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Reply to comments on 'A programmable digital signal averager'

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1981 J. Phys. E: Sci. Instrum. 14 1104 (http://iopscience.iop.org/0022-3735/14/9/418) View the table of contents for this issue, or go to the journal homepage for more

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J. Phys. E : Sci. Instrum., Vol. 14, 1981. Printed in Great Britain ~~~

Comments and Correspondence

Comments on ‘A programmable digital signal averager’ In a recent paper, Srinivasan et al (1980) have described a programmable digital signal averager with facility for programming the sampling period, number of channels and number of sweeps. We have examined this paper in some detail and find that some points need clarification. I n signal processing, averaging techniques are utilised to improve the signal-to-noise ratio for repetitive signals, as the signal adds coherently, while noise remains incoherent. Hence the average value of noise reduces to zero while that of the signal approaches its average value. For subsequent scans to be coherent, the scan must start with a definite relationship with the input signal. In the system described by the authors, the sampling interval bears no correlation with the input signal. The authors should indicate how, if any, such synchronisation is achieved in their signal averager. The authors have indicated that the sweep counter is loaded into a cascade of three 7495 (SR2 shift registers), which is shifted in synchronism with SRI. To achieve division by 2N, the dividend has to be shifted AT positions down. In the scheme mentioned, the number of sweeps is limited to 256 which is only eight-bits, while the authors are using 3x7495 which would imply a sweep capability of 4096 sweeps. There is no facility for selecting the number of channels or having programmable resolution for different channels. No facility is incorporated for selecting different averaging algorithms, which reduces the versatility of the instrument as a programmable signal averager. University of Bombay Western Regional Instrumentation Centre University Campus (Vidyanagari) Kalina Santacruz (East) Bombay-400 098 India 21 January 1981

S Govind K Ramani

The authors replied to the comments by Govind and Ramani as follows: As a short communication, the circuit details could not be included in the article, which seems to have resulted in the communication gap. The clarifications to the points raised are as follows. For samples of various scans to remain coherent, the ‘sync’ pulse (which forms one of the essential inputs to any 0022-3735/811091104+01 $01.50 0 1981 The Institute of Physics

signal averager) is used to clear the decade timing counters. This will ensure that the counters start with zero reference time, for every repetition of the input signal. The sampling period is calculated by dividing the signal period with the number of signal channels chosen. In the system described, a ten-bit bipolar ADC (which means nine-bit, if we exclude the sign bit) has been used for the digitiser. This will mean that for sweeps of 512 and above the term ( f ( t m + k T )- 1Mm-1k]1/2-\rbecomes zero and hence no further improvement in S/N ratio can result for larger number of sweeps. However, in our system we used 3 x 7495 as we were interested in using, at a later time, a higher resolution ADC to permit a longer number of sweeps. For the present, 2 x 1495 would be adequate. The number of channels is programmable, in binary steps, from a front-panel rotary switch. The address counter value is compared with the set member of channels and on coincidence a pulse is generated for self-clearing the counters, to enable data-collection for the next sweep. The coincidence pulse also advances the sweep-counter. The article aims at covering the stable averaging algorithm with the specified programmable parameters and does not include the other averaging algorithms. Department of Physics IISC Bangalore- 12 India NAL Bangalore-17 India 27 February 1971

J Ramakrishna

R Srinivasan S R Rajagopalan R Ramesh

Reference Srinivasan R, Ramakrishna J , Rajagopalan S R and Ramesh R 1980 A programmable digital signal averager J . Phys. E: Sci. Instrum. 13 1263-5