This Test Definition for the Evaluation of Digitizing Waveform Recorders (DWR) defines the process that can be performed as part of the evaluation and testing of geophysical sensors, digitizers, sensor subsystems and geophysical station/array systems. General DWR Tests Static Performance Tests Static tests provide a constant or non time-varying stimulus to the DWR under evaluation. The purpose of these tests is to determine specific parameters such as: gain (accuracy at nominal, full-scale and

more »

... verscale), DC offset, short-term and long-term stability, relationship to quantizing noise floor, and correlated/uncorrelated spurious signals. The results of these tests include measurement of dynamic range and resolution. DC Accuracy, Nominal (DWR-DCA) Purpose: The purpose of the DC accuracy test is to determine and verify the accuracy of the DWR. The bit-weight (LSB) of a non-gain-ranged digitizer is its resolution. Configuration: The DWR inputs are connected to a +/-DC voltage source, usually set to +/-1 volt. Evaluation: The DC gain (accuracy) of the DWR, DC offset, bit-weight (LSB)/resolution and counts/volt are measured. DC Accuracy, Full-Scale (DWR-DCFS) Purpose: The purpose of the DC full-scale test is to determine and verify the accuracy of the DWR at full-scale. The full-scale value is used in calculating Maximum Potential Dynamic Range (MPDR). Configuration: The DWR inputs are connected to a +/-DC voltage source set to the manufacturers specification for full-scale. Evaluation: The DC gain (accuracy) of the DWR, DC offset, bit-weight (LSB)/resolution and counts/volt at full-scale are measured. DC Accuracy, Over-Scale (DWR-DCOS) Purpose: The purpose of the DC over-scale test is to determine and verify the accuracy of the DWR at a specified value over full-scale. Configuration: The DWR inputs are connected to a +/-DC voltage source set to the manufacturers specification for over-scale. Evaluation: The DC gain (accuracy) of the DWR, DC offset, bit-weight (LSB)/resolution and counts/volt at over-scale are measured. AC Clip (DWR-ACC) Purpose: The purpose of the AC clip test is to determine and verify the maximum signal or clip level of the DWR. Configuration: The DWR input is connected to an AC voltage source set to 1 Hertz. The amplitude of the sinusoid is increased until the value of hard clip is reached. Evaluation: The AC clip voltage is measured. Input Terminated Noise (DWR-ITN) the oscillator is set to greater than one-half full-scale of the DWR. The frequency of the oscillator is set to a frequency unrelated to the sample rate and with nine or more harmonics observable. Evaluation: A power density spectrum provides a measure of the non-linearity of the DWR. THD is calculated by integrating the power density spectral peaks at the fundamental and all harmonics (up to nine) below the Nyquist frequency. Crosstalk (DWR-CTK) Purpose: The purpose of the crosstalk test is to determine the extent of crosstalk between channels on a multi-channel DWR. Configuration: The DWR channel under test is terminated with 50 ohms. All other DWR inputs are connected to a large amplitude sinusoidal test signal. Evaluation: A power density spectrum provides a measure of crosstalk. The ratio of test signal to crosstalk signal is calculated using integrated power density spectra around the test signal frequency. Common-Mode Rejection Ratio (DWR-CMR) Purpose: The purpose of the common mode rejection test is to determine the ability of the DWR to reject a common mode signal on differential inputs. Configuration: The individual inputs of each channel of the DWR are connected to an isolated, large amplitude sinusoidal test signal. The test generator common is connected to the appropriate signal reference or signal common on the DWR. Evaluation: A power density spectrum provides a measure of the un-rejected common-mode signal. The ratio of test signal to common-mode signal is the common-mode rejection ratio. Broadband Dynamic Performance Tests Dynamic tests are those that provide a time-varying stimulus to the DWR under evaluation. The purpose of these tests is to determine the DWR performance when digitizing time-varying signals. Multitudes of tests are available to determine the DWR digitizer's self noise, deviation from ideal performance and conversion distortions. Broadband tests are dynamic tests that use Gaussian pseudo-random signal