A Sampling Unit for an X Y RecorderTodd I. Smith Citation: Review of Scientific Instruments 44, 288 (1973); doi: 10.1063/1.1686108 View online: http://dx.doi.org/10.1063/1.1686108 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/44/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Using a sample and hold circuit as a boxcar integrator/ultrasonic recording unit Am. J. Phys. 47, 1012 (1979); 10.1119/1.11668 Time Mark Generator for an XY Recorder Rev. Sci. Instrum. 39, 127 (1968); 10.1063/1.1683090 X-Y Recorder Am. J. Phys. 33, xvii (1965); 10.1119/1.1971129 Sampling in High Frequency Recording Rev. Sci. Instrum. 36, 869 (1965); 10.1063/1.1719743 A Simple Type of X(t) — Y Recorder Rev. Sci. Instrum. 22, 541 (1951); 10.1063/1.1745997

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288 R. CLAUDE WOODS

Rastrup-Anderson, and J. Rosethal, J. Chern. Phys. 40, 3378 (1964).

22M. A. Heald and C. B. Wharton, Ref. 14, p. 388. 23 A. von Engel, Ionized Gases (Oxford U.P., London, 1965),

pp. 243,247.

THE REVIEW OF SCIENTIFIC INSTRUMENTS

2'p. Llewellyin-Jones, The Glow Discharge (Methuen, London, 1966), p. 84.

25 A. von Engel, Ref. 23, pp. 124, 243. 26A. von Engel, Ref. 23, p. 33. 27M. A. Heald and C. B. Wharton, Ref. 14, p. 245.

VOLUME 44, NUMBER 3 MARCH 1973

A Sampling Unit for an X - Y Recorder

Todd I. Smith Departments of Physics and Electrical Engineering, University of Southern California, Los Angeles, California 90007

(Received 6 October 1972; and in final form, 27 November 1972)

An inexpensive sampling circuit is described which allows high frequency repetitive signals to be plotted on an x-y recorder. The minimum sample width of 100 nsec is short enough that the effective high frequency 3 dB down point is about 6 MHz. The circuit includes provisions for signal averaging which allows significant improvement in the signal-to-noise ratio in many situations.

INTRODUCTION

The oscilloscope camera used by many laboratories for obtaining permanent records of waveforms is simple to use, but the results are often not ideal. The photographs are generally awkward to store or file, and it is difficult to use them to obtain data accurate to better than about 5%.

The instrument to be described has been used in our laboratory to reproduce high frequency repetitive wave­forms on a standard x-y recorder. The trace on the recorder reproduces the waveform with an accuracy which can approach 0.1%.

The circuit will be recognized as a version of a boxcar (or gated) integrator, a versatile instrument typically used in reducing noise in NMR experiments. Indeed, the boxcar integrator as described in the literature,t-6 or available commercially, can be used to trace waveforms. The present circuit, making use of integrated circuits and eliminating some of the flexibility of the general purpose boxcar, is very inexpensive and noncritical to construct.

CIRCUIT DESCRIPTION

The unit is normally operated in conjunction with an oscilloscope on which the waveform to be recorded is dis­played. A sample and hold circuit obtains the value of the waveform at some horizontal deflection, x, of the oscillo­scope. As the value of x is slowly increased from zero to full scale deflection of the oscilloscope by a ramp which also drives the x axis of the recorder, the output of the sample and hold circuit reproduces the waveform on the yaxis.

Signal averaging is accomplished by requiring that the sampling circuit, which looks at the waveform for a time r" feed an R-C integrating network. If r»RC, a single

sample is sufficient to charge C to its final value. If T«RC, many samples will be required and C will be charged to a value representing a running average of the samples.

