Digital phase meter apparatus
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates broadly to a phase meter, and in particular to a digital phase meter apparatus. 
     Phase measurements are utilized to determine the relative times at which alternating currents and voltages in a circuit take on zero values. If two voltages v 1  and v 2  are zero at the same instant, they are in phase, with zero phase difference (or out of phase with 180° difference). If one voltage v 1  passes through zero 1/8 cycle before a second voltage v 2 , it leads by 360°/8 or 45°. The common phase meter, a commercial device for determining the angle between current and voltage, can be used when its presence will not disturb the circuits under measurement. When phase angles to be measured are in high-impedance or low power circuits, this common device may be unsatisfactory and other measurement methods must be used. 
     The electronic phase-angle meter provides the angle (π-θ) directly. One such prior art instrument converts the two voltage waves to square waves by repeated amplification and limiting. The zero crossings of the square waves are identical to the zero crossing of the original voltage waves. The two square waves are applied to the input of a circuit that will pass current only when both square waves are positive. In this case the greater the lag of one voltage, the smaller the overlap of the positive portions and the lower the average current. The current in this case is proporational to θ. 
     This type of circuit has the theoretical limitation that each input voltage must be greater than a critical minimum value. In practice the critical value is determined by the noise on the amplifier input. If the voltage is too low, this noise causes a random zero-crossing shift and the results would be subject to this uncertainty. 
     A precision phase-angle meter for high frequency voltages uses a variable delay line, and its operation is based on the fact that the difference of two voltages of constant amplitude is a minimum when the two are in phase. One of the two voltages to be compared is connected to both inputs of a variable-delay line, which is then adjusted to give a minimum output. The two voltages to be compared are then connected to the two terminals and the delay line is readjusted to give a minimum output. The change in the delay-line setting indicates the time delay of one voltage relative to the other. When the frequency, is known, the time delay can be computed as angle of lag. If Δt is the change in the delay-line setting and the frequency is f hertz, the phase angle is given by 2πfΔt radians or 360fΔt degrees. 
     Some prior art devices for electronically or digitally measuring phase difference are disclosed in the following U.S. Patents. For example, U.S. Pat. No 3,469,196 discloses a circuit for detecting the phase difference between two signals which includes means for squaring the two signals, and, first and second gates connected thereto for developing outputs whenever the two signals are simultaneously negative or positive respectively. U.S. Pat. No. 3,553,579 discloses a circuit for measuring the difference in phase between a reference signal and an unknown sinusoidal signal which includes a digital counter which provides a digital indication of the amount of phase displacement. A couple of references which are cited for general interest are U.S. Pat. Nos. 3,820,022 and 3,953,794 each of which discloses a circuit for measuring the phase difference between two signals. 
     The present invention, which utilizes a combination of digital elements to perform the function of measuring the phase difference between an input signal and a reference signal, is not disclosed by the prior art. 
     SUMMARY OF THE INVENTION 
     The digital phase meter utilizes a digital phase measurement circuit to measure the phase difference between an input signal and a reference signal of the same frequency, and output this phase difference information in the form of an eight bit number. The digital phase meter has control lines which are digitally compatible with microprocessor or computer-based control operations. The digital phase meter utilizes three analog inputs: a clock input, a signal input, and a reference input. The clock input, which can be either sinusoidal or square, is used to synchronize and drive the digital components of the circuit. The signal and reference inputs are the signals whose phase difference is measured. The digital control lines for the digital phase meter are start, clear, read ready, and read data. All control lines are active low and can be sequentially addressed for a digitally controlled phase measurement routine. The digital phase meter is operational for input frequencies of up to 10 MHz and clock frequencies of 40 MHz. The maximum level of precision of the digital phase meter is wavelength (λ)/256. (The precision is proportional to the signal-to-clock frequency ratio. Greater precision can be realized by cascading counters to accommodate lower signal-to-clock frequency ratios.). 
     It is one object of the present invention, therefore, to provide an improved digital phase meter apparatus. 
     It is another object of the invention to provide an improved digital phase meter apparatus which is completely digitally compatible and controllable. 
