MEASUREMENTS OF ELECTRICAL AND MAGNETIC QUANTITIES

S T A T E OF THE ART OF O S C I L L O G R A P H T E C H N O L O G Y

V. L. G o r d i n UDC 621.317.75 (047)

According to our information, there are about 60 types of o6cillographs in use in the Soviet Union at the pres- ent time, including both old types taken out of production and the very latest oscillographs made only by an experi- enced party.

The basis of the classification proposed below (only for reasons of convenience) is the most important parame- ter of the remaining technical characteristics of an oscillograph: the passband of the vertical deflection channel. Among oscillographs with passbands to 10 MHz, there are 11 types with passbands to 1 MHz, and 10 types with pass- bands to 5 MHz.

There are 13 types of oscillographs with passbands to 25 MHz, and one oscillograph, the C1-31, which is mass- produced and built on the usual principles of amplification, has a passband up to 100 MHz. There are another 8 types of high-speed oscillographs whose passbands are in excess of 100 MHz.

Seven of the types of oscillographs enumerated above use semiconductor devices. Complete conversion to semiconductors in the future is foreseen, which will reduce the size of the instruments and the required power.

The existing classification of oscillographs in accordance with the requirements, of GOST 9810-61 TT classi- fies oscillographs according to:

a) the number of simultaneous processes; there are 9 types of dual-beam oscillographs; C1-18, 51, 16, 34, 42, 15, 17, 7, 39;

b) the accuracy with which the wave shape is reproduced and the accuracy of measuring U (amplitude values) and t (time intervals); there are four accuracy Classes with error from 3% (class I) to 10% (class III).

At the present time there is only one oscillograph, the C1-40, that is close to the requirements of class I accu- racy. Only five types out of 59 have been taken out of production at present (obsolescent and not conforming to con- temporary requirements). Here we should discuss one ~rend, incorrect in our view, of assigning a new type (designa- tion) to a newly designed oscillograph. If various operational requirements are not taken into consideration, there might be a number of instruments in stock that have similar technical characteristics:

1. Among the single-beam oscillographs with passbands up to 1 MHz, C1-19 = C1-30 = C1-48 = C1-59. The C1-19 is identical to the C1-30 in class and group as defined in GOST 9763-67. Moreover, the C1-30 weighs 35 kg instead of the 21 kg of the C1-19.

2. Among the single-beam oscillographs with passbands up to 10 MHz, C1-49 = C1-22 = C1-35.

3. Among the wide-band oscillographs with passbands to 20 MHz, C1-20 = C1-54 = C1-50.

4. Among the dual-beam oscillographs, C1-16 = C1-34 (three times worse in its sensitivity characteristics).

5. Among storage oscillographs, C1-37 = C1-46 = C1-51.

6. Among television oscillographs, C1-52 = C1-13 = C1-57.

From a survey of the analogous oscillographs we can draw the following conclusions. The development of a new device similar to one already in use, has often been clearly uneconomical and could have been completely avoided by modernization of the basis of an already existing instrument (C1-49, C1-35, and others).

Translated ~omIzmer i te l 'naya Tekhnika, No. 9, pp. 59-61, September, 1970. Original article submitted September 16, 1969.

�9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 1 7th Street, New

York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever

without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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The manufacture of similar instruments at various factories is due to weak central control of design subjects. Weak control has also been responsible for failure to ca] /a halt to the production of old types of oscillographs and to encourage the issuance of new ones. Our industry peorIy satisfies the requirements in osciUographs. A not insig- nificant role in this matter is played by the unwarranted expenditures by government agencies on multiple designs for similar types of instruments. This includes problems associated with the manufacture and designs of oscillographs.

It is advisable to discuss the basic characteristics of oscillographs and their calibration. The question of which characteristics shouM be provided for oscillographs in general use has been decided in GOST 9810-61, "Electron beam oscillographs. Technical Specifications." This does not include specific parameters of storage, stroboscopic, and high-speed oscillographs. The following can be related to several fundamental characteristics of oscillographs: passband, signal amplitude measurement error; time interval measurement error; sensitivity (range); synchronization parameters; and scan length range.

Oscillographs are classified into classes of accuracy according to the error in reproducing the signal waveform and measuring 5(t) and 5(U) in accordance with the standard specifications. Here it is appropriate to recall the disa- greement between the technical specifications on oscillographs and on electron-ray tubes, and also the differences in methods of testing the tube parameters.

In the technical specifications on electron ray tubes, the working part of the scale is not estimated from the line widths as in GOST 9810-61, and the focusing quality is also determined differently. Such a disagreement in evaluations of the tube parameters results in degradation of the important characteristics of the oscillographs: the working part of the scale and beam width. Existing interdepartmental barriers inhibit a general agreement on the solutions to these problems between the ministries of electronic technology and the radio industry. Storage oscillo- graphs, besides the common characteristics already enumerated, have a number of additional characteristics: maxi- mum recording speed, reproducing time, and storage time of the recorded image. These specific characteristics are not covered by the existing GOST 981g-61.

In view of the prospects of stroboscopic oscillographs, it is feasible to work out a standard for the technical requirements on these oscillographs.

