EDN Access--03.03.97 Pulse-width trimmer outdoes one-shots

-March 03, 1997

EDN logoDesign IdeasMarch 3, 1997

Pulse-width trimmer outdoes one-shots

Shyam Sunder Tiwari, IGCAR, Kalpakkam, India

The simple circuit in Figure 1a trims the trailing portion of a large-width input pulse to a size as narrow as 10 nsec. The input to the circuit is an active-low TTL pulse longer than 10 nsec or a continuous low level after a high-to-low transition. The output is a fixed 10-nsec pulse, irrespective of the input-pulse width.

The circuit doesn't require a capacitor to generate the timing pulse. Thus, its reliability over long periods is far greater than the timing pulses that RC-based, TTL, monostable multivibrators generate. These ICs have other limitations, too: They generally require an input pulse longer than 50 nsec and can't produce an output pulse shorter than 50 nsec.

In Figure 1a's circuit, a fast 74F04 inverter drives the fast 2N3960 transistor switch. A second 74F04 buffers the output to obtain a signal polarity identical to the input pulse's. The transistor switch sinks about 10 mA while conducting. The transistor turn-on delay, which is characteristic of the transistor's switching time, forms the output pulse. The 74F04 gates delay the output by approximately 6 nsec. (DI #1998)

Figure 1
This pulse-width trimmer (a) uses fast inverters and a transistor switch to trim the width of the output pulse to 10 nsec (b).

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