   The device under test may be any of a variety of devices in which optical signals are used, developed, conducted, switched, etc. The device under test may be a fiber optic device, such as a fiber optic cable, individual optical fiber, multiplexer, optical switch or cross connect device, or other optical source, optical device, and/or optical communication device, system or component or communication to, in and/or from another computing device. The light  14  provided by the device under test  15  for measurement by the optical power meter and measurement circuitry  10  may be a single wavelength, more than one wavelength, etc., and may be in a narrow or broad spectrum range, for example, in the visible range, in a range that is other than visible, or in a range that is partly visible and/or partly other than visible, etc. 
   The range selection circuit  13  is coupled to the detector  11  and in parallel relation to the measurement circuitry  12  to set the range of the measurement circuitry according to the magnitude of the light signal detected by the detector  11 . As was mentioned, it is antics that the signal strength of the light signal may vary over a wide range, and the cooperative relation of the measurement circuitry  12  and the range selection circuit  13 , which receive the conditioned signal that is proportionally representative of the light signal, allows for fast, accurate measurement of the light signal. Accordingly, the range selection circuit  13  is a nonlinear circuit able to respond to a wide range of sigal strengths detected by the detector  11  and to set the measuring range of the linear-ranging measurement circuitry  12  allowing the measurement circuitry to maintain good sensitivity while measuring signals with good accuracy, as is described in further detail below with respect to  FIG. 2 . 
   Output device  16  may be used for several purposes. One example, is a meter that displays the signal strength values measured by the measurement circuitry  12  or some other values measured by the measurement circuitry. The output  16  may be a computer that stores, uses and/or analyzes data from the measurement circuitry. The output  16  may be a display, such as a liquid crystal display, cathode ray tube display, or other display that displays the data graphically, numerically or both. The output  16  may take other forms, too, as is known in the art and may be developed in the future. 
   The present invention utilizes a non-linear amplifier described further below and associated circuitry configured to directly drive linear ranging circuitry in the measure circuitry  12 . This arrangement allows the optical power meter to change ranges relatively fast, for example, as fast as one (1) ties the hardware settling time. Using the non-linear range selection circuit  13  in parallel with the linear-ranging measurement circuitry  12  to select the linear ranging thereof, ranging happens in real time. Therefore, the sometimes long cycles required for range setting using convent software based ranging or using exclusively linear hardware ranging approaches are not encountered. 
   The range selection circuit  13  is illustrates in greater detail in  FIG. 2  in the optical power meter and measurement circuitry  10 . An input signal in the form of light  14  ( FIG. 1 ) impinging on the detector  11  causes an electrical signal produced by the detector. The electrical signal from the detector  11  is conditioned by an appropriate amplifier or pre-amplifier circuit  20  and is provided by lines  21 ,  22  as an input signal, which is proportionally of the light signal strength, simultaneously to the parallel connected measurement circuitry  12  and the range selection circuitry  13 . The measurement circuitry  12  and the range selection circuit  13  are conned in parallel, which contributes to the increased speed of the range selecting function and making of measurements, compared to prior ranging systems. 
   The measurement circuitry  12  may be conventional measurement circuitry for the signals intended to be measured by the optical power meter and measurement circuitry  10 . Several examples of such measurement circuitry are mentioned above, and others also exist. In an embodiment of the present invention the measurement circuitry is of the type which includes linear ranging capability to obtain good accuracy in the measurements, for example, compared to some measurement circuitry that use non-linear measurement techniques, which may have reduced accuracy at certain signal ranges, for example, at lower signal strengths. 
   In the range selection circuit  13  illustrated, the non-linear amplifier  23  is a logarithmic amplifier. The logarithmic amplifier  23  receives the conditioned input signal on line  22  and provides its output on line  24  as an input to a number of parallel connected comparators  25   a – 25   f  (collectively designated by reference numeral  25 ). 