generators as stimuli. Relative Transfer Function (DWR-RTF) Purpose: The purpose of the relative transfer function test is to determine the relative gain, and phase between channels in a multi-channel DWR. Configuration: The DWR inputs are connected to a bandwidth-limited Gaussian signal generator. The signal generator output amplitude is set to greater than one-half the full-scale range of the DWR. Evaluation: Coherence analysis computation provides a measure of relative gain, and relative phase. Channel to channel time skew is calculated. Analog Bandwidth (DWR-ABW) Purpose: The purpose of the analog bandwidth test is to verify the bandwidth of the DWR analog/digital filter. 12 Configuration: The DWR inputs are connected to a bandwidth-limited Gaussian signal generator. Evaluation: A power density spectrum provides a measure of the DWR bandwidth. The 3 dB point and relative attenuation at the Nyquist are measured. Modified Noise Power Ratio (DWR-MNPR) Purpose: The purpose of the modified noise-power-ratio test is to determine the DWR performance compared to n-bit ideal digitizers. This test determines the performance of the DWR at all amplitudes from small signal to clip level. Configuration: The DWR inputs are connected to a bandwidth-limited Gaussian signal generator. The bandwidth of the signal generator is set to avoid aliasing the DWR and to maximize the power within the DWR passband. The signal generator output is varied from a low level to past the DWR clip level. Evaluation: Coherence analysis computation provides a noise-power-ratio value for each level of input signal to the DWR. (Note that if the relative phase between the DWR channels is significant, it must be corrected to zero.) These estimated noise power ratios are compared to the performance model of n-bit ideal digitizers. Broadband Signal-to-Noise Ratio (DWR-BSNR) Purpose: The purpose of the broadband signal-to-noise ratio is to determine the maximum broadband dynamic range of a DWR. This test also provides the Large-Signal Noise (LSN) floor of the DWR. This is a relevant test for geophysical applications as signals are more approximated by Gaussian broadband signals than by sinusoids. Configuration: The DWR inputs are connected to a bandwidth-limited Gaussian signal generator. The bandwidth of the signal generator is set to avoid aliasing the DWR and to maximize the power within the DWR passband. The signal generator output amplitude is set to approximately the full scale of the DWR inputs without clipping. Evaluation: Coherence analysis computation provides a signal-to-noise ratio for each pair of digitizer channels. This Broadband SNR is an indication of the maximum broadband dynamic range of the DWR. Coherence analysis noise-power computation provides the signal-induced Large-Signal Noise (LSN) floor of the DWR for a full-scale broadband noise signal. Broadband Large-Signal Noise Floor (DWR-BLNF) Purpose: The purpose of the large-signal noise-floor test is to determine the DWR noise floor in the presence of a full-scale broadband signal. Configuration: The DWR inputs are connected to a bandwidth-limited Gaussian signal generator. The signal generator output amplitude is set to the full scale of the DWR inputs without clipping. Evaluation: Coherence analysis noise-power computation provides the signal-induced noise-floor of the DWR for a full-scale broadband noise signal. Timing Tests Geophysical digitizing waveform recorders utilize a Universal Time Code (UTC) source, typically GPS, to time-tag the digitizer data samples. The DWR internal clock is usually synchronized to or phaselocked to this UTC receiver. The following timing tests determine the accuracy of this time-tag, sample 13 rate verification, the response of the DWR to UTC loss and power cycling, and the short-term stability of the DWR time base. Time-Tag Accuracy (DWR-TTA) Purpose: The purpose of the time-tag accuracy test is to verify the ability of the DWR to accurately time-tag the data samples with respect to the DWR inputs. Configuration: The DWR inputs are connected to a 1 Pulse per Minute (PPM) or 1 Pulse Per Hour (PPH) output of an independent running GPS Timing Reference. Evaluation: The time-tags of the data from the DWR are analyzed for correct time on the hour and minute transitions. DWR sample rate (samples per second/minute) is verified. Time-Tag Accuracy Drift (DWR-TTD) Purpose: The purpose of the time-tag accuracy drift test is to verify the ability of the DWR to accurately time-tag the data samples with respect to GPS receiver variations. Configuration: The DWR input is connected to a 1 PPM output of an independent running GPS Timing Reference. The DWR GPS receiver is allowed to stabilize. Data are collected over an extended time-period. During this period, the GPS antenna is covered to "hide the sky" for approximately one hour and then uncovered. Data are acquired during the GPS recovery and re-lock period. Evaluation: The time-tags of the data from the DWR are analyzed for correct time on the minute transitions while stable and during GPS interruptions. Calibrator Performance Tests Digitizing Waveform Recorders frequently include a programmable voltage or current calibrator source to calibrate the internal DWR or drive a sensor calibration input. The calibrator may generate sinusoidal, step, white/pink noise, random binary telegraph (RBT) or a combination of these. The calibrator performance can be tested to determine parameters such as amplitude accuracy, frequency/duration, and distortion. Sine-Calibrator Amplitude (DWR-CAT) Purpose: The purpose of the calibrator amplitude test is to determine and verify if the DWR accurately programs the correct amplitude for sensor calibrations. Configuration: The DWR calibrator output is connected to a signal source measurement system. The amplitude and frequency of the DWR calibrator are set to known levels. Evaluation: Measured amplitudes are compared to the programmed amplitudes. Sine-Calibrator Frequency (DWR-CFT) Purpose: The purpose of the calibrator frequency test is to determine and verify if the DWR accurately programs the correct frequency for sensor calibrations. Configuration: The DWR Calibrator output is connected to a frequency counter. The calibrator is programmed through its range of frequencies. Evaluation: Measured frequencies are compared to the programmed frequencies. Sine-Calibrator THD (DWR-CHD) Purpose: The purpose of the calibrator THD test is to determine and verify the linearity of the sensor calibration generator. Configuration: The DWR calibrator output is connected to a signal source measurement system. The amplitude and frequency of the DWR calibrator are set to known levels. Evaluation: A power density spectrum provides a measure of the linearity of the calibrator. THD is computed. Calibrator Loopback THD (DWR-CLB) Purpose: The purpose of the calibrator loop back THD test is to determine and verify the linearity of the sensor calibration generator using the DWR. Configuration: The DWR calibrator output is looped back to the DWR input. The amplitude and frequency of the DWR calibrator are set to known levels. Evaluation: A power density spectrum provides a measure of the linearity of the calibrator. THD is computed. The THD using the DWR should be the same as the CHD test. Step-Calibrator Amplitude/Duration (DWR-SCA) Purpose: The purpose of the step calibrator amplitude/duration test is to determine and verify if the DWR accurately programs the correct amplitude and duration for sensor calibrations. Configuration: The DWR calibrator output is connected to a signal source measurement system. The amplitude and duration of the DWR calibrator are set to known levels. Evaluation: Measured amplitudes and duration are compared to the programmed amplitudes/duration. White-Noise-Calibrator Amplitude/Duration (DWR-WCA) Purpose: The purpose of the white-noise amplitude/duration test is to determine and verify if the DWR accurately programs the correct amplitude and duration for sensor calibrations. Configuration: The DWR calibrator output is connected to a signal source measurement system. The amplitude and duration of the DWR calibrator are set to known levels. Evaluation: Measured amplitudes and duration are compared to the programmed amplitudes/duration. Random-Binary-Calibrator Amplitude/Duration (DWR-RCA) Purpose: The purpose of the RBC amplitude/duration test is to determine and verify if the DWR accurately programs the correct amplitude and duration for sensor calibrations. Configuration: The DWR calibrator output is connected to a signal source measurement system. The amplitude and duration of the DWR calibrator are set to known levels. Evaluation: Measured amplitudes and duration are compared to the programmed amplitudes/duration.