     It is another object of the invention to provide an improved digital phase meter apparatus which utilizes leading/lagging signal edge detection and thus is immune to modulo π ambiguities 
     It is another object of the invention to provide an improved digital phase meter apparatus which has to 8-bit precision (wavelength, λ/256). 
     It is another object of the invention to provide an improved digital phase meter apparatus which is constructed with off-the-shelf, low cost, standard TTL integrated circuit units. 
     These and other advantages, objects and features of the invention will become more apparent after considering the following description taken in conjunction with the illustrative embodiment in the accompanying drawings 
    
    
     BRIEF DISCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the digital phase meter apparatus according to the present invention, 
     FIG. 2 is a flow diagram for the operation of the digital phase meter apparatus, 
     FIG. 3 is being comprised of FIGS. 3a-3c which are a schematic diagram of the digital phase meter apparatus, and, 
     FIG. 4 is a block diagram of a distance measuring configuration using the digital phase meter apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 there is shown a block diagram of the digital phase meter apparatus to measure and determine the phase difference between an applied input signal and a reference signal. A plurality of high speed differential voltage amplifier/dual differential line receiver units 10, 12 and 14 which comprise signal squaring units, respectively receive a clock signal, signal under test and a reference signal. The clock signal which is applied to signal squaring unit 10 appears at the output as a square wave which is applied to the clock input of the 8 bit counter unit 18. The output of signal squaring unit 12, which is a square wave representative of the input signal under test, is applied to the leading edge phase detection unit 16. The output of signal squaring unit 14 is a square wave which is representative of the input reference signal and it is applied to the leading edge/phase detection unit 16. The leading edge/phase detection unit 16 receives two control signals, a START and a CLEAR signal. The leading edge detection portion of unit 16 provides an output signal to the 8 bit counter unit 18 at its counter enable input. The phase detection portion of unit 16 provides an output signal to the input of latch 2 unit number 20. The latch 2 unit 20 receives a control signal entitled &#34;READ READY&#34;. The output of the 8 bit counter unit 18 is applied to the latch 1 unit 22. The latch 1 unit 22 receives a control signal entitled &#34;READ DATA&#34;. When the read ready signal and the read data signal are respectively applied to latch unit 2 unit 20 and latch unit 1 unit 22, the output from the latch 2 unit 20 is combined with the output of the latch 1 unit 22 and is applied, as shown, to the data bus. However, the data bus will sense the data from only one of the appropriately strobed latches at a time. Turning now to FIG. 2 there is shown a flow diagram of the logical operation steps that occur in the digital phase meter apparatus of FIG. 1. The phase measurement process is initialed by first applying a CLEAR signal. The CLEAR signal clears the 8-bit counter unit 18 and the leading edge/phase detection unit 16. The START signal is applied to the leading edge/phase detection unit 16 to initiate the detection of the leading edge of the reference signal that is utilized to measure the phase deviation therefrom of the signal under test. The READ READY signal is applied to the latch-2 unit 20 to determine if the phase measurement process is completed. When the latch-2 unit 20 indicates that the phase measurement process has been completed, the READ DATA signal can be applied to the latch-1 unit 22 which puts the phase measurement data from latch-1 unit 22 onto the data bus line. 
     In FIG. 3 there is shown the manner in which FIGS. 3a, 3b and 3c are combined to form a complete schematic diagram of the digital phase meter apparatus. As described with respect to FIG. 1, the digital phase meter apparatus receives three analog input signals which are, respectively, the CLOCK signal, the SIGNAL-A, and the REFERENCE signal. In FIG. 3a, there is shown the CLOCK signal and the SIGNAL-A signal being respectively applied to high speed differential voltage comparator (LM361) units 50, 52. FIG. 3b, shows the REFERENCE signal applied to a high speed differential voltage comparator (LM361) unit 54. The outputs of units 50, 52, and 54 are respectively connected to dual differential line receiver (UA9637) units 56a, 56b and 56c. The combination of the differential voltage comparator (LM361) and the dual differential line receiver (UA9637) comprise the signal squaring units of FIG. 1. There are five digital control lines which are utilizes in the digital phase meter apparatus. These digital control signals are designated START, CLEAR, READ READY and READ DATA. The clock input, which can be as high as 40 MHz, is used to synchronize and drive the digital components in the circuit. The SIGNAL-A and REFERENCE inputs are the two signals between which the phase difference is measured. The data acquisition and phase measurement which is performed by the digital phase meter apparatus is divided into four basic functions which are: Signal Squaring, Leading-Edge Detection, Phase Measurement, and Digital Conversion. The sequence and explanation of these basic functions will be described with respect to FIGS. 3a-3c. In addition, reference will be made to FIG. 1 wherein the simplicity of that Figure will provide a further aid. 