   The respective outputs  26   a – 26   f  (collectively referred to as output  26 ) from each of the comparators  25  are coupled to the measurement circuitry  12  to select the range of the measurement circuitry. The range can be selected by an appropriate logic circuit in the measurement circuitry  12 . The range can be selected using appropriate analog or digital circuitry able to respond to the output from the non-linear amplifier  23  to provide to the measurement circuitry  12  a signal of the type to which the linear-ranging circuitry thereof can respond. The range may be selected by software that responds to a digital word composed by signals on the outputs  26  from the comparators  25 . In any event, though, the outputs from the comparators  25  in effect are range selection signals that determine the range setting of the met circuitry  12  to measure the signal then being detected by the detector  11 . The number of comparators  25  and/or their arrangement along with logic decoding circuitry, software or the like may be coordinated with the number of different ranges that can be selected in the measurement circuitry  12 . 
   In an exemplary embodiment of the invention, the conditioned input signal provided by the detector  11  and pre-amplifier  20  to the lines  21 – 22  may vary over several orders of magnitude, for example five or six, say, from on the order of about 1 to about 100,000 or even 1,000,000, or still greater. The logarithmic amplifier  23  may in effect compress the input signal thereto, which varies over a range of 1 to 100,000, so that the output signal therefom varies over a range of, for example, 1 to 50. The output signal from the logarithmic amplifier  23  triggers respective comparators to switch the range of the measurement circuitry at decade intervals (or at some other intervals, as may be desired). Thus, for example, only the comparator  25   a  may be triggered to produce a prescribed output when the output from the logarithmic amplifier  23  is below (or perhaps is either at or below) a prescribed magnitude, such as, for example, 10% of the maximum output from the logarithmic amplifier. The other comparators  25   b – 25   f  would be triggered at different respective percentages of the maximum output signal from the logarithmic amplifier  23 . The output signal from the logarithmic amplifier would be representative of the signal to be measured, e.g., the output from the pre-amplifier  20  or the light signal impinging on the detector  11 . 
   In an exemplary embodiment the number of comparators that are triggered will represent and determine the range setting for the measurement circuitry  12 . Alternatively, if the comparators are set up so only one comparator would be triggered exclusively, then that sole comparator would be the controlling factor for setting the range in the measurement circuitry  12 . 
   As will be appreciated, the comparators  25  may be set up to provide a prescribed output when the magnitude of the logarithmic amplifier  23  (or other non-linear device) provides an output that is a given value or a given range of values. The comparators  25  may be set up so that when the signal to be measured is within one range only a single comparator provides the prescribed output to set the range of the measurement circuit  12  and when the signal to be measured is in a different range only a different single comparator provides the prescribed output. Alternatively, the comparators may be set up so that for one or more of the ranges several comparators provide the prescribed output to set the range of the measurement circuitry. 
   It also will be appreciated that although the invention is disclosed using comparators  25  to respond to the output form the non-linear amplifier  23  (or other non-linear device), other devices, such as logic circuits, threshold detecting circuits, digital voltmeters, or other analog or digital devices or circuits may be used to respond to the non-linear device to provide an input to the measurement circuitry  12  to set the range thereof. 
   Although the range selecting circuit  13  is illustrated and described with respect to a logarithmic amplifier  23 , it will be appreciated that the amplifier  23  may be another non-linear device. Such non-linear device is able to respond to the input signal, to compress that input signal and to provide appropriate inputs to the comparators  25  so they can operate to cause the measurement circuitry to select the appropriate range for measuring the input signal. 
   The optical power meter and measurement circuitry  10  operates in relatively short time to select the range of the measurement circuitry  12  and to measure the input signal. The amplifier  23 , whether logarithmic or otherwise non-linear, compresses the range of its input signal so the comparators can be set to operate in parallel easily and promptly to trigger and thereby to set the range of the measurement circuitry  12  so accurate and prompt measurement can be made of the input signal. 
   For convenience, measurement circuitry  12  may include ranging in respective decades, as is conventional, or the ranging my be in other increments. Therefore the amplifier  23  and comparators  25  are coordinated to affect, select, control or adjust the ranging circuitry in the circuitry  12  at respective decades in accordance with the signal strength of the input signal detected by the detector  11 . In a sense the range selection circuit  13  uses the non-linear function of the amplifier  23  to compress the dynamic range of the measured signal and to control the measurement circuitry so that the measured signal is measured at an appropriate range setting of the measurement circuitry. Thus, the combination of the non-linear range selection circuitry  13  to select the measuring range of a linear-ranging measurement circuitry  12  in effect improves the dynamic range of the optical power meter and measurement circuitry  10 . The ranges of the measurement circuitry may be substantially linear or in any event may be relatively linear ranges compared, for example, to those ranges encountered in non-linear measuring instruments, such as logarithmic measuring instruments or the like. 
