PATENT ABSTRACT
A system and method is designed to measure its own environmentally caused inaccuracies and, based upon these measurements, adjust itself to compensate for the inaccuracies. In one embodiment, a test system first measures the signal loss through a model “long” path constructed in the same substrate as is the main test circuit. Since the test path is constructed on the same substrate it then represents the actual environmental impact on the test circuit. The test signal is then sent through a “short” test path and the ratio difference from a reference measurement condition between the two paths yields the necessary compensation which is then used to calibrate the test circuit. In another embodiment, a test signal is applied across a capacitance made up of copper on different layers of substrate material. The actual environmental conditions on the substrate layers modify the measured capacitance value, which is then provided along with temperature as input to a model which determines compensation for the test circuit. Both embodiments can be applied to individual circuits or to systems that are subject to environmentally induced changes to their transmission line loss characteristics.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION  
       [0001]     Changes in environmental humidity and temperature cause drift in the calibrated accuracy of high frequency signal generators, power meters, measuring receivers and other electronic test equipment. This equipment is expected to perform to specification in climates ranging from hot and dry to cold and wet. Typically this equipment is constructed using printed circuit boards made of dielectric materials which are affected by changes in temperature (dimensionally and electrically) and which absorb water from the environment. As a result, the insertion loss and characteristic impedance of transmission line structures fabricated on these boards will vary with changes in environmental conditions. This variation affects the calibrated accuracy of the test equipment. Since the environment in which the test equipment is calibrated can differ from that in which it is to be used, allowances must be made in the equipment specification setting process to be able to guarantee the specified level of performance over a range of environmental conditions. These allowances result in poorer performance specifications for the equipment than would be possible if the environmental variation did not exist.  
         [0002]     Typically, some form of temperature compensation is incorporated into the equipment design. Ambient temperature is fairly easy to sense and the equipment performance is characterized as a function of this temperature. During operation, corrections are made to compensate for ambient temperature variation. Many instrument specifications require that the instrument must be powered on for some period of time to allow the relationship between ambient temperature and the instrument internal temperature to stabilize. Depending on the instrument&#39;s design, this time period can range from minutes to hours. The effectiveness of this temperature compensation is limited because not all points in the equipment chassis are at the same temperature, the temperature characteristics of various printed circuit assemblies differ, and the effects of moisture absorption are uncompensated.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     It has been observed that not only do the current environmental conditions impact equipment inaccuracies but the cumulative past environmental conditions also act to change the accuracy. Taking this observation into consideration, a system and method is designed to first measure parameters related to its own environmentally induced inaccuracies and then based upon these measurements, the system adjusts itself to compensate for the inaccuracies.  
         [0004]     In one embodiment, an insertion loss sensing system is formed by a long transmission line and a short transmission line. An RF source and detector are used to measure the difference between the insertion losses of these two transmission lines. This difference in insertion loss, and the difference in length between the two transmission lines, provides a measure of the loss per unit length of transmission lines formed on the same substrate (or similar substrates) as the insertion loss sensing system. By capturing the loss per unit length data at the time the electronic test equipment is calibrated, and again at time intervals during operation of this equipment, it is possible to determine changes in the equipment&#39;s calibration due to changes induced by the environmental conditions.  
         [0005]     In another embodiment, the capacitance of parallel plate capacitors formed by copper areas on the printed circuit boards are measured. Capacitance and board temperature are measured at the time the equipment is calibrated, and the data is stored in non-volatile memory. During operation, capacitance and temperature are measured again (at time intervals). The values measured at calibration time and those during operation are fed into an algorithm which models the board&#39;s environmental behavior. This algorithm then produces a correction factor which is used to compensate for the environmentally induced change from the original calibrated performance.  