     The signal squaring function will be readily understood with respect to FIG. 1 wherein the clock signal, reference input signal are applied to the signal squaring units 10, 12, 14 respectively. Each of the signal squaring units 10, 12, 14 comprise, as indicated in FIG. 3a, 3b, an LM361 unit in combination with a UA9637 unit. The detailed schematic diagram for the digital phase meter apparatus circuit is shown in FIGS. 3a through 3c. 
     The CLOCK, SIGNAL, and REFERENCE input signals are sinusoidal waveforms with a typical amplitude of approximately 600 millivolts peak to peak. The SIGNAL-A is the input signal of which the phase is to be measured. In order to condition the SIGNAL-A for a more defined leading edge, a high speed differential voltage comparator (LM361) and a dual differential line receiver (UA9637) are used to square up and amplify the incoming signals to make them TTL compatible. FIG. 1 shows this function wherein three sine waves are applied respectively to the units 10, 12, 14 and the output signal is a well-defined square wave. The clock frequency, while higher than the SIGNAL and REFERENCE frequencies, should not exceed these frequencies by more than a multiple of 256. This is to ensure that the maximum phase difference measured can still be represented by an 8-bit output. The SIGNAL-A and REFERENCE signals should be of the same frequency. 
     The phase measurement process is accomplished in the leading edge/phase detection unit (FIG. 1, unit 16) which are part of integrated circuit unit 74S74 in FIGS. 3a, b and are labelled respectively units A, B, C and D. The integrated circuit unit 74S74 comprises a series of D flip-flops which are used to detect the leading edges of the SIGNAL-A and REFERENCE inputs. The edge detection process is initiated when the flip-flops A, B, D and the counters (units 60, 62) are cleared due to the strobing of the Clear control line (Pl-2 in Figure 3c). When the Start control line (Pl-15 in FIG. 3b) clocks flip-flop unit A, the flip-flop unit B goes high when the leading edge of the REFERENCE signal is detected (the leading edge clocks the flip-flop). The output of the flip-flop unit C, which is initially high, goes low when the leading edge of the SIGNAL-A input is detected. When the complement output of the flip-flop unit C goes high, the flip-flop unit D is clocked and a logical &#34;1&#34; is entered to the least significant bit input of latch 2 unit 20. In general, the flip-flop units B and C, in conjunction with the AND gate 58, act as a switch that is activated on the leading edge of the REFERENCE signal and is deactivated on the leading edge of the SIGNAL-A input. During this activation time, the counter unit (units 60, 62 in Figure 3c) is enabled and the CLOCK signal drives the counter unit 60, 62. 
     The phase measurement is accomplished by the operation of the flip-flop units B and C, the AND gate 58 and the counter units 60, 62. The time interval between the two positive leading edges of the SIGNAL-A and the REFERENCE signal specifies the phase difference. The AND gate 58 which is in the high state for the phase measurement duration, enables the counter units 60, 62 to accumulate a count which is representative of the phase differences between the two signals (the SIGNAL-A and the REFERENCE signal). As was earlier pointed out, the CLOCK signal continues to increment the counter units 60, 62 for the duration of the phase difference period. The measurement is completed when the leading edge detection units flip-flops B and C disables the counter units 60, 62. 