   Thus, the linear ranging effected in the measurement circuitry  12  by the range selection circuit  13  allows for fast, accurate measuring of input signals that vary over wide ranges. 
   Briefly referring to  FIG. 3 , an optical testing apparatus  30  is illustrated. The optical testing apparatus has a number of measuring channels  31 ,  32  for measuring simultaneously or substantially simultaneously light  14  from different respective optical channels of the device under test  15  or light from different optical devices, etc. Although only two measuring channels are illustrated, it will be appreciated that there may be more than two channels. Each measuring channel includes an optical power meter and measurement circuitry  10  of the type described herein, which function as was described above. Although the outputs  16  are shown separately for each measuring channel, it will be appreciated that the output data may be combined for several of the measuring channels for use in data analysis by a computer, storage, graphical display, comparison, or other purpose. 
   Briefly referring to  FIG. 4 , a modified optical power meter and measurement circuitry  10 ′ is illustrated. In this embodiment a range signal decoder  40  is used to respond to the output from the non-linear device  23  (here illustrated as a logarithmic amplifier). The range signal decoder  40  responds to or decodes the output from the logarithmic amplifier and provides a direct input to the linear-ranging circuitry in the measurement circuitry to select the operational range of the measurement circuitry to obtain an accurate measurement of the input signal. It will be appreciated that the range signal decoder  34  may be any of a number of devices, for example, comparators as were described above, analog or digital logic circuitry, a computer for decoding the output from the logarithmic amplifier  23  to provide the range selection for the measurement circuitry, etc. 
   Turning to  FIG. 5 , a sign measuring system  100  including a multiple stage ranging circuitry  101  is illustrated. The system  100  includes an input  102 , a signal conditioning circuit  103 , the multiple stage ranging circuitry exemplified as a two (2) stage ranging circuit (referred to below for convenience as “2-stage ranging circuit” or “circuitry”), measuring circuit and analog to digital (A/D)  104  and output  105 . The output  105  may be the same as or similar to the output  16  described above. The input  102  may be, for example, a photosensitive diode or some other device that provides an input signal to the signal conditioning circuit  103 . If light is to be measured by the signal measuring system  100 , the input  102  may be a photosensitive diode. If some other parameter is to be measured, the input  102  may be some other sensor or the like. The signal conditioning circuit  103  may be a conventional pre-amplifier or some other signal conditioning device used in measurement instruments. 
   The two stage ranging circuitry  101 , which is described in greater detail below, receives an input signal from the signal conditioning circuit  103  and/or input device  102 , adjusts the range for amplification and/or measurement of that input signal, and delivers the signal and/or range setting adjustments to the measuring circuit, A/D  104 . The measuring circuit, A/D  104  measures the signal provided it and delivers to the output  105  information representative of that measurement for display, recording or other use. 
   Turning to  FIG. 6 , portions of the signal measuring system  100 , including the 2-stage ranging circuitry  101 , which has a first stage  110  and a second stage  111 , are shown in greater detail. The first stage  110  of the 2-stage ranging circuitry  101  includes a number of amplifiers, each having a different gain, such as, for example, x1, x10, x100, x1000, x10000, x100000, and 1000000 or more. In the illustrated embodiment of  FIG. 6 , there are seven such amplifiers  112   a–   112   g  (collectively referred to by reference  112 ) in the first stage  110 , although there may be more or fewer amplifiers; and the gain of the respective amplifiers may be different than presented in this example. The amplifiers  112  are connected by respective switches  113   a – 113   g  (collectively referred to by reference number  113 ) as respective amplification or range setting channels in parallel between an input  114  and an output  115 . The input  114  may be from the signal conditioning circuit  103  and/or input device  102 ; and the output  115  may be coupled to the measuring circuit, A/D  104 . 
   A switch controller  116  controls operation of the switches  113  based on a prescribed input to the switch controller. For example, the switch controller  116  is coupled to the input  114  to sense the magnitude or other characteristic of the signal thereon as the prescribed input, and based on that sensing the switch controller operates via one of its outputs  117  to turn on (or off) respective switches  113  to couple a respective amplifier  112  in circuit between input  114  and output  115 , thus setting the range of the first sage  110 . Another output  118  from the switch controller  116  is coupled to the measuring circuit, A/D  104  to indicate the range at which the first stage  110  is set. 