A schematic of the sampling circuitry is shown in Fig. 1 (a). Field effect transistors Ql and Q2 form an analog switch which is normally off, but is turned on by a 2V negative pulse applied to point A. The switch is on for r" the duration of the pulse, and has a resistance of approxi­mately 60 n. The range of input values which the switch can handle is limited to ±1O V, and the pulse width cannot be less than about 100 nsec without having too much charge capacitively transferred from the driving circuitry to the storage capacitor during each sample. This effect is a problem even with 100 nsec pulses but is helped by the trimmer capacitor C1, which is adjusted for zero output when the input is shorted. Even using C1, the zero level will depend somewhat upon the signal repetition rate when using 100 nsec pulses.

When the switch is ON the signal is connected through a resistor to a storage capacitor which is used to hold the signal value until the next sample. The total series re­sistance and the capacitor value are selected by switch SI and determine the time required for the capacitor volt­age to approach the signal voltage. Note that any series resistance in the source will affect the capacitor charging rate. To prevent significant discharge of the capacitor during the interval between sample pulses an analog devices AD 503 J FET-input operational amplifier is used as a unity gain buffer between the capacitor and the output.

The circuitry which forms the sampling pulses and which triggers the pulses at the proper times so that the wave­form is traced by the recorder is shown in Fig. l(b). The timing logic is performed by IC2, an MC 1035 triple differential comparator with a Schmitt trigger output.

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SAMPLING UNIT 289

One input to the comparator is proportional to the hori­zontal deflection of the oscilloscope and the other is pro­portional to the x deflection of the recorder. Typically, the oscilloscope deflection is provided by a high speed saw­tooth, while the recorder deflection is produced by a slow ramp. The resistor network between rcz and the inputs provides scaling to keep the signals with the proper range for the integrated circuit as well as allowing calibration of the deilection of the x axis of the recorder in terms of the horizontal deflection of the oscilloscope. Whenever the fast sawtooth at the input to lC 2 is increasing and its value equals the value of the ramp at the other input, a step is produced which triggers the one-shot multivibrator rCa after amplification by Q3. The pulse from lCa drives the sampling switch, and its width, Ts, is determined by the resistor and capacitor selected by switch Sz.

Since the ramp is increasing with time, the sample pulse will appear at successively larger horizontal deflec­tions for successive cycles of the sawtooth. Since the saw­tooth is synchronized with the waveform, and the x deflec­tion of the recorder is proportional to the ramp voltage, the recorder trace will duplicate the oscilloscope display. It should he pointed out that linear sweeps and ramps are not necessary for proper operation, as the only require­ments are that the inputs to the comparator be propor­tional to the horizontal deflections of the respective devices.

The resistor network indicated in Fig. 1 (b) between IC z and the fast and slow sweep inputs is appropriate for use with the 0~100 V sawtooth output available from each independent time base of a Tektronix 556 dual beam oscilloscope. One time base is used to display the waveform while the other drives the x~y recorder from the X IN/OUT

terminal. If, however, the recorder contains its own time base, it is generally more convenient to apply a 0~5 V

Fo .. t Sweep 82K 2$ 4.7K

82K 25K 10K 4.7K SlOw Sweep In

x In/out

(b) -ISV

FIG.!. (a) Schematic of sample and hold circuits; (b) schematic of pulse and timing logic circuits.

FIG. 2. Oscilloscope and recorder traces of test waveform. The oscilloscope horizontal deflection was 10 ,usee/em and the vertical deflection was 1 V /cm. The recorder sweep rate was 1 sec/div and the vertical sensitivity was 1 V /div. The sample width was 100 nsec and the averaging time was 10 ,usee.

ramp derived from the recorder to the x IN/OUT terminal of the unit and not bother with the dual beam oscilloscope.

The values of resistance and capacitance shown with switch Sl provide 10 averaging times ranging from 1 fJ.sec to 30 msec, and the values shown with switch Sz provide six sample widths from 100 nsec to 10 msec.

The power supply for the unit must provide ± 15 V at 60 mA and +6 V at 75 mAo Although the average currents are small, rather large transients are present during the switching intervals. Consequently, each section should be well decoupled from the power supply to prevent un­desired feedback.

