with reference to fig1 , the data acquisition instrument 10 has two analog measurement sensors 12 and 14 as inputs . typically the sensors are microphones but could also be vibration sensors , such as accelerometers , or other types of analog sensors . the two sensors 12 and 14 may be located at the distance from each other , with each sensor connected to a processor 16 through a two - path data channel . sensor 12 detects analog signals directed into data acquisition instrument 10 where a low pass filter 22 limits the bandwidth of the incoming signal prior to splitting the signal into two paths 24 and 26 at a splitter junction 25 . the two paths are characterized be a first path 24 with a high fixed gain amplifier 32 and a second path 26 with a low fixed gain amplifier 42 . each amplifier 32 and 42 is followed by a low pass filter , 34 and 44 respectively , for anti - aliasing purposes . the filters are followed by a / d converter 36 in path 24 . each a / d converter has a clock adjust circuit for applying a time clock delay . considering one path in comparison to the other , the relative delay corresponds to a phase match . a / d converter 46 has a clock adjust circuit 48 , while a / d converter 36 has a clock adjust circuit 38 . path selection is governed as described in the &# 39 ; 354 patent . the amount of clock adjustment sets the phase correction from master clock 50 . each clock adjust circuit 38 , 48 is addressed as a phase match pair , a differential signal , with the proper time clock delay on a respective line 39 , 49 . the specific time clock delay is computed by processor 16 and sent out on clock adjust transmit block 52 , described below . using the clock adjustment from blocks 38 and 48 the a / d converters 36 and 46 , respectively , are able to stitch data from the two paths into one stream in the stitcher 54 . the processor 16 computes the two path delay as follows . fig2 illustrates the calibration mode of the instrument 10 shown in fig1 . the input vibration sensors are not shown because switches , not shown in fig1 , block them from respective low pass filters 22 and 72 . instead , the calibration signal is switched into low pass filter 22 from d / a converter 61 through amplifier 62 and the calibration enable switch 63 . when the instrument 10 is not in the calibration mode , switch 63 blocks signal from the d / a converter . in fig2 , all clock adjust and stitch data circuit are not shown because they are not enabled in the calibration mode . similarly , other dual path channels are not enabled and are not shown . a calibration signal from switch 63 passes through low pass filter 22 and is split into two paths between high gain amplifier 32 and low gain amplifier 42 . after filtering by respective low pass filters 34 and 44 , the split signals pass through a / d converters 36 and 46 , with synchronization by master clock 50 , before entering processor 16 for a first channel for spectrum analysis . it is well - known that the phase difference in frequency domain of two sine signals can be translated to the time delay between these two signals in the time domain . for example , a 90 degree phase difference at 1 khz indicates a quarter millisecond delay in time between the two measured signals . if we feed identical signals in two paths , the calculated phase difference will indicate the time delay between the two signals in the paths . the processor 16 generates a phase match value for each path in the spectrum analyzer 60 when data from each path is used to compute phase delays in computed phase delay block 61 . phase match values are stored in memory 63 . the phase match is a differential signal , one phase related to another , that will be transmitted to the clock adjust circuits of fig1 when the instrument is out of the calibration mode . the phase match values are queued for transmission in clock adjust transmit buffer 52 . in processor 16 phase match is computed assuming that in a typical dynamic signal analyzer or vibration data collector , the group time delay of a signal conditioning filter and an anti - aliasing filter , phase - linearity and time delay of the a / d converters in difference of high gain versus low gain paths can be measured by one signal value : phase match between paths . phase match , a differential signal , is the value of the maximum phase deviation between each pair of paths at a certain frequency . phase match reflects the difference of the time delays in time domain of the signals between each pair of paths . previous studies by others teach that the time delay of two signals can be found from the phase spectrum of the cross spectrum . assume x ( ω ) is the fourier spectrum of the input signal x ( t ); y 1 ( ω ) and y 2 ( ω ) are the fourier spectra of measured signals from two input paths : h 1 ( ω )= m 1 ( ω ) e jφsub1 ( ω ) and h 2 ( ω )= m 2 ( ω ) e jφsub2 ( ω ) where h 1 ( ω ) and h 2 ( ω ) are the transfer functions of the front end of two input paths m 1 ( ω ) and m 2 ( ω ) are the magnitude functions and φ 1 ( ω ) and φ 2 ( ω ) are the phase functions . the magnitude and phase functions indicate how the magnitude and phase of the transfer function vary with frequency . if we calculate the cross - spectra g 21 ( ω ) between y 1 ( ω ) and y 2 ( ω ): g 21 ⁡ ( ω ) = conjugate ⁡ ( y 1 ⁡ ( ω ) ) * y 2 ⁡ ( ω ) = x ⁡ ( ω ) * m 1 ⁡ ( ω ) ⁢ ⅇ - j ⁢ ⁢ φ ⁢ ⁢ sub ⁢ ⁢ 1 ⁢ ( ω ) * x ⁡ ( ω ) * m 2 ⁡ ( ω ) ⁢ ⅇ j ⁢ ⁢ φ ⁢ ⁢ sub ⁢ ⁢ 2 ⁢ ( ω ) = x 2 ⁡ ( ω ) * m 1 ⁡ ( ω ) * m 2 ⁡ ( ω ) ⁢ ⅇ j ⁡ ( φ ⁢ ⁢ sub ⁢ ⁢ 2 ⁢ ( ω ) - φ ⁢ ⁢ sub ⁢ ⁢ 1 ⁢ ( ω ) ) then we see that the phase of the cross - spectrum φ 2 ( ω )- φ 1 ( ω ) is a perfect way to measure the time delay . although the phase spectrum is a frequency dependent function , it can be shown that a constant time delay will make a constant slope of φ 2 ( ω )- φ 1 ( ω ) function , or note that the phase value should be normalized against 360 degrees . for example , a phase of 10 degree at frequency of 10 khz indicates a time delay of : returning to fig1 , the time delay in the paths ( i . e ., phase match ), is the differential signal applied from the clock adjust transmit block 52 shown in both fig1 and fig2 to the clock adjust circuits 38 and 48 on lines 