Noise Suppression by Time Exposure Oscilloscope PhotographyG. S. Huchital and M. Young Citation: Review of Scientific Instruments 43, 759 (1972); doi: 10.1063/1.1685751 View online: http://dx.doi.org/10.1063/1.1685751 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/43/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Comment on ``Noise Suppression by Time Exposure Oscilloscope Photography'' Rev. Sci. Instrum. 44, 924 (1973); 10.1063/1.1686286 Erratum: Notes on Oscilloscope Photography Rev. Sci. Instrum. 32, 1158 (1961); 10.1063/1.1717199 Exposure Meter for CathodeRay Oscilloscope Photography Rev. Sci. Instrum. 32, 1143 (1961); 10.1063/1.1717184 Notes on Oscilloscope Photography Rev. Sci. Instrum. 32, 741 (1961); 10.1063/1.1717486 A Time Interval and Reference Line Marker for Moving Film Photography of Oscilloscope Traces Rev. Sci. Instrum. 19, 805 (1948); 10.1063/1.1741162

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 43. NUMBER 5 MAY 1972

Noise Suppression by Time Exposure Oscilloscope Photography

G. S. HUCHITAL AND M. YOUNG

Electrophysics Division, Rensselaer Polytechnic Institute, Troy, New York 12181

(Received 20 January 1972; and in final form, 8 February 1972)

We describe a simple and inexpensive means of enhancing repetitive signals obscured by noise with roughly equal amplitude. The signal and noise are displayed on an oscilloscope, and we perform a time average over many traces by time exposure photography. If the oscilloscope triggering is synchronous with the signal, the result is a significant suppression of the offending noise.

We were recently faced with the common problem of detecting a repetitive electrical signal, partly or wholly submerged in noise. Significant noise existed at approxi­mately the same frequency as the signal and thus made direct observation difficult. In such cases, the presence (or absence) of the signal, its frequency, and so on are easily inferred by electronic auto- or cross-correlation methods. Nevertheless, it is relatively difficult to get detailed infor­mation (waveform, decay time, and so on) from these correlation techniques.

If the signal is sufficiently consistent and repetitive, it can be observed by a well known averaging process. Briefly, this may be done by taking a large number n of samples of the signal with noise. The samples are taken periodically, synchronous with the repetition rate of the signal, and averaged. The signals add coherently and the noise incoherently; the signal to noise ratio (SIN) is thus improved by a factor of n!.

In this paper, we describe a simple, inexpensive photo­graphic technique that we have used to enhance repetitive signals obscured by noise of roughly equal amplitude:

FIG. 1. Photographs of oscilloscope traces with a SIN ratio of about 2.5. The upper trace shows a single sweep, the lower averaged over approximately 1000 sweeps. In the latter case, the general form and amplitude of the signal are evident. For comparison, the last three divisions show the noise alone.

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SIN,...., 1. In one sense, we have used photographic film to perform the average described in the preceding paragraph. We display the signal and noise on an oscilloscope triggered externally and synchronized with the signal. Averaging is performed by taking a time exposure. The noise exposes the film randomly, while the signal exposes certain parts of the film preferentially. The noise thus on average gives rise to a low exposure, while the signal brings about a higher exposure. Control of the net exposure will allow enhancing the signal at the expense of the noise.

The technique, while not new, is little known, and we have found it to be quite useful in our application. Figure 1 shows a typical result with SIN,...., 2.5. The upper part shows a single oscilloscope trace, in which the signal is largely swamped by the noise. The lower photograph is averaged over approximately 1000 sweeps and shows the signal relatively clearly. The noise is displayed without the signal at the end of the trace. The frequency of the signal is immediately apparent, and some details of its form can be seen. (Note that the waveform is not sinusoidal.) We were interested in knowing whether the waveform ended ab­ruptly or decayed over several cycles. The lower oscillo­graph shows that it stops abruptly.

