Right here is an easy frequency doubler circuit that produces a sq. wave output with a exact 50 p.c obligation cycle. There are related circuits within the literature [References 1, 2, 3] which require changes or choice of some elements to set the obligation cycle to 50 p.c.
With this circuit, only a matched pair of resistors produces a 50 p.c obligation cycle output pulse, and, as well as, the heart beat obligation cycle shouldn’t be affected by adjustments within the provide voltage.
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The circuit could be seen in Determine 1. It has been examined from 500Hz to 2.8MHz, and I’m assured it’ll work at greater frequencies when you use a quicker one-shot for U1.
Determine 1 The frequency doubler circuit with a 50 p.c obligation cycle.
The system makes use of suggestions through op amp U5A to power the one-shot to provide an output sq. wave with a nominal common worth of two.5 volts that equals the DC reference established by the matched pair (or a set of precision resistors), R26 and R27.
The reference for op amp U5A is derived from the identical provide voltage that’s utilized by one-shot U1A which swings rail-to-rail as a result of it drives a really gentle load. The result’s that the common worth of the one-shot’s output and the worth of the reference voltage change by the identical proportion for provide voltage adjustments, so the obligation cycle doesn’t change.
Unique NOR gate U4 is used to buffer the sq. wave enter sign and to double the frequency of the sign. The sq. wave alerts on the enter of U4D are delayed relative to one another by two gate delays. The output is a nominal 20 nanosecond pulse practice having twice the frequency of the enter sq. wave. These pulses set off the one-shot to provide doubled output frequency pulses, and the suggestions loop forces the obligation cycle of the pulses to 50 p.c.
Transistors Q5 and Q6 and related elements are a continuing present supply which prices capacitor C11 on the RX/CX enter of the one-shot. The present is proscribed to about 5 milliamps, which is the max that’s implied by the information sheet. My LTspice simulation wouldn’t run with out the present limiting circuitry. The present limiting additionally prevents the enter of the one-shot from being hit with a big present surge at turn-on or when capacitor C11 is switched for various frequency ranges.
The worth of capacitor C11 is left to the consumer’s discretion…
To my shock, the circuit will function with C11 omitted! At low frequencies, the present provided by the fixed present supply is within the nanoamps vary, which can be within the vary of part leakage currents in some purposes, and this may occasionally trigger erratic circuit operation. So, omit C11 with warning.
Omitting C11 could also be okay within the frequency vary of 1MHz and above. Right here, the enter capacitance of the one-shot plus stray circuit capacitance could also be ample.
Desk 1 exhibits the noticed frequency vary of operation for a number of values of C5.
For correct operation, the enter should be a sq. wave with 50 p.c obligation cycle. A pulse with one thing apart from a 50 p.c obligation cycle can be utilized if the extra circuitry is applied (see Determine 1). The extra circuitry produces an output sq. wave with 50 p.c obligation cycle that’s used because the enter to the frequency doubler.
A Schmitt set off and quad nand gate U3 are used to offer orderly startup for the frequency doubler part. The frequency doubler doesn’t begin up till the heart beat enter is sort of at 50 p.c obligation cycle.
I’ve not constructed this extra circuitry, however I’ve simulated it with LTspice, it’s largely a reproduction of the frequency doubler circuit which I did construct and check.
Reference 4 is an alternate frequency doubler circuit which operates with a pulse enter with lower than a 50 p.c obligation cycle, and supplies a sq. wave output with a 50 p.c obligation cycle.
References:
- Extensive-range pulse-shaping circuit provides sq. waves with 50% obligation cycle
R. M. Stitt and R. L. Morrison, Burr-Brown Analysis Corp., Worldwide Airport Industrial Park, Tucson, Ariz.
400 Concepts for Design quantity 3, 1976, web page 178. - One-shot with suggestions loop maintains fixed obligation cycle
H. P. D. Lanyon, Worcester Polytechnic Institute, Worcester, Mass.
Electronics Designer’s Casebook, web page 122. (No quantity quantity or printed date given.) - Frequency-doubler produces square-wave output
Robert L. Taylor, I & F Electronics, Nashville, Tenn.
Electronics Designer’s Casebook No 1, web page 23. (No printed date given.) - Convert any sign to precisely 50% obligation cycle
Jim McLucas
EDN Design Concepts, June 25, 2013
https://www.edn.com/convert-any-signal-to-exactly-50-duty-cycle/
Jim McLucas retired from Hewlett-Packard Firm after 30 years working in manufacturing engineering and on design and check of analog and digital circuits.
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