         [0006]     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0008]      FIG. 1  shows one embodiment of an RF signal trace on a board;  
         [0009]      FIG. 2  shows one embodiment of a method for calibrating electronic equipment;  
         [0010]      FIG. 3A  illustrates one embodiment of a system and method for using an equivalent circuit path for determining environmental loss error;  
         [0011]      FIG. 3B  illustrates one embodiment of a circuit for utilizing the concepts of the invention;  
         [0012]      FIG. 4A  illustrates one embodiment for using capacitance and temperature measurement to determine dielectric characteristic changes which are then applied to a model to determine environmentally induced performance (gain) changes  
         [0013]      FIG. 4B  illustrates one embodiment of a circuit and method block diagram which utilizes the capacitance and temperature measurement concept of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  shows one embodiment of a representative circuit board  12  in an RF instrument illustrating representative signal path  11  extending from input  101  through the board and through various circuits thereon (shown in  FIG. 3B ) to signal output  102 . Note that, if desired, the input signal could be generated on board  12  instead of on a separate circuit.  
         [0015]     In operation, in one embodiment, a signal (such as from source  31   FIG. 3A ) is selected as an input to the RF test circuitry  300  (shown in  FIG. 3B  and represented on  FIG. 1  as path  11 ). In one embodiment, an output from path  11  is applied to device under test (DUT)  103 . An output from DUT  103  is then applied to test receiver  104  to determine if the DUT is within a range. Alternately, DUT  103  could output its own signal which is then received by test receiver  104 . In some situations the test signal generator and the test receiver are in the same housing of a measurement test system.  
         [0016]     A typical board dimension for board  12  would be 11.2″ wide and 5.2″ high, with the typical RF signal path  11  having a length between 15″ and 24″. PC board  12  is typically constructed from one of several different board materials such as, FR4, GETEK™, or Rogers™ 4350. These materials will absorb moisture over a period of time and this moisture affects the loss characteristic of RF signals propagating on transmission lines formed on these boards which is also dependent on temperature for any given moisture content.  
         [0017]     RF System designers are putting more and more functionality into a single RF module, which typically contains one of these boards. The RF path on a board will typically contain amplifiers, mixers, filters, modulators, switches, and power splitters to generate an RF signal having a desired frequency and other parameters. Signals are isolated from one another by ground planes and internal walls with gaskets on the front and back covers. Typical overall path losses for these types of paths in GETEK™ are from 0.75 to 1.5 dB at 500 MHz, from 1.5 to 2.4 dB at 1,000 MHz and from 3.0 to 4.8 dB at 2,500 MHz. The loss variation depends on the type of PC board dielectric material. For example, the path losses for FR 4 material are a little more than the values shown above and the path losses for Rogers™ 4350 material are about one-half these values.  
         [0018]     The loss variation also depends on the type of RF path. Microstrip, on an outer surface of the board, has the lowest loss and stripline, inside a multilayer board between two ground planes, is higher in loss. Different types of shielding and matching require the use of both microstrip and stripline structures. Using a GETEK™ design and depending on the RF path length, the loss on a board can vary as much as 1.5 dB at 2,500 MHz due to environmentally induced changes caused by temperature and humidity.  
         [0019]     In a specific example of an RF signal generator design, present calibration procedures can take out most of the observed 0.6 dB variation down to a level below 0.1 dB uncertainty immediately following the calibration. However, since calibration is intrusive, it is normally limited to being performed once per day. Under such a once a day procedure it has been observed that environmental loss uncertainty can be lowered to only 0.3 dB. By adding together all the uncertainties of measurement, manufacturing and yield, a typical RF source accuracy using the once per day calibration procedure yields a +/−1.0 dB accuracy specification. Note that with only a factory calibration and no further once a day calibration, the accuracy spec would be +/−1.3 dB due to environmental conditions. Using the compensation concepts described herein it is anticipated that as much as 0.4 to 0.5 dB error can be removed so as to achieve an overall RF source accuracy specification of +/−0.8 to 0.9 dB from 500 MHz to 2500 MHz. Circuit designs with longer traces and/or with more stripline traces could achieve even greater improvement than in this example. Since environmental compensation can be applied for each test performed, if desired, the initial (or subsequent) device calibrations need not be performed as often. Also, since the compensation adjusts for environmental conditions, such as moisture, there is no need to allow the circuitry to “dry out” prior to running a test protocol on a piece of equipment.  
