A simple frequency doubler circuit produces a square wave output with a precise 50 percent duty cycle.
Here is a simple frequency doubler circuit that produces a square wave output with a precise 50 percent duty cycle. There are similar circuits 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, just a matched pair of resistors produces a 50 percent duty cycle output pulse, and, in addition, the pulse duty cycle is not affected by changes in the supply voltage.
The circuit can be seen in Figure 1. It has been tested from 500Hz to 2.8MHz, and I am confident it will work at higher frequencies if you use a faster one-shot for U1.
The system uses feedback via op amp U5A to force the one-shot to produce an output square wave with a nominal average value of 2.5 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 same supply voltage that is used by one-shot U1A which swings rail-to-rail because it drives a very light load. The result is that the average value of the one-shot’s output and the value of the reference voltage change by the same proportion for supply voltage changes, so the duty cycle does not change.
Exclusive NOR gate U4 is used to buffer the square wave input signal and to double the frequency of the signal. The square wave signals at the input of U4D are delayed relative to each other by two gate delays. The output is a nominal 20 nanosecond pulse train having twice the frequency of the input square wave. These pulses trigger the one-shot to produce doubled output frequency pulses, and the feedback loop forces the duty cycle of the pulses to 50 percent.
Transistors Q5 and Q6 and associated components are a constant current source which charges capacitor C11 at the RX/CX input of the one-shot. The current is limited to about 5 milliamps, which is the max that is implied by the data sheet. My LTspice simulation would not run without the current limiting circuitry. The current limiting also prevents the input of the one-shot from being hit 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 user’s discretion…
To my surprise, the circuit will operate with C11 omitted! At low frequencies, the current supplied by the constant current source is in the nanoamps range, which may be in the range of component leakage currents in some applications, and this may cause erratic circuit operation. So, omit C11 with caution.
Omitting C11 may be okay in the frequency range of 1MHz and above. Here, the input capacitance of the one-shot plus stray circuit capacitance may be sufficient.
Table 1 shows the observed frequency range of operation for several values of C5.
For proper operation, the input must be a square wave with 50 percent duty cycle. A pulse with something other than a 50 percent duty cycle can be used if the additional circuitry is implemented (see Figure 1). The additional circuitry produces an output square wave with 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 provide orderly startup for the frequency doubler section. The frequency doubler does not start up until the pulse input is nearly at 50 percent duty cycle.
I have not built this additional circuitry, but I have simulated it with LTspice, it is largely a duplicate of the frequency doubler circuit which I did build and test.
Reference 4 is an alternative frequency doubler circuit which operates with a pulse input with less than a 50 percent duty cycle, and provides a square wave output with a 50 percent duty cycle.
This article was originally published on EDN.
Jim McLucas retired from Hewlett-Packard Company after 30 years working in production engineering and on design and test of analog and digital circuits.
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