Unknown Frequency Quiz
September 1965 Popular Electronics

September 1965 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Lots of old-time sci-fi movies and TV shows liked to display Lissajous patterns on oscilloscopes when an authentic high tech look was needed in a scene. Every engineer, technician, and physicist in the world - me included - roll his or her eyes at the sight of such a lame attempt to impress the public. Of course the truth is the first time I saw a Lissajous pattern gyrating on an o-scope screen, I was mesmerized. The need to crank on the signal generator knobs and take control of the electroluminescent object d'art was overwhelming. Now, in the same manner that watching the first couple Space Shuttle launches was a thrill not to be missed and then became just another launch, so, too, is watching a live Lissajous pattern on a scope - it's just another Lissajous. Except, well, that's not quite so - at least for me. I always eagerly viewed Shuttle launches and landings (in the good old days when the U.S. had a manned space program), and I still can't pass up the opportunity to play with Lissajous patterns on an oscilloscope.

Unknown Frequency Quiz

By Robert P. Balin

An unknown frequency can be accurately determined by analyzing the Lissajous pattern it forms when combined with a known frequency. Lissajous patterns can quickly tell you the ratio of the frequencies applied to the vertical and horizontal inputs of an oscilloscope. See if you can do problems 1-10. Ratios and unknown frequencies are given on page 95.

(Hint) : In problem 1, for example, there are three-and-one-half vertical cycles as indicated by the peaks at the top and bottom of the pattern, and two horizontal cycles as indicated by the peaks on either side. This combination or vertical-to-horizontal frequency ratio is 3.5 to 2, or 7 to 4. Therefore, if the vertical frequency is 2800 cycles, the unknown horizontal frequency must be 1600 cycles to produce the pattern shown.

See answers below.

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

Quiz Answers

  1)  7:4, 1600 cycles

  2)  3:2, 750 cycles

  3)  7:3, 600 cycles

  4)  9:4, 540 cycles

  5)  5:6, 1200 cycles

  6)  4:5, 48 cycles

  7)  7:6, 3600 cycles

  8)  4:3, 160 cycles

  9)  5:7, 4900 cycles

10)  8:3, 2400 cycles

Posted September 21, 2018