Here is a simple frequency doubling circuit that produces a square wave output with an accurate 50 percent duty cycle. There are similar schemes in the literature [References 1, 2, 3] which require adjustments or selection of some components to set the duty cycle to 50 percent.

With this circuit, only one matched pair of resistors produces a 50 percent duty cycle output pulse, and in addition, the pulse duty cycle is unaffected by supply voltage changes.

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The chain can be seen in Figure 1. It’s been tested from 500Hz to 2.8MHz and I’m confident it will work at higher frequencies if you use a faster one-shot for U1.

Figure 1 The frequency doubling circuit with 50 percent duty cycle.

The system uses feedback via op amp U5A to force the single shot to produce a square wave output with a nominal average of 2.5 volts, which equals the DC reference established by the matched pair (or set of precision resistors), R26 and R27.

Op-amp reference U5A is drawn from the same supply voltage used by one-shot U1A, which swings from rail to rail as it drives a very light load. The result is that the average value of the single-shot output and the value of the reference voltage change in the same proportion to the supply voltage changes, so that the duty cycle does not change.

The exclusive NOR gate U4 is used to buffer the input square wave signal and to double the signal frequency. The square wave signals at the input of U4D are delayed relative to each other by two gate delays. The result is a nominal 20 nanosecond pulse train with twice the frequency of the input square wave. These pulses trigger the one-shot to produce pulses with twice the output frequency, and the feedback loop forces the duty cycle of the pulses to 50 percent.

Transistors Q5 and Q6 and their associated components are a DC source that charges capacitor C11 at the RX/CX input of the one-shot output. The current is limited to about 5 milliamps, which is the maximum listed in the datasheet. My LTspice simulation wouldn’t work without the current limiting circuit. The current limit also prevents the one-shot input from hitting with a large current surge at turn-on or when capacitor C11 is switched for different frequency ranges.

The value of capacitor C11 is left to the discretion of the userâ€¦

To my surprise, the circuit will work with C11 omitted! At low frequencies, the current supplied by the DC source is in the nanoamp range, which can be in the range of component leakage currents in some applications, and this can cause unstable circuit operation. So skip C11 with caution.

C11 bypass can be fine in the frequency range of 1MHz and above. Here, the input capacitance of the one-shot plus the stray circuit capacitance may be sufficient.

table 1 shows the observed frequency range of operation for several values â€‹â€‹of C5.

For proper operation, the input should be a square wave with a 50 percent duty cycle. A pulse with something other than 50 percent duty cycle can be used if the additional circuit is implemented (see Figure 1). The additional circuit produces a square wave output with a 50 percent duty cycle that is used as the input to the frequency doubler.

A Schmitt trigger and quad nand gate U3 are used to ensure proper startup of the frequency doubling section. The frequency doubler does not start until the pulse input reaches almost 50 percent duty cycle.

I haven’t built this additional circuit, but I’ve simulated it with LTspice, it’s pretty much a duplicate of the frequency doubling circuit I built and tested.

Benchmark 4 is an alternative frequency doubling circuit that operates with a pulse input with less than 50 percent duty cycle and provides a square wave output with 50 percent duty cycle.

References:

1. A wide range pulse shaping circuit produces square waves with a 50% duty cycle
RM Stitt and RL Morrison, Burr-Brown Research Corp., International Airport Industrial Park, Tucson, Ariz.
400 Design Ideas, Volume 3, 1976, Page 178.
2. One-shot with feedback maintains constant duty cycle
HPD Lanyon, Worcester Polytechnic Institute, Worcester, MA.
Electronics Designer’s Casebook, page 122. (No volume number or publication date given.)
3. A frequency doubler produces a square wave
Robert L. Taylor, I & F Electronics, Nashville, Tenn.
Electronics Designer’s Casebook Number 1, page 23. (No published date given.)
4. Convert each signal to exactly 50% duty cycle
Jim McLucas
EDN Design Ideas, 25 Jun 2013

Jim McLucas retired from Hewlett-Packard Company after 30 years in manufacturing engineering and the design and testing of analog and digital circuits.

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