Let us discuss the problems of controlling the fundamental characteristics of oscillographs.

It is not feasible to check all the characteristics applicable to general-use oscillographs; rather we should con- sider only those that affect the attainment of a certain accuracy in reproducing a wave shape and those that charac- terize the oscillograph as a measuring instrument.

The accuracy with which a signal is reproduced can be established if the frequency response characteristics of the amplifying channel, the overshoot introduced by the oscillograph itself, and pulse droop are measured. The scan length range and sensitivity range of the instrument are automatically checked with 5(t) and 5(U) are deter- mined. Synchronization conditions can be related to a check of the operating capability of the oscillograph. In our opinion, it is not economically feasible to check the working part of the screen, the minimum scan repetition fre- quency at which a fast signal can be observed, beam width, maximum allowable Uin, the parameters of the oscillo- graph input and the plate output, dc amplifier drift, and a number of other characteristics, in the state standards and measurement technology laboratories, since all of these characteristics are tested during stage inspections and also during periodic tests.

We should discuss separately the necessity and possibihty of determining pulse overshoot and droop.

At the present time, the existing COST 9810-61 quite categorically requires that overshoot introduced by the

oscillograph itself should be determined by applying a pulse to the front equal to 1.5 r r of the oscillograph (rise time) or a pulse with Tf for which there is no overshoot; intrinsic overshoot in the test pulse should not exceed 0.2 of the oscillograph overshoot (~,). These requirements were probably imposed by the writers of the test pulse para- meter specs on the reasoning that a suitable standard tester would be designed by the time GOST 9810-61 came in- to effect. Unfortunately, up to now there has been no standard tester to measure the whole wide range of oscillo- graphs in use according to the requirements of GOST 9810-61 on such parameters as rise time, overshoot on a pulse image, droop of the image pulse tip, and ringing. It is clear that it is most often impossible to test these parameters.

To make all newly developed devices satisfy the specifications, designers are either applying their own non- standard calibrating devices or, where it is impossible to come completely up to specifications (such as in the entire range of sensitivity on rr) , are attempting to obtain decisions on deviations from standard.

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Recently almost every newly developed oscillograph has a number of deviations from standard. It turns out that the specification requires more than is possible at the present state of the art. This situation with oscillograph testers often has the result that the designers write in the oscillograph characteristics data that cannot be verified on all the positions of the input divider with standard instruments.

The basic documents that govern oscillographs at the present time are GOST 9810-61 and Methodical Instruc- tions No. 246.

In setting up the methodical instructions, it was not taken into account that extension of all the recommended testing methods to certain old types of oscillographs (in particular, with reference to passband, CI-1, C1-2, C1-3, G1-4, C1-5, and C1-~3) is not feasible. The methodical instructions should only recommend testing means, i.e., notes should have been added concerning the possibility of using instruments similar to those recommended with a metrologiea! reserve for measurement errors.

Let us discuss the testing means required for testing the basic characteristics of the device, and then we shall consider the available instruments that may be recommended for testing oscillographs.

P a s s b a n d T e s t i n g

The standard apparatus (according to GOST 9810-61) for this test is a voltmeter used to monitor voltage fluc ~ tuations at the input. The basic requirement relative to the voltmeter concerns the uniformity of the frequency re- sponse. The specification calls out a voltmeter that is simple to use, with a frequency range 0-100 MHz, and with a measurement error close to 1% in the voltage range from 0.2 to 100 V. The V3-24 voltmeter is quite close to these specifications.

The V3-24 voltmeter can be used for oscillographs for which a section of the passband is specified with low frequency nonuniformity. For the majority of existing oscillographs with nonuniformity held to 3 dB, it is permissible to use other voltmeters, such as the VKT-9, V3-12, V3-25, and V7-2 (VLU-2). For high sensitivities (for C1-15/5) it is advisable to use a V3-28 voltmeter. It is also permissible to use high-frequency dividers, 1-350, D2-17, AC-1, D2-18, and ]92-5 for monitoring the high amplitude at their input.

We should also mention the types of generators, sources of sinnsoidal voltages, to be umd for determining the passband AF. The basic specification on the generator includes a wide range of frequencies, 20 Hz to 100 MHz, low nonlinear distortion factor, and high range of generated amplitudes (from 0.3 mV to 100 V). The following types of generators can be recommended: G3-41, G3-33, GK3-40, G3-39. It is permissible to use other types of generators with the required frequency and amplitud e ranges.

In testing passband it is sometimes a problem to decide which generators should be used to check/xF to 30 MHz or sensitivity on the order of 10-50 V/cm. In this case it is advantageous to test AF at sensitivities to 2-5 V/cm using one of the generators mentioned above, and then determine the rise time rr of the oscillograph, which is as- sociated with the passband by the weI1-known empirical formula AF = 0.35/~- r.