In Fig. 2, SIN is approximately 1. The lower trace (again

FIG. 2. As in Fig. 1, with a SIN ratio of about 1.

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760 G. S. HUCHITAL AND M. YOUNG

averaged over 1000 sweeps) nevertheless displays much information about the waveform.

All of the photographs were exposed on Polaroid type 47 roll film. While this is a fairly high contrast film, it is designed primarily for continuous tone photography. Noise suppression would be considerably improved with a line copy film, with the noise level adjusted to fall on the toe of the characteristic curve of the film.

Finally, we note that this method can be applied to

THE REVIEW OF SCIENTIFIC INSTRUMENTS

relatively fast signals by using the persistence of vision (integration time of the eye) or the persistence of the phosphorescing oscilloscope screen. However photography (and Polaroid photography in particular) provides an easy way of integrating for any length of time. High contrast film and careful exposures should allow processing even when the SIN ratio is significantly less than unity.

This work was supported in part by the National Aeronautics and Space Administration.

VOLUME 43, NUMBER 5 MAY 1972

Vibrating Sample Magnetometer for Protein Research

ALFRED G. REDFIELD* AND CHARLES MOLESKI

IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598

(Received 30 April 1971; and in final form, 21 December 1971)

A simple magnetometer for use with weakly paramagnetic frozen solutions is described. It uses the same super­conducting coil for generating the field and detecting the vibrating magnetization. A nearly translationally in­variant sample holder and low frequency vibration of large amplitUde are advantageous. In a sample volume of 0.7 ml, 2XlO-7 moles of a species having a single g=2, s=! spin produces a signal reproducibly different from that of pure water, when the temperature is varied from 77 to 2 K.

INTRODUCTION

In studies of metalloproteins and other paramagnetic molecules in frozen solution it is useful to use magnetic susceptibility measurements to determine the magnetic moment, if any, of the bound metaU The high (1OL 105)

molecular weight of such proteins means that the con­centration must often be in the range of a millimole per litre, so that for low spin species the magnetic susceptibility contribution of interest, even at 2 K, may be appreciably less than the diamagnetic susceptibility of the water solvent and the sample container (",7X 10-7 cgs). Some­times the amount of sample is limited, and occasionally it is desirable to extend measurements to moderately high fields (20-30 kG) in order to distinguish a low spin protein magnetization from a possible high spin impurity by their different saturation behavior.

This paper describes our methods for studying such samples with a vibrating sample magnetometer2 of unconventional design, as used for the past three years on a variety of samples.3 Our particular variation of the vibrating sample magnetometer is simple and inexpensive. This simplicity is possible because no very strongly field dependent phenomena, such as the de Haas-van Alphen effect in metals, are expected in proteins so that great field homogeneity is unnecessary and a small superconduct­ing solenoid can be used to generate the field.

APPARATUS

A field of up to 30 kG is produced by a coil of about 6000 turns of Supercon wire (0.25 mm diam niobium­titanium core; 0.33 mm diam copper cladding). The dimensions are shown in Fig. 1. The coil form was brass, and the coil was wound from a single piece of wire whose midpoint was soldered to the coil form. The two halves of the wire were wound4 as physically separate coils, L1 and L2 in Fig. 1, separated from each other by only a thin plastic spacer whose thickness was chosen to flatten the field at the center. The sample reciprocates along the coil axis, and generates in-phase currents in the low impedance transformer primary. The high impedance secondary is connected to a lock-in amplifier. The lock-in reference is generated by an alternator geared 1: 1 on a motor driven shaft which reciprocates the sample by means of an eccentric linkage.

Thus, we use the same coil to generate the field and to detect the changing flux. This leads to adequate sensitivity, and is simpler and less expensive than separate detector coils. However, the main problem in measurements on proteins is sample purity and position variation, not thermal or microphonic noise.

Since the magnetic susceptibility of most protein samples is comparable to the diamagnetic contribution of the holder, it is impossible to adjust the sample position by maximizing the signal. This must be done ahead of time

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