OPERATION AND EXAMPLES

The signal to be recorded is connected to the SIGNAL IN

terminal of the sampling unit as well as to the vertical input of an oscilloscope adjusted for the desired display. The horizontal sweep output of the oscilloscope is con­nected to the FAST SWEEP INPUT. The SI(;NAL OUT ter­minal of the sampling unit is connected to the y axis of the recorder and the x IN/OUT terminal to the time base output from the recorder.

The display on the oscilloscope can be traced by the recorder once the sample width, averaging time, and re­corder sweep rate have been determined. As the fidelity of the recorder trace depends upon these quantities, some care must he exercised in their choice. The effect of the sample width is fairly straightforward, as most structure in the waveform which occurs during a sample will be lost. In practice, the sample width is usually chosen to be be­tween 0.1% and 1% of the time required for full scale horizontal deflection of the oscilloscope.

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290 TODD I. SMITH

FIG. 3. Oscilloscope and recorder traces of a noisy test waveform. The noise reduction evident in the recorder trace was provided by averaging with the sampling unit. The sample width was 100 nsec and the averaging time was 1 msec. The recorder sweep rate was 100 sec/div and the vertical sensitivity was 0.5 V /div. The oscilloscope horizontal deflection was 10/tsec/cm and the vertical deflection was 5 V/cm

The averaging time can be set to any desired value. However, it must be remembered that the averaging time is one RC time constant of the storage network, and that only the time during which a sample is being taken is effective in changing the stored value. Accordingly, if the averaging time is longer than the sample width, many samples must be obtained in the vicinity of each point along the waveform to allow the capacitor to charge to the correct value. The advantage of this is that the capacitor stores a running average of several samples, providing a reduction in the effect of superimposed noise. The disadvantage is a consequence of the fact that the number of samples is given by the product of the time required for a recorder sweep and the oscilloscope sweep

repetition rate. Thus, the requirement of many samples can lead to impractically long recorder sweeps if too much averaging is attempted on low repetition rate signals.

The performance of the sampling unit is illustrated in Figs. 2 and 3. The waveform shown in the oscilloscope photograph at the top of Fig. 2 was produced by ringing in an L-C circuit coupled to a pulse generator. The gen­erator output was a 40 j.Lsec wide step which was turned on 10 j.Lsec after the beginning of the oscilloscope sweep. The oscilloscope time base was set to 10 j.Lsec/ cm and as the sweep was free running the repetition rate was slightly less than 104 Hz. The vertical scale factor was 1 V / cm. The line drawing in Fig. 2 is a direct reproduction of the recorder trace obtained with a 100 nsec sample width, 10 j.Lsec averaging time, and a recorder sweep rate of 1 sec/div. The horizontal ticks correspond to 1 cm oscilloscope de­flection (10 j.Lsec in this case). The vertical scale is 1 V /div.

Figure 3 illustrates the noise reducing ability of the sampler. The signal shown in the oscilloscope photograph was composed of about 5 V rms of noise superimposed upon a signal of one-half the amplitude of that in Fig. 2. The horizontal deflection is again 10 j.Lsec/cm, while the vertical deflection is 5 V / cm. The recorder trace in Fig. 3 shows the noise reduction obtained with a sample width of 100 nsec, an averaging time of 1 msec and a recorder sweep rate of 100 sec/div. The vertical scale is 0.5 V /div.

lD. F. Holcomb and R. E. Norberg, Phys. Rev. 98, 1074 (1955).

'R. J. Blume, Rev. Sci. Instrum. 32, 1016 (1961). 3J. Reichert and J. Townsend, Rev. Sci. rnstrum. 35, 1692

(1964). 'D. Ware and P. Mansfield, Rev. Sci. Instrum. 37, 1167 (1966). 5W. G. Clark and A. L. Kerlin, Rev. Sci. Instrum. 38, 1593

(1967). 6G. L. Samuelson and D. C. Ailion, Rev. Sci. Instrum. 40, 676

(1969).

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