   The second stage  111  of the 2-stage ranging circuitry  101  may be similar to the first stage  110 . The second stage  111  is coupled to the output  115  of the first stage  110  to receive the output signal therefrom as an input to the sed stage. If desired, there may be one or more additional circuits, ranging circuitry stages, or the like coupled between the first and second stages. For brevity, simplicity and convenience, though, the two ranging stages  110 ,  111  are shown coupled directly together, whereby the output  115  of the first stage  110  is the input to the second stage  111 . 
   The second stage  111  includes a number of amplifiers, each having a different gain, such as, for example, x1, x10, and x100. In the illustrated embodiment, the second stage  111  has three amplifiers  120   a ,  120   b ,  120   c , which are coupled in respective amplification or range setting channels, each of which includes a respective amplifier  120  and a respective switch  121   a ,  121   b ,  121   c . The respective amplifiers  120  and switches  121  are coupled in parallel between the input  115  of the second stage and the output  122  thereof. The second stage  111  also includes a switch controller  123 , which turns on (or of respective switches  121  based on a prescribed input to the switch controller. For example, the prescribed input to the switch controller  123  may be the signal, on the input  115  of the second stage, i.e., in the illustrated embodiment output from the first stage  110 . The switch controller  123  has a number of outputs collectively indicated at  124  that operate respective switches  121  and an output  125  to provide an input to the measuring circuit, A/D  104  to indicate the range at which the second stage  111  is set. 
   As was described above, the source resistance and capacitance, e.g., the circuitry coupled to the input  114  of the first stage  110  of the 2-stage ranging circuit  101 , and the various resistance and capacitance ms of the respective amplifier channels, which include amplifiers  112  and switches  113 , for ample, may be incompatible and inconsistent. Therefore, time involved in establishing signals for measurement, the taking of measurements, and the switching of ranges may result in inaccuracies and/or inconsistencies. Therefore, the switch controller  116  switches the respective amplification channels of the first stage  110  at a speed that is relatively slow, for example sufficiently slow as to allow for the timing out of respective time constants and settling of signals for measurement. As was indicated above, the first stage  110  handles a relatively large range of signal variations or magnitudes, e.g., front x1 to x1,000,000 gain (or smaller or even larger, if desired). 
   However, the second stage  111  of the 2-stage ranging circuitry  101  may be faster in its switching operation than the first stage  110  and/or may have a smaller range over which gain is varied, e.g., from x1 to x100. Since the time constants associated with a smaller gain amplifier usually are shorter than the time constants associated with at least some of the larger gain amplifiers in the first stage  110 , the impact on time constants and speed of measurements by the second stage  111  is less than by the first stage  110 . Therefore, the second stage  111  can be switched to different ranges by the switch controller  123  faster than the gains in the first stage are switched, while still providing for accurate measurements. Furthermore, since the second stage  111  is serially connected with the first stage  110 , the dynamic range of the measurement system  100  is increased relative to a measurement system with only a single stage ranging circuitry, while still being able to provide for high speed measurements. Exemplary measurements may be taken at a speed on the order of 100,000 per second while providing a dynamic range of on the order of 50 dB. 
   The operating gains of the two stages  110 ,  111  of the 2-stage ranging circuitry  101  are combined to provide a transfer function for the measurement system  100 . For example, the gains in effect due to respective amplifiers  112 ,  120  being switched into operation by respective switch controllers  116 ,  123 , may be multiplied to provide the transfer function and, thus, the effective gain of the 2-stage ranging circuitry. The measuring circuit, A/D  104  is in of the transfer function by the collections of respective outputs  118 ,  125  from the switch controllers  116 ,  123  to thereto, as is illustrated. Therefore, the measuring circuit, A/D  104  provides a digital output representative of the signal being provided at the input to the measurement system  100 , e.g., as detected by the photosensitive diode  102 . 
