Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes Calculators Education Engineering Magazine Articles Engineering software RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising Magazine Sponsor RF Cafe RF Electronics Symbols for Visio RF Electronics Symbols for Office Word RF Electronics Stencils for Visio Sponsor Links Saturday Evening Post NEETS EW Radar Handbook Microwave Museum About RF Cafe Aegis Power Systems Anritsu Alliance Test Equipment Amplifier Solutions Anatech Electronics Axiom Test Equipment Berkeley Nucleonics Bittele Centric RF Conduct RF Copper Mountain Technologies Empower RF everything RF Exodus Advanced Communications Innovative Power Products ISOTEC KR Filters Lotus Systems PCB Directory Rigol RF Superstore San Francisco Circuits Reactel RFCT TotalTemp Technologies Triad RF Systems Windfreak Technologies Withwave LadyBug Technologies Wireless Telecom Group Sponsorship Rates RF Cafe Software Resources Vintage Magazines Thank you for visiting RF Cafe!
Triad RF Systems Amplifiers - RF Cafe

Wave-Shape Plots for Checking Amplifier Distortion
May 1939 QST Article

May 1939 QST

May 1939 QST Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from QST, published December 1915 - present (visit ARRL for info). All copyrights hereby acknowledged.

Most people engaged in circuit design and adjustment in a professional environment own or have access to a spectrum analyzer and/or digital oscilloscope with an FFT function, so measuring the harmonic content of a signal is a fairly simple job. A lot of instruments will display a listing of frequency makeup and the percentage of the whole signal it occupies. Many, though, particularly hobbyists, use simple analog o-scopes where determining harmonic content requires a largely subjective assessment of the displayed waveform. In 1939 when this article appeared in QST magazine, almost nobody, whether amateur or professional, had even an analog spectrum analyzer available, and therefore relied on drawings and a trained eye to discern harmonic content. The original article included two sets of full-size drawings of the distorted waveforms - one for comparing to a 2-inch CRT display and another for a 3-inch CRT. The user could trace the shapes onto a piece of onion paper and overlay it directly on the CRT for comparison.

Wave-Shape Plots for Checking Amplifier Distortion

By George Grammer

A Simple Method of Determining Whether Distortion Exceeds Acceptable Limits

Harmonic distortion is something which cannot wholly be avoided in an audio amplifier, but it must be kept within tolerable limits if the quality of reproduction is to be good. It is of considerable interest, therefore, to know the order of distortion present in an amplifier, but its measurement requires rather expensive equipment. On the other hand, for amateur work it is less important to know the exact amount and type of distortion generated than it is to know whether it lies above or below limits which represent "good," "fair," or "poor" performance.

 - RF Cafe

Wave-shape plots for use with a 2·& 3 inch oscilloscope tube.

If one has an oscilloscope with a linear sweep the order of distortion can be found quite readily with the help of the accompanying plots. These drawings, which were prepared by John L. Stiles, W8PLN, give typical cases of the types of harmonic distortion ordinarily encountered in audio equipment. The second-harmonic cases are characteristic of single-ended stages using triode tubes, while the third-harmonic drawings are representative of push-pull amplifiers or Class-B modulators. The sixth case, marked "7% second plus 5% third" is typical of a single-ended pentode amplifier working at rated output.

To utilize the drawings it is necessary to have a rather pure single-frequency signal for reference. This signal can be compared with the sine-wave curve to make sure that it meets the specifications. If no audio-frequency generator is available, the power-line wave-shape usually will be pretty close to a pure sine wave, and in case there is an appreciable discrepancy the harmonics can be filtered out by connecting a fairly good-sized condenser across the source. With the power line as the signal source, a step-down transformer should be used both for insulation and to reduce the voltage to a suitable value. The wave-shape of the source should be checked directly against the oscilloscope, of course, and should not be fed through the speech amplifier until after the purity of the wave has been established.

It is generally more convenient to use a higher frequency than 50 or 60 cycles, since a good many speech amplifiers will not respond well to such low frequencies - nor is it necessary that they should, since speech seldom contains any components below 100 cycles. The simple sine-wave oscillator shown in the Handbook1 will serve nicely for this purpose. It costs very little to make and is a handy gadget for testing purposes.

The two sets of figures shown are suitable for use with 3-inch and 2-inch tubes respectively. They are about as large as is practicable without running too close to the circumference of the cathode-ray tube screen. To use them, lay a sheet of transparent paper or celluloid over the drawing and carefully trace the plots, using as fine a line as possible. The tracing can then be held or fastened to the screen of the tube with the plot appropriately centered. With the signal applied to the oscilloscope, the horizontal and vertical controls should be manipulated until the screen pattern coincides as closely as possible with the tracing. It is not necessary to pay any particular attention to the dotted base-line, since this may or may not correspond to the horizontal sweep line on the screen when there is no vertical input.

The figures show a characteristic difference in form between waves containing even and odd harmonics. When even harmonics are present the lower half-cycle is not the same shape as the upper; in this case one half-cycle is more peaked while the second is broadened. With odd-harmonic distortion, however, the two half-cycles are identical, both being flattened at the peaks. The general rule that the wave is symmetrical in shape with odd-harmonic distortion and asymmetrical with even-harmonic distortion is true in all cases, although the actual shapes shown here are applicable only in the special case of distortion generated by the ordinary tube amplifier. A shift in the phase of the harmonic with respect to the fundamental will change the wave-shape considerably, even though the relative amplitudes are the same. When both even and odd harmonics are present, the resultant wave-shape is naturally a combination of the two effects, and the type of distortion is not easily recognizable.

By trying various of the plots against the pattern on the screen it should be possible to determine quite readily whether the distortion is excessive. If the output wave is not quite a sine wave but fails between the sine and 5 per cent second-harmonic curves, then the distortion is certainly less than 5 per cent, which is good. Distortion between 5 per cent and 10 per cent is tolerable enough, but if it exceeds 10 per cent it would be advisable to look into the speech amplifier. In general, attempt to make the lines coincide as far as possible starting from the center, letting the peaks indicate the order of distortion.

1 "Instruments and Measurements" chapter.



Posted June 3, 2016

Anritsu Test Equipment - RF Cafe
KR Electronics (RF Filters) - RF Cafe
Innovative Power Products Passive RF Products - RF Cafe
PCB Directory (Manufacturers)

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free


About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024


    Kirt Blattenberger,


RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website: