Modern Data Acquisition Systems Reinoud Sleeman Modern data

Modern Data Acquisition Systems – Reinoud Sleeman Modern data acquisition systems: digitizers and dynamic range Reinoud Sleeman ORFEUS Data Center Royal Netherlands Meteorological Insitute sleeman @ knmi. nl (KNMI) IRIS - ORFEUS Workshop Managing Waveform Data and Related Metadata for Seismic Networks Helwan, Cairo, Egypt 8 – 17 November 2009 Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Layout • introduction • digitizing theory (dynamic range, oversampling) • ADC - delta sigma modulator • decimation (SEED) • measuring and representing instrumental noise • instrumental noise of today’s dataloggers • instrumental noise of the STS-2 Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Introduction and motivation Modern Data Acquisition Systems – Reinoud Sleeman seismic background noise (m/s 2) power spectral density STS-2 N-S • • • E-W STS-1 Z seismic station: Heimansgroeve (HGN), Netherlands sensors: STS-1, STS-2 time windows: 2002 (302 - 309) and 2003 (029 – 043) Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Introduction and motivation Modern Data Acquisition Systems – Reinoud Sleeman from STS-2 manual el mod e s i o n w Lo l eve l e s i 2 no STS- Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

• Johnson-Mathiesen seismometer > 1 Hz • STS-1 < 0. 01 Hz STS-1 STS-2 noise (Wielandt, 1991)

Introduction and motivation Modern Data Acquisition Systems – Reinoud Sleeman analog signal How do we make a digital (bit stream) representation from an analog signal ? digital representation …. . 0011011110101000011010111010011 01111001001011101010010101011… Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Introduction and motivation Modern Data Acquisition Systems – Reinoud Sleeman • How do we get a digital (bit stream) representation from an analog signal ? • How accurate is the representation ? • Does the digitizing system bias the digital data ? • What does ‘dynamic range’ mean and how must we interpret these numbers (e. g. 145 d. B) given by vendors ? • How can we measure the noise level (dynamic range) ? • What information about the recording system is useful or important for seismologists and needs to be stored as (SEED) metadata ? …. . 001101111010100001101011101001101111…… Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Digitizing theory Modern Data Acquisition Systems – Reinoud Sleeman Dynamic range of a N-bit digitizer Quantization: Variance of error: x, q(x) ∆ : smallest discrete step (LSB) q(x) x e +∆/2 -∆/2 t Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Digitizing theory Modern Data Acquisition Systems – Reinoud Sleeman Dynamic range of a N-bit digitizer Quantization: Variance of error: Quantization levels in a n-bit ADC: Managing Waveform Data and Related Metadata for Seismic Networks ∆ : smallest discrete step (LSB) 2 A: full scale input Cairo, Egypt, Nov 8 – 17, 2009

Digitizing theory Modern Data Acquisition Systems – Reinoud Sleeman Dynamic range of a N-bit digitizer Quantization: Variance of error: Quantization levels in a n-bit ADC: ∆ : smallest discrete step (LSB) 2 A: full scale input Sine wave (amp A): Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Digitizing theory Modern Data Acquisition Systems – Reinoud Sleeman Dynamic range of a N-bit digitizer Quantization: Variance of error: Quantization levels in a n-bit ADC: ∆ : smallest discrete step (LSB) 2 A: full scale input Sine wave (amp A): Dynamic range: (Bennett, 1948) Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Digitizing theory Modern Data Acquisition Systems – Reinoud Sleeman Dynamic range of a N-bit digitizer Quantization: Variance of error: Quantization levels in a n-bit ADC: ∆ : smallest discrete step (LSB) 2 A: full scale input Sine wave (amp A): Dynamic range: (Bennett, 1948) 6 d. B per bit Dynamic range digitizer: Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling PSD vs. sampling rate • In an ideal digitizer (assuming white digitizer noise) the quantization noise power is uniformly distributed between [0 – f. NYQ] Hz. • The noise power does not depend on the sampling rate. • For higher sampling rates the power spreads over a wider frequency range, so decreasing the power spectral density (and thus decreasing the quantization error) ! • Higher sampling rate improves the accuracy of the estimate of the (analog) input signal. Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling Ts = sampling interval (s) Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling Ts = sampling interval (s) noise power Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling Ts = sampling interval (s) noise power Theoretical expression for the (one-sided) PSD of quantization noise in a n-bit digitizer: PSD of quantization noise depends on the (initial) sampling rate, and so does the dynamic range ! Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling • oversampling factor 4 leads to increase of SNR of ~ 6 d. B (or 1 bit) Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling • oversampling factor 4 leads to increase of SNR of ~ 6 d. B (or 1 bit) • 1 -bit ADC with 256 x oversampling achieves a resolution of 4 bits Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling • oversampling factor 4 leads to increase of SNR of ~ 6 d. B (or 1 bit) • 1 -bit ADC with 256 x oversampling achieves a resolution of 4 bits • to achieve 16 bits resolution (96 d. B) you must oversample with factor 4^16 (~ 4000 000), and for 24 bits resolution this factor is 4^24 (~280 000 000 ); both can not be realized ! Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Oversampling • oversampling factor 4 leads to increase of SNR of ~ 6 d. B (or 1 bit) • 1 -bit ADC with 256 x oversampling achieves a resolution of 4 bits • to achieve 16 bits resolution (96 d. B) you must oversample with factor 4^16 (~ 4000 000), and for 24 bits resolution this factor is 4^24 (~280 000 000 ); both can not be realized ! • this problem is overcome by the delta-sigma modulator with the property of noise shaping, to enable a gain of more than 6 d. B for each factor of 4 x oversampling. Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Delta-Sigma Analog-Digital (A/D) Modulator (one-bit noise shaping converter) Inose and Yasuda, University of Tokyo, 1946 Comperator: Feedback: Integrator: Pulse train: ADC or quantizer average of y follows the average of x accumulates the quantization error e over time pulse density representation of x Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Delta-Sigma Modulator ei xi si ui Managing Waveform Data and Related Metadata for Seismic Networks q(ui) Cairo, Egypt, 8– 2009 IRIS Workshop, 21 -26 Oct 2007, Kuala Nov Lumpur - 17, R. Sleeman

Modern Data Acquisition Systems – Reinoud Sleeman Delta-Sigma Modulator ei xi si ui q(ui) noise shaping 2 -nd order: Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, 8– 2009 IRIS Workshop, 21 -26 Oct 2007, Kuala Nov Lumpur - 17, R. Sleeman

Modern Data Acquisition Systems – Reinoud Sleeman PSD noise Assumption: quantization noise is white noise with feedback without feedback 0 100 output sample rate (Hz) 32000 initial sample rate The feedback loop in the quantizer shapes (differentiates) the quantization noise, with the result of smaller quantization noise at lower frequencies at the price of larger quantization noise at higher frequencies. Noise shaping does not change the total noise power, but its distribution. The downsampling (decimation) process uses digital anti-alias filters (FIR) which characteristics must be known by seismologists. Therefore the FIR coefficients must be part of the metadata. Improved resolution (reduced quantization error) at a lower effective sampling rate Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, 8– 2009 IRIS Workshop, 21 -26 Oct 2007, Kuala Nov Lumpur - 17, R. Sleeman

Modern Data Acquisition Systems – Reinoud Sleeman Delta-Sigma Modulator / Modulator Decimation Delta-Sigma SEED Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Layout • introduction • digitizing theory (dynamic range, oversampling) • ADC - delta sigma modulator • decimation (SEED) • measuring instrumental noise • instrumental noise of today’s dataloggers • instrumental noise of the STS-2 Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Measurement of instrumental noise (dynamic range ) • 50 ohm shortened input recording (digitizer) • common input recording (digitizer or sensor) Ø Ø coherency analysis of 2 channels (Holcomb, 1989) coherency analysis of 3 channels (Sleeman et. al. , 2006) triplet method Holcomb, L. G. , A direct method for calculating instrument noise levels in side-by-side seismometer evaluations. U. S. Geol. Surv. , Open-File Report 89 -214 (1989) Sleeman, R. , A. van Wettum and J. Trampert. Three-channel correlation analysis: a new technique to measure instrumental noise of digitizers and seismic sensors. Bull. Seism. Soc. Am. , 96, 1, 258 -271 (2006) Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman 2 -channel vs. 3 -channel model Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Three-channel technique: § direct method for estimating instrumental noise and relative transfer functions (relative calibration!) based on the recordings only § no a-priori ð information required about transfer functions method not sensitive for errors in gain Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, 8– 2009 IRIS Workshop, 21 -26 Oct 2007, Kuala Nov Lumpur - 17, R. Sleeman

Modern Data Acquisition Systems – Reinoud Sleeman The digitizer experiment Q 4120 STS-2 Q 4120 Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, 8– 2009 IRIS Workshop, 21 -26 Oct 2007, Kuala Nov Lumpur - 17, R. Sleeman

Modern Data Acquisition Systems – Reinoud Sleeman PSD of self-noise Q 4120 measured with common STS-2 vertical signal (@ 20 sps) Power Spectral Density graph 143 d. B Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, 8– 2009 IRIS Workshop, 21 -26 Oct 2007, Kuala Nov Lumpur - 17, R. Sleeman

Modern Data Acquisition Systems – Reinoud Sleeman Does the quantization error increases during quantizing a seismic signal ? Quanterra Q 4120 139 d. B NARS datalogger Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

Modern Data Acquisition Systems – Reinoud Sleeman Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

STS-2 self noise measurement needs Q 330 HR with pre-amp enabled ! STS-2 noise (Wielandt) Q 330 -HR pre-amp enabled

Modern Data Acquisition Systems – Reinoud Sleeman The sensor experiment Q 330 -HR; pre-amp Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009

The Conrad Observatory Experiment • NERIES framework (TA 5) (funding) • Conrad Observatory (infrastructure, local conditions) • 4 STS-2 (same generation) • 4 Q 330 -HR, enabled pre-amplifier • Antelope ® acquisition

Q 330 -HR

Without thermal isolation Background noise LNM Wielandt (STS-2) Triplet (STS-2) 12 d. B !

With thermal isolation Background noise LNM Wielandt (STS-2) Triplet (STS-2) LH BH

Background noise LNM Wielandt (STS-2) Triplet (STS-2) no isolation thermal isolation

Seismic PSD Triplet (STS-2) julian day (2008)