39 and 49 , respectively in the upper channel of fig1 . in processor 16 , a real time filter 56 and a data buffer 58 are used to queue and bandwidth limit data for the spectrum analyzer 60 . in order to look at the phase match at all concerned frequency areas in the calibration mode , we can use various signal excitations , such as a single sine wave , white noise , rectangular wave , etc . as set by a command from processor 16 to signal source 61 , a d / a converter as a calibration signal source . to measure the phase , the requirement is that these excitation signals must have certain energy at high frequencies . a dc signal , i . e ., a signal with constant voltage , cannot serve the purpose . the d / a converter 61 together with the data processor 16 provides the maximum flexibility and programmability therefore is preferred . the instrument 10 of fig1 has a second channel associated with sensor 14 . the sensor 14 detects analog signals directed into instrument 10 with different circumstances than sensor 12 that can arise from a different position or perhaps a different sensor mechanism . in any event the circuitry of the second channel is the same as the circuitry of the first channel , including a low pass filter 72 and the two paths 74 and 76 feeding the high gain amplifier 72 and low gain amplifier 92 , respectively . each amplifier 82 and 92 is followed by a low pass , filter 84 and 94 , respectively , for anti - aliasing purposes . the filters are followed by respective a / d converters 86 and 96 in the two paths 74 and 76 for cross path amplitude calibration . each a / d converter has a respective clock adjust circuit 88 and 98 , with phase match inputs 89 and 99 . the phase match is computed by processor 16 and transmitted to clock adjust block 52 . using the clock adjustments from master clock 50 , a crystal oscillator , each clock adjust circuit 88 and 98 is addressed with a phase match differential signal applied on lines 89 and 99 based upon use of the switched calibration signal described above . with the phase match information , the a / d converters of blocks 86 and 96 are able to stitch data from the two paths into one stream in the stitcher 55 so that the processor 16 can compute the two path delay for clock adjustment . time delays of the sampling clock are established by internal calibration from a reference source 61 , as previously mentioned with reference to fig2 . in review the reference source is preferably a d / a converter , but could be a dc source or an analog signal source . a d / a loop is to generate a calibration source signal to compute the phase match value between two paths of each measurement channel . a switch 63 is used to turn on or off the process . during the time of extracting phase match values , the analog source signal is put into each measurement channel . the spectrum analyzer 160 will compute the phase match values which are stored in the processor and can be translated into the time delay of a / d converters . this assures us that in the measurement stage the real signals from the sensors will be phase matched by adjusting the sampling clock delay of each a / d converter . the calibration signal could be a sine wave , square wave or rectangular shape waveform , sawtooth waveform or white noise , as mentioned above . the signal is amplified in amplifier 62 to the desired amplitude . a switch 63 is connected to amplifier 62 so that the analog excitation signal from amplifier 62 can be fed to a selected one of the low pass filters 22 and 72 at the same time . it is important that the connection from switch 63 to each of the low pass filters 22 and 72 be arranged such that the excitation signal arrives at only one of the two channels so that each channel is calibrated independently . once calibration is established in each channel , the dual paths to the a / d converts allow phase match signals to be applied to the paths prior to stitching in fig1 using stitch circuits 54 and 55 . after stitching , signals go to real time filters 56 and 57 . the real time filters maybe either iir or fir filters . a real time filter is one in which each incoming data point is processed without a time gap . the data buffers 58 and 59 connected to real time filters 56 and 57 respectively , allow accumulation of data words of desired length prior to forwarding the data words to a spectrum analyzer 60 . the spectrum analyzer uses a fast fourier transform analyzer or discrete fourier transform to transform the time domain signals into the frequency domain . for each channel , a spectrum analysis is performed and a phase match differential signal is produced using the two paths in each channel in the calibration mode and stored in memory for transmission to the clock adjust transmitter 52 that sends the phase match signals to clock adjust circuits 38 and 48 of the first channel and clock adjust circuits 88 and 98 of the second channel , with each pair of clock adjust circuits receiving one phase match signal on respective input lines 39 , 49 of the first channel and 89 , 99 of the second channel . once the instrument is calibrated , each channel takes data from an input sensor that is then cross path amplitude and phase range corrected data . the spectrum analyzer 52 produces output signals on line 77 that are available for general use outside of instrument 10 . signals from both input channels are available for further processing . note that amplitude and phase calibration values are computed at different times . usually , amplitude and phase calibrations are conducted when the system is just turned on , or right before measurements are taken . once values are computed , these parameters will be applied when data measurements are taken . the switch 63 is used to turn on and off the calibration process . when it is turned on , a calibration source signal will be applied to each input ; otherwise , the sensor signals will come in . also , note that the phase match value is calculated using the spectral analysis method when the signal source is applied to two paths of each channel simultaneously . in other words , the data from both paths of a measurement channel comes into the processor for computation simultaneously .