Electronic – Ignore Pulses Less than a Given Width in a Pulse Train


Let's say I have a 5V pulse train that varies between 0 and 75000 pulses per second. Pulse width, let's say, is 5 microseconds. How could I calculate or determine the best way to filter out all pulses less than 4 microseconds in width?

Best Answer

This circuit should do what you want:

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When a pulse comes in on the lead marked IN, it starts the first 74221 monostable (there are two in the same package) which is set for 4 µs. The output of the monostable \$\small \overline{\text{Q}}\$ pin will be 1 until it is started, then it goes to 0. So during the 4 µs period, the AND gate (74HCT11) is inhibited so there is no output. After 4 µs, the AND gate is enabled again, so the rest of the pulse (if longer than 4 µs) goes through to OUT. See the timing diagrams below.

Since 4 µs has been cut off the beginning of any valid pulse, the second monostable is started on the falling edge of the input pulse (but only if it is longer than 4 µs), and the output remains high to add an additional 4 µs to the end of the output using the OR gate (74HCT32). Thanks to stefandz for pointing out the need for this additional functionality.

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The buffer marked DS1100Z at the bottom is actually a delay line with a delay of 100 ns; this is to compensate for the propagation delay in the 74123 from trigger to output.

Note because of the tolerance of the capacitor, this timing could be off by 10%, so the resistor should be tweaked as necessary.

By using monostables, it is trivially easy to modify the timing value from 4 µs to something else just by changing a the resistors and/or capacitors.

Power pins (Vdd and Vss) and decoupling caps are not shown.

I was originally going to use a 555 timer, but it turns out the 555 is not supposed to be used for pulse widths less than about 10 µs. So I turned to the 74221, which can generate pulses down to the ns range.

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