Abstract:
In a data acquisition instrument with dual A/D converters in each measurement channel, phase corrections between the paths are computed using a locally generated calibration signal. The calibration signal goes through dual paths simultaneously and is stitched into a single signal for analysis by a processor running a Fourier transform where phase differences are easily identified and used to establish a phase correction signal applied to a single master clock supplying clock signals to both paths. A switch allows the calibration signal to be applied to each channel or to no channels when actual data is being collected.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional application Ser. No. 61/870,659, filed Aug. 27, 2013. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to high dynamic range measurement method and system, and more particularly to high dynamic range measurement for a multiple path data acquisition system. 
     BACKGROUND ART 
     In a conventional high dynamic range measurement system of the type used, for example, in shock wave and vibration measurement, the input range setting is one of the most important settings. For example, in an analysis system there may be a number of different input voltage range settings for each input channel. The input range setting has a direct impact on the quality of measurement, which is mainly reflected by SNR (Signal-to-Noise Ratio) or dynamic range. Users are often troubled by being unable to set the optimum range because the measured signal either is non-stationary or has an unknown amplitude. For a high channel count system having multiple input ranges, it is even more difficult to get all the input ranges to a suitable value. To deal with this situation, many instruments are designed with an intelligent auto-ranging capability. “Auto-ranging” tries to set the best input range based on an estimated measurement before the test actually begins. Auto-ranging can only deal with stationary or repetitive signals, i.e., those signals without many magnitude changes. For non-stationary signals such as electrical transients, shock waves, impacts, earthquake signals, and the like, auto-ranging usually does not work because each pulse may take a different magnitude. For a signal with long time history and a large range of amplitude change, auto-ranging cannot be applied at all because during the measurement procedure the signal input range, i.e., the amplifier gain setting, cannot be changed. 
     As described in the publication “New Technology Increases the Dynamic Ranges of Data Acquisition Systems Based on 24-Bit Technology,” in SOUND AND VIBRATION, April 2005, pages 8-11, Andersen et al. state that sound and vibration transducers (e.g., microphones) have outperformed other analysis systems in linearity and dynamic performance. For such a system, the ratio between the highest and lowest signal level the system can handle is defined as its “dynamic range.” The publication states that if the dynamic range is too low, high range signals will typically be clipped and distorted while the low range signals will typically be buried in system noise that originates from the transducer element and the electronics conditioning the transducer. As a solution, the publication describes utilizing a specialized analog input designed to provide a very high dynamic range of analog circuit pre-conditioning the transducer signal before forwarding the signal to a pair of specially designed 24-bit analog-to-digital converters (ADCs) in two paths. Both data streams from the ACDs are forwarded to a digital signal processing environment, where dedicated algorithms in real-time merge the signals. 
     In U.S. Pat. No. 7,302,354, assigned to the assignee of this invention, J. Zhuge describes dual A/D (analog-to-digital) signal paths and cross-path amplitude calibration to provide accurate and reliable measurements in a data acquisition system. 
     In the &#39;354 patent, the input signal is directed to two paths, e.g., Path A and Path B. The first path measures the full range (e.g., +/−10 volts), while the second path includes a high-gain amplifier, such as one having a gain factor of 1024. Each path includes an analog-to-digital converter (ADC). Thus, the preferred embodiment includes a measurement channel with a one-to-one correspondence between the number of paths and the number of ADCs, which sample the input signal simultaneously. 
     After the ADCs of the different paths convert the input signal into the digital domain, the system selects among measurement points. When the input signal is within the amplitude range of high gain Path B, the system selects the values from Path B. On the other hand, when the magnitude of the input signal is outside the amplitude range of Path B, the system selects the values from Path A. Thus, a subset of measurement points is selected from Path B, the default path, and the remaining measurement points are selected from Path A, so that the selected values at the measurement points are stitched into a final data stream. The total dynamic range of the measurement is increased by roughly 60 dB at full range input. 
     If Path B will be saturated when a signal is greater than a certain amplitude level, the digitized value from the ADC of Path B should not be used in forming the final data stream. Instead, the value at the corresponding measurement point of Path A is used. The selection of measurements occurs on a point-by-point basis. 