T i m e - I n t e r v a l M e a s u r e m e n t E r r o r

This requires a standard time interval generator in the range from 0.1 psec to 10 sec with an error of 1-2%. The available stock of standard instruments to test this characteristic is quite wide. Here note that 5 (t) must be tested over the whole range of lengths at ,the minimum and maximum of the linear dimensions of the test interval. In the majority of oscillographs, only the time interval measurement is given. If the value of 5 (t) at the maximum and minimum dimensions is plotted on the X axis and in the whole range of calibrated lengths (r p sec / cm) , then it is not feasible to conduct another test on the scan nonlinearity since this will only determine one of the components of the total time measurement error. If there are time traces, then it is not feasible to test 7, the scan nonlinearity in a testing laboratory. Otherwise, 5(t) is tested in accordance with Methodical Instructions No. 246.

Here we can recommend the following standard apparatus: I2-9A, G5-4B, G5-17, and G5-26 with an e lec- tronic-counter frequency meter, any pulse generator with the required rp range with an I2-8 time interval meter, and any sinusoidal voltage generator with the required frequency range and an electronic-counter frequency meter.

V o l t a g e A m p l i t u d e M e a s u r e m e n t E r r o r

The standard instruments for conducting this test are unsatisfactory. It reqttires a pulse voltage source with pulse length range from 0.2 psec to 10 /tsec, 0.5% output amplitude error, and dynamic range from 50 mV to 100V.

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At the present time standard generators with calibrated amplitudes are not being produced industrially. De- signers at the sample test stage are using I i - 1 certified nonstandard stations.

The OKV-5A standard compensation voltmeter with measurement error 0.2-0.4% i n t h e range 1.2-150 V has not received widespread acceptance; accordingly the developers of Methodical Instructions No. 246 directed them- selves to the situation, including this voltmeter in a list of standard instruments. Now the V4-7 digital pulse volt- meter 6(U) from 1.5% for U e 1 V to 0.5% for greater U in the range from 0.1 gsec, F = 10 Hz-100 kHz can be rec- ommended for the amplitude range from 1 to 150 V.

The following pulse voltmeters can also be recommended: V4-11 with amplitude range 1.5-150 V and pulse voltmeter IIN-3.

In the absence of these instruments, the G5-26 generator with a range of U from 50 mV to 50 V and guaranteed ~2% error in the output U at the number 1 setting of the even control element can also be recommended.

In testing for the amplitude measurement error over the guaranteed U d and l u of the peak- to-peak amplitude, the nonlinearity of the amplitude characteristic and the error of the built-in amplitude calibrator cannot be esti- mated. Of course, it is most correct to test amplitude measurement error according to a pulse signal, since nonuni- formity of the frequency response affects the pulse amplitude relatively little. All the same, in our opinion it is worthwhile to estimate the errors involved in testing the calibrated sensitivities and amplitude error or the built-in calibrators with an equivalent exchange of the meander of the calibrators by a peak- to-peak amplitude of the sinu- soidal voltage from the Vl -4 (V1-2) device on the basic repetition rate of the built-in amplitude calibrator.

The experience of our institute and the L 'vov factory indicates that such an exchange is possible for the m a - jority of oscillographs, since the existing possibility of error due to nonuniformity of the frequency characteristic overlaps subjective error in reading out the linear dimensions. The use of the V1-2 would permit considerable sim- plification of the method of testing the built-in calibrators in old oscillographs that do not have external outputs and the error of calibrating the sensitivity of the majority of oscillograph types.

We must note that such standard testing devices as the OKV-5M, V4-7, V4-11, and IIN-3 have a common fault: in their resolution capability they are not suitable for use as standards for measuring small signals, less than 1 V. They can only be used in this application in combination with a set of precision dividers.

R i se T i m e , O v e r s h o o t , a n d D r o o p

Although these tests are not called out in Methodical Instructions No. 246, they should be discussed briefly.

Tests of rise, time, overshoot, and droop require a pulse generator with variable-slope front in the range from 2 nsec to 0.5 ~sec and amplitude range from 100 mV to 100 V. Standard generators with such characteristics are not produced industrially. Several organizations use nonstandard generators with variable front, but this is no solution for the mass of testing installations. To obtain the required leading edge length now requires a generator with a steep front and integrating circuits to adjust the length. The following can be recommended as standard generators for this purpose: G5-26, G5-9 (GKI-1), G5-11 (GNI-1), G5-16, G5-17, and G5-13 (GKI-5). These generators can be used to determine the value of overshoots. A GK-26 generator is recommended for determining droop in the whole range

of pulse lengths.

New T r e n d in S p e c i f i c a t i o n s on O s c i l l o g r a p h C h a r a c t e r i s t i c s

a n d M e t h o d s o f T e s t i n g T h e m

If there is now a somewhat confused testing procedure and frequency analysis of the elements of the transient response: overshoot, rise time, and droop, then in conjunction with the development of an apparatus that will pro- duce standard-form test pulses, it would be possible to go to a more rational method of testing the characteristics of an osci l lograph-to a test of the transient response. There are already plans to standardize the technical require- ments on oscillographs on the basis of che transient response and a plan to standardize the testing methods. Of course, these projects will only come into force after a broad appropriation for the production of the required generators.

A test of the transient response of linear four-terminal networks provides a more correct estimate of the qual- ity of transmission of a complex-wave signal and reduces the technical difficulties invo!ved in the measurement as

well.

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