         [0020]     Since PC board transmission line losses are the biggest source of the humidity and temperature induced errors, systems that have more PC boards or longer PC board RF path lengths, can achieve much improved calibration accuracy using the concepts discussed herein.  
         [0021]      FIG. 2  shows one embodiment  20  of a method for calibrating electronic equipment, such as, for example, signal generators, signal measuring receivers, power meters and the like. In the embodiment shown, the equipment to be compensated is test equipment in a frequency range between 500 MHz and 2,500 MHz, but the procedures discussed herein can be utilized for any equipment having RF signals that are affected by environmental effects on a circuit board.  
         [0022]     Process  202  determines if it is time for an environmental compensation to be run on the circuit according to certain parameters. These parameters are determined when the circuit is designed and characterized over the expected environmental conditions. This step can be avoided, if desired and the compensation can be performed on a continuous or periodic basis. If the compensation is not to be performed, then the test signal is produced (or in the case of a measurement device, measured) using the selected test frequency via process  207  by, applying the last correct test protocol. If environmental compensation is to be performed, then process  204  selects a calibration signal frequency based upon the selected frequency of the test protocol. Process  205  applies the calibration signal as will be described to determine the cumulative environmental effect on the RF circuit trace. Using this cumulative effect determination, process  206  determines the loss error to the RF signal based upon the environmental conditions. Process  207  applies correct compensation to the test protocol at the selected test frequency or adjusts the receiving circuitry by compensating the receiving circuitry for the effects of the environmental conditions. Process  208  then performs the test on the actual equipment (not shown) according to the test protocol selected for the test RF signal.  
         [0023]     Note that since the compensation can be done internally, processes  204 - 207  could be initiated at any time and in fact can be done at times when the system is not being utilized for actual testing thereby further maintaining the accuracy of the system by reducing compensation related downtime as well as inaccurate readings.  
         [0024]      FIG. 3A  illustrates one embodiment  30  of a system and method utilizing a measured board loss change in an equivalent circuit path ( 34 ) to determine the change in the loss in the actual RF path  300  ( FIG. 3B ). Since the PC board accumulates loss changes from moisture as absorbed by the board in its particular environment over time, it is possible to create within the PC board (or on a separated board if desired) a representative path  34 , herein called the long path, which is used to determine a ratio between path  34  and short path  33  which effectively allows for the monitoring of environmental differences since a prior calibration. The long (or mock) path is created in the same substrate (or in a substrate having the same physical properties when exposed to moisture over time) as is the actual RF path so that it is representative of the moisture and temperature effects over time experienced by the actual RF path.  
         [0025]     This procedure can be accomplished in one of many ways. For example, calibration source  31  is applied to RF power splitter  32  which sends the calibration signal through short trace  33  and through long trace  34 . RF switch  35  under control of self calibration process  302 , which in turn is under control of control program  301 , switches back and forth between the short path (trace) and the long path (trace). The outputs from each trace are detected via RF level detector  36 , converted to digital values via A to D converter  37  and presented to microprocessor  38 . Control program  301  then determines the ratio between the short trace and the long trace to arrive at a loss approximation as to how environmental conditions have changed actual test circuit  300  (shown in  FIG. 3B ). Note that long path  34  and short path  33  can be constructed on the same substrate as the actual circuit to be compensated (circuit  300 ) or they can be created on a separate board using materials that react similarly to the environmental conditions as the materials used in the boards of the actual RF circuitry  300  to be compensated.  
         [0026]      FIG. 3B  shows RF circuitry  300  to be compensated which is adjusted under control program  301  to yield proper test results regardless of environmental conditions. Thus, as shown in  FIG. 3B , signal source or synthesizer  310  is provided to input amplifier  311  which goes to filters  312 , modulators  313  and other signal conditioning circuits  314  to output amplifier  315 . Output amplifier  315  or any of the other elements, in circuit  300  have been adjusted by the control program  301  to compensate for the current environmental conditions as determined by the circuitry of  FIG. 3A  based on a measured difference due to humidity and temperature working on the substrate. In this manner output  102  of test circuit  300  is compensated for the environmental effects which have accumulated over a period of time.  
