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What's Your EQ?
July 1964 Radio-Electronics

July 1964 Radio-Electronics

July 1964 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

My guess at the solution for the "Unsquare Waves" challenge in "What's Your EQ" feature of the July 1964 Radio-Electronics magazine was wrong, but would have been reasonable for a more modern oscilloscope. I thought maybe the compensation capacitor in the o-scope probe was way out of adjustment. Since the author provides a schematic of the oscilloscope input circuit, you will probably spot right off what the cause of his unexpected waveform was. The other problem is a fairly simple, first-year electronics course deal. As the title of it suggests, you'll need to take into account the charge on each capacitor to most easily arrive at the answer. Bon chance.

What's Your EQ?

Conducted by E. D. Clark

Two puzzlers for the students, theoretician and practical man. Simple? Double-check your answers before you say you've solved them. If you have an interesting or unusual puzzle (with an answer) send it to us. We will pay $10 for each one accepted. We're especially interested in service stinkers or engineering stumpers on actual electronic equipment. We get so many letters we can't answer individual ones, but we'll print the more interesting solutions - ones the original authors never thought of.

Write EQ Editor, Radio-Electronics, 154 West 14th Street, New York, N. Y. 10011

Answers to this month's puzzles are on page 58.

Unsquare Waves - RF CafeUnsquare Waves

My audio generator was producing very bad square waves. It was new and I expected better from it, but the scope trace was rounded even at 1,000 cycles, worse at 5,000, and a misshapen sine wave at 10,000. My first impulse was to distrust the simple clipper-type shaping circuit. I recalled a similar circuit used in a Lafayette audio generator which gave very good square waves to 10 kc; the chief difference being small chokes used as peaking coils at two points.

I bought suitable chokes, wired them in - nothing. No improvement whatever. Still blaming the clipper, and knowing that my scope couldn't be as bad as all that, I even tried two Schmitt-trigger squaring circuits. When they failed to do any better, I began to wonder about my scope, and I decided to feed the signal direct to the CRT vertical-deflection plates, bypassing the amplifier. If the scope amplifier's response was really so bad, that test would show it.

Before I actually tried the direct feed, I found the trouble. It was the scope, but there wasn't really anything wrong with it! Can you figure out what happened, with the help of the partial schematic.

- Peter E. Sutheim

 

Capacitor voltages - RF Cafe"Q"

Three capacitors, each charged initially as shown, are connected in series with a resistor and a switch. Determine the steady-state voltage on each capacitor after the switch is thrown.

- Oscar D. Anderson

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, Radio-Electronics, 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.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Answers to What's Your Eq?

This month's puzzles are on page 33

 

Unsquare Waves

As I prepared to feed the square-wave signal to the vertical plates directly, I reached for the INT-EXT switch at the back of the scope. It had been in the EXT position for months!

Why hadn't I noticed? Because of R13 and R14, the deflection circuit was electrically continuous even though the switch was set to EXT. Because scope plates draw virtually no current, there was little voltage drop across R13 and R14. But the resistors, together with stray shunt wiring capacitance and the capacitance of the deflection plates themselves, formed a very effective low-pass filter that rounded off the square waves by progressively attenuating higher harmonics. On sine waves and most other waveforms, the discrimination wasn't noticeable.

I slid the switch back to INT, and found I had beautiful square waves-even from the original clipper circuit!

 

"Q"

Capacitor voltages - RF CafeThe resistor has no effect on the steady-state values - RF CafeThe resistor has no effect on the steady-state values, so it can disregarded. By using the reciprocal of the sum of the reciprocals (the capacitors-in-series formula), and the sum of the voltages, the effective capacitance of the three is found to be 1 μf at 300 volts, and a charge Q of 300. (Q = CE)

This is the total charge obtainable from the circuit after switch is thrown. The charge remaining on each capacitor in the steady state equals its original charge minus 300.

The polarity of C2 is reversed in the final or steady state.  

 

That Two-Meter Puzzler (from the April edition)

The solution to this April puzzler evoked some comment about the correctness of the de ammeter reading. Reader Richard Mirdas came up with a solution using "ideal" components (zero meter resistance-diode with zero forward, infinite back resistance). When diode conducts, the current divides equally between diode and meter, allowing a meter peak-reverse current of Imax/2. With Irms = 1.15 amp, and using the formula: lave = Irms/4 x 0.637/0.707 = de amps, the dc ammeter reads 0.2587 amp. Reader Jesse T. Hancock Jr. suggests (again using ideal components), that the original given solution could be gotten by inserting an additional diode in series with the meter, connected so that meter current is cut off while "black- box diode" conducts. We ran bench tests with practical components (two different de ammeters, two different-type diodes). Due to the diode's requiring a few-tenths-volt forward bias before it conducts, reverse current flows in the meter during part or all of the negative half-cycle. There would be no definite de value indicated; it depends on the meter sensitivity and the diode characteristics. A sensitive meter would read zero, and an insensitive one (high voltage-drop for full-scale deflection) would approach the given reading of 0.515 amp.

- Editor

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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 lady's voice announced "You've Got Mail" when a new message arrived...

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