     There are a number of potential concerns with this implementation. One concern is whether the small phase difference between the different paths will cause difficulties. Previously it was known that by using the same clock source to control the sampling rate of each ADC, the phase match between paths can be optimized. 
     When addressing this concern, the values that are of greatest importance are those at transition measurement points when the final data stream transitions from one path to another path during a “stitching” process. Without proper treatment, there will be discontinuities at the transitions. The &#39;354 patent uses a special cross-path amplitude calibration process. It is not necessary that the cross-path calibration eliminate, or even reduce, the absolute measurement error of measurement paths. Instead, the calibration is designed to match the errors among the different paths, so that the paths will generate the measurement values as close as possible. This will allow the transition of the signal from one region to another to be very smooth during the “stitching” process. 
     Cross path amplitude calibration solves the issue of how to adjust the amplitude difference coming from two A/D converters. In an ideal environment and with perfect electronic circuits, there is no phase mismatch between two or multiple A/D converters in different paths. Amplitude adjustments in the time domain would be sufficient. In reality, there is always phase error or phase mismatch between the two paths, in either analog circuitry or inside of the A/D converters. A large mismatch in phase will make the “stitching process” of digital signals coming from two A/D paths difficult. 
     With current commercially available data acquisition circuitry, when the signals of interest in a lower frequency range, say below 10 kHz range, the phase mismatch is usually insignificant. When the signals of interest are in a higher frequency range, such as 20 kHz or above, the phase mismatch may be more significant. 
     An object of the invention is to achieve cross path phase calibration in a dual path data acquisition system involving multiple data channels with phase matching. 
     SUMMARY OF THE INVENTION 
     In the &#39;354 patent, cross path amplitude calibration is achieved using a single time clock source driving the A/D converters in a dual path instrument system. The present invention retains the cross path amplitude calibration of the &#39;354 patent but improves the performance of the circuitry by adding certain time adjustments to the clock that drives each A/D converter in each path. By slightly adjusting the time clock delay for each of the A/D converters, the phase mismatch of all A/D paths can be greatly reduced. 
     To make adjustments to the clock delay for each of the A/D converters, it is necessary to determine how much adjustment is needed. To do so, a locally generated signal can be fed into the analog input end of all A/D converter paths simultaneously, then allowing a data processor to receive the raw data from the two paths. The raw data is not stitched during this process. Once the data is received, a discrete Fourier transform (DFT) or fast Fourier transform (FFT), can be applied to data in the two data paths to determine phase differences or a phase match for the A/D converters. With the knowledge of phase differences, the clock time delay can be known, and later applied to the two paths to adjust the clock signal. 
     While the cross path amplitude calibration and phase calibration can be conducted manually and with an external excitation source, it is preferred to have circuitry that is housed internally in an instrument so that the calibration process can be conducted at any time automatically. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a dual channel instrument with the capability of cross-path phase calibration. 
         FIG. 2  is a block diagram of a cross-path phase calibration dual channel instrument in a phase calibration mode. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , 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. 
       FIG. 2  illustrates the calibration mode of the instrument  10  shown in  FIG. 1 . The input vibration sensors are not shown because switches, not shown in  FIG. 1 , 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  FIG. 2 , 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  FIG. 1  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:
 
 Y   1 (ω)= H   1 (ω)* X (ω) and  Y   2 (ω)= H   2 (ω)* X (ω)
 
where
 
 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
 
Time delay=(1/ω)*(φ 2 (ω)−φ 1 (ω)
 
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:
 
Time delay=(1/10,000 Hz)*(10/360)=2.77 us
 
Returning to  FIG. 1 , 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  FIG. 1  and  FIG. 2  to the clock adjust circuits  38  and  48  on lines  39  and  49 , respectively in the upper channel of  FIG. 1 . 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  FIG. 1  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  FIG. 2 . 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  FIG. 1  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.