Posted on Sep 2, 2012

The circuit uses one-half of a dual D flip-flop as an inverter. When the input decreases, the flip-flop resets, and its Q output increases. When the input increases, the reset line is released and Q gets clocked low. The rc delay between applying the input signal to the flip-flop`s reset input and its clock input enables clocking the flip-flop on the input`s positive edge. A 74HC74 dual D flip-flop, for example, requires a minimum recovery time of 5 ns after releasing the reset input before strobing its clock input.

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Therefore, speccing rc at greater than 7.5 ns provides adequate margin. The slight slowing of the clock edge presents no problem, because the clock input"s maximum allowable rise time is a much longer 500 ns. To prevent skewing ofthe output"s symmetry, limit the maximum input frequency to less than 10 MHz.

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