         [0027]      FIG. 4A  illustrates one example of a system and method using measured capacitance and temperature changes as inputs to a model to estimate the actual loss to be expected in the RF path. Structure  40 , a multi-layer printed circuit board, absorbs moisture from the environment. As this moisture enters the board, it changes the dielectric constant of the board material since the permittivity of water is higher than that of the board material. A capacitor is formed between copper area  405  and ground plane  403 . Board dielectric layer  401  forms the dielectric for this capacitor. Sensing changes in the capacitance of this capacitor structure provides information about the moisture content in board dielectric layer  401  which will affect surface microstrip transmission line losses. Similarly, a capacitor is formed between copper area  406  and ground planes  403  and  404  with board dielectric  402  forming the capacitor dielectric. Sensing changes in capacitance of this capacitor structure provides information about the moisture content in board dielectric  402 , which will affect internal stripline transmission line losses.  
         [0028]     Capacitance measurement circuitry  41  is connected to copper area  405  by a surface printed circuit trace and to copper area  406  by plated printed circuit via hole  407 . Temperature measurement circuitry  410  senses the temperature of the printed circuit board. Capacitance measurement circuitry  41  and temperature measurement circuitry  410  can both be realized advantageously using ADC model AD7747 available from Analog Devices, Inc. This ADC is a two channel capacitance to digital converter which provides high resolution capacitance measurement and also contains an on-chip temperature sensor.  
         [0029]      FIG. 4B  illustrates an environmental compensation system  400 . Microprocessor  42  receives input from temperature sensor  410  and capacitance sensor  41 . This information is provided to calibration process  45  at the time process  45  generates calibration data for RF circuitry  420  on the printed circuit board associated with system  400 . This calibration data is typically RF gain as a function of RF frequency, and is used by control process  48  to make hardware control settings in RF circuitry  420 , via microprocessor  42 . The capacitance and temperature data presented to calibration process  45  represent the board environmental condition at the time the RF circuitry calibration data is generated.  
         [0030]     During normal operation, microprocessor  42  collects temperature and capacitance data periodically and presents the data to moisture estimation algorithm  44 . Moisture estimation algorithm  44  provides an estimate of the change in printed circuit board moisture content since calibration to loss model  46 . Loss model  46  takes the moisture change and the temperature change since the original RF circuitry calibration data was generated and produces a set of data  47  which predicts the change in RF circuit performance as a function of operating frequency. Data  47  is then used, along with the RF circuitry calibration data produced by calibration process  45 , by operational control process  48  to make settings in the RF circuitry to produce calibrated operation with compensation for the environmental effects.  
         [0031]     Since various dielectric substrate materials may be used to fabricate printed circuit boards in a test instrument, different moisture estimation algorithms ( 44 ) may be required for circuit boards of differing construction. Loss model  46  is not only circuit board construction dependent; it is dependent on the RF circuit design itself. Thus, each design will require a unique loss model. This model is typically generated by correlating moisture and temperature changes, during controlled environmental characterization testing, to measured RF circuit performance.  
         [0032]     Placement of the capacitive and temperature sensors can impact the accuracy of the environmental compensation. Water absorption by the board dielectric is a relatively slow process and absorption rates may differ from one area of a board to another. For example, water incursion will occur faster near the edges of a PC board. For maximum accuracy, the sensors need to be placed such that conditions in critical circuit areas are accurately reflected by the sensor data.  
         [0033]     Note also that while the calibration of a test signal output (signal generator) has been discussed, a receiving circuit (measuring receiver), or a power meter, or any other type of equipment that is sensitive to calibration parameters, can also be calibrated. In fact, the signal generator, the signal receiver or both can be calibrated, if desired, in the same system.  
         [0034]     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. One of ordinary skill in the art will readily appreciate from the disclosure of the present invention, any processes, machines, manufacture, compositions of matter, means, methods, or steps, that presently exist or that will be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.