     The phase difference that is measured in the leading edge/phase detection unit is converted into an 8-bit binary number via two 4-bit cascaded counters. The output of the AND gate 58 is used to enable the counters 60, 62 for the duration of the phase difference. After the phase detection/measurement has been completed, a logical &#34;1&#34; is latched onto the least signficant bit of the 8-bit data bus output of latch 2 unit 72. When an external controller (e.g. microprocessor) strobes the READ READY line, the logical &#34;1&#34; is detected on the data bus and the controller can then strobe the READ DATA control line to read the phase data from the counter unit 60, 62 that had been latched in latch 1 unit 71. 
     In FIG. 4 there is shown a phase delay measurement circuit in which the digital phase meter apparatus 100 is utilized. The operation of the digital phase measurement circuit is now described. For a generic phase measurement routine, the digital control lines are strobed (active low) as follows: 
     1. Clear: When Clear is strobed, flip-flop A, B, D and the counters are cleared. 
     2. Start: When Start is strobed, the circuit is initialized and initiates the phase measurement when the first positive leading edge of the Reference input is detected. 
     3. Read Ready: This line is continuously being monitored (strobed) by the microprocessor controller or controlling microcomputer and will put the least significant bit from latch on to the data bus. 
     4. Read Data: When this line is strobed, the output phase data from the tri-state latch 1 (units 74S373) can be read. 
     There is shown in Table 1 the input/output levels of the various flip-flops, the AND gate and the latches (components shown in FIG. 3c) when the digital phase meter conducts a phase measurement. 
     
                                           TABLE I                                 
__________________________________________________________________________
DIGITAL PHASE METER TRUTH TABLE                                           
                                 `AND` GATE                               
Control                                                                   
     Control Line                                                         
             FLIP-FLOP INPUT/OUTPUT                                       
                                 OUTPUT                                   
Line OR      A   B     C     D   (enable DATA                             
Sequence                                                                  
     Input Signal                                                         
             D Q D Q Q D Q Q D Q counter)                                 
                                         LINES                            
__________________________________________________________________________
1    CLEAR   H L L H L H L H H L L       ALL                              
                                         LOW                              
2    START   H H H H L H L H H L L       ALL                              
                                         LOW                              
     LEADING H H H L H L L H H L H       ALL                              
     Edge of                             LOW                              
     REFERENCE                                                            
     LEADING H H H L H L H L H H L       ALL                              
     Edge of                             LOW                              
     SIGNAL                                                               
3    READ    H H H L H L H L H H L       Least                            
     READY                               Sign.                            
                                         BIT                              
                                         = H                              
4    READ    H H H L H L H L H H L       PHASE                            
     DATA                                DATA                             
                                         WORD                             
__________________________________________________________________________
 L = Logical `0                                                           
 H = Logical `1                                                           
 Dark bordered squares indicate at what step a transition takes place.    
 
    
     The following is a parts list for the digital phase meter: 
     
         ______________________________________                                    
                                    QUAN.                                 
ITEM  PART NO.   PART NAME          REQ.                                  
______________________________________                                    
1     74LS161N   SYNCHRONOUS        2                                     
                 4 BIT COUNTERS                                           
2     SN74S373N  TRANSPARENT LATCHES                                      
                                    2                                     
3     74S74PC    DUAL D POSITIVE EDGE                                     
                                    2                                     
                 TRIGGERED FLIP-FLOPS                                     
                 WITH PRESET AND                                          
                 CLEAR                                                    
4     SN7408N    QUAD 2-INPUT AND   1                                     
                 GATES                                                    
5     UA9637ARC  DUAL DIFFERENTIAL  3                                     
                 LINE RECEIVER                                            
6     LM361N     HIGH SPEED DIFFEREN-                                     
                                    3                                     
                 TIAL VOLTAGE COM-                                        
                 PARATORS                                                 
7     DM7414N    HEX SCHMITT TRIGGER                                      
                                    2                                     
                 INVERTER                                                 
8                10K RESISTORS      6                                     
9                .1 mf CAPACITORS   15                                    
10               5K 10 turn potentiometer                                 
                                    1                                     
______________________________________                                    
 
    
     Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.