Showing posts with label Clock multiplier. Show all posts
Showing posts with label Clock multiplier. Show all posts

Design Quiz: multiply by 2 clock circuit

Design problem: Make a simple circuit whose output clock is twice in frequency to the input clock.


An XOR gate with one of its input getting delayed version of the other input can act as a frequency multiplier. Since, an XOR gate produces a ‘0’ when both inputs are same, and ‘1’ when both inputs are different; if it gets delayed version of one input at the other, every time input toggles, a pulse is produced at the output. The duration of the pulse is equal to the delay introduced by delay element. The circuit and the resulting waveform is shown in figure 1 below. This circuit arrangement is also known as pulse generator as it produces a pulse on every toggle of input.


An XOR pulse generator circuit can act as a multiply by two, but it does not guarantee a duty cycle
(a) Multiply-by-2 clock circuit                                      (b) Input and output clock waveforms of multiply-by-2                                                                                                 clock circuit


Characteristics of XOR multiply by 2:
  • The output pulse duration is equal to the delay introduced by delay element.
  • For duty cycle to be equal to 50%, the delay element’s delay must be half that of input clock. Since, this cannot be guaranteed, the output duty cycle will not be 50%.
  • The delay element’s delay must be less than half the input clock period; otherwise it will not work
  • The inactive state of XOR multiply-by-2 will be 0 as it produces a '0' when both inputs are same. To implement a multiply-by-2 circuit with '1' as inactive state, you will have to use an XNOR gate.

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