   Turning to  FIG. 7 , another example of 2-stage ranging circuitry  110   a  is illustrated. The 2-stage ranging circuitry  101   a  is similar in construction and operation to the 2-stage ranging circuitry  101  described above. However, in the 2-stage ranging circuitry  110   a  the switch controller  116   a  is a logarithmic amplifier and the switch controller  123   a  is a number of comparators  130   a ,  130   b ,  130   c . The logarithmic amplifier may function similarly to the logarithmic amplifier  23  described above, for example, with respect to the embodiment illustrated in  FIG. 1 ; and operation of the first stage  110  of the 2-stage ranging circuitry  101  is carried out generally as was described above with respect to the description of  FIGS. 5 and 6 . The logarithmic amplifier switch controller  116   a  provides an output on respective lines, which are collectively illustrated and referred to as  117 , to control respective sits  113 , such as field effect transistor (FET) switches, for example; and an output is provided on line  125  to the measuring circuit, A/D  104 , here illustrated as an A/D  104   a  and an A/D control-processor  104   b.    
   An example of the AID control-processor  104   b  is illustrated in  FIG. 8 , including a processor  131 , such as a microprocessor, and associated programming, memory  132 , input/output channels  133 , etc. The A/D control-processor  104   b  may be a specially designed circuit, sometimes referred to as an ASIC. The AID  104   a  receive special inputs, as is shown at  134  in  FIG. 8 , from the logarithmic amplifier switch controller  116  and the comparators switch controller  123  via respective lines  118 ,  125 , to indicate the current transfer function of the 2-agc ranging circuitry  101 . This way the output information from the A/D  104   a  is directly representative on the appropriate scale of the input signal to the measuring system  100 , e.g., from the photosensitive diode  102 . 
   In the second stage  111 , the switch controller  123  ( FIG. 6 ) and the switch controller  123   a  ( FIG. 7 ) may be a logarithmic amplifier, such as the logarithmic amplifier  116   a . However as the number of ranges to be switched in the second stage  111  is relatively small and the total span of the range of the second stage and/or the range span of each amplifier  120  is relatively small, especially compared to the total span of the first stage  110  and/or of the larger gain amplifiers of the first stage, a switch controller  123   a  may be composed of several comparator circuits  130   a ,  130   b ,  130   c , as is illustrated in  FIG. 7 . 
   Each comparator  130  has an input  136  coupled to line  115 , which is the input to the second stage  111  of the 2-stage ranging circuitry  101  to monitor the signal thereon. Each comparator also has a reference signal input  137   a ,  137   b ,  137   c , which is a percentage function related inversely to the gains/ranges of the amplifiers  120   a ,  120   b ,  120   c , for example, 100%, 10%, and 1%, respectively, of a reference signal from a signal source, not shown. Therefore, depending on the signal on line  115 , a respective comparator  135   a ,  135   b ,  135   c  will produce an output that will turn on a respective FET switch  121   a ,  121   b ,  121   c  to couple a respective amplifier  120   a ,  120   b ,  120   c  in circuit between line  115  and line  122 . Other techniques also may be used to select which of the amplifiers  120  is to be turned on, e.g., coupled in operation by a switch  121 , in the second stage  111 . 
   Referring to  FIG. 9 , a computer program, flow chart or logic diagram  150  is illustrated. The program  150  may be used to prepare appropriate computer program or software to operate the processor  131 , for example, to carry out the measuring functions of the measuring system  100  using the 2-stage ranging circuitry  101 , e.g., as is illustrated in  FIGS. 7 and 8 . At block  151  and  152  the first and second stages  110 ,  111  are checked to determine what is the range sing thereof. At block  153  the range settings are provided to the processor  130 . At block  154  the A/D  104   a  takes a measurement by counting the analog signal provided it on line  122  and converts that analog signal to a digital signal representing the value thereof. At block  155 , based on the range information obtained at blocks  151 ,  152  the digital signal is coordinated with the transfer function of the 2-stage ranging circuitry  101 , which identifies the counted digital signal value according to the transfer function of the 2-stage ranging circuitry  101 , thus indicating the real magnitude of the signal detected by the input device  102 , for example. 
   At block  156  the digital signal is provided to the output  105  of the measurement system  100  as an indication of the detected signal by the input device  102  and for further use, as may be desired. 
   In the illustrated and described embodiments hereof the invention is described with respect to optical measurement instruments, such as optical power meters. However, it will be appreciated that the features of the invention may be used with other measurement systems. 
   INDUSTRIAL APPLICATION 
   The present invention may be used to measure signals over a wide dynamic range at relatively high speed.