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.
 |
| (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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