# How to obtain the widest pulse on an oscilloscope?

I need to get the widest pulse from a train of pulses. In some tester you have the option to show only the highest measure, There's a way to do that with the pulse measure in a DSO?

The DSO is an Atten ADS1022C, the pulse train is normally around 112 kHz, and I need to know if those pulse ever last more than 1 ms, no matter if stay high or low.

• You can set a trigger on pulse width greater than a value. Set this value initially to a very high one. And then, when your pulse train is running gradually reduce this value manually. Jun 3 '16 at 13:58
• And why are you concerned with a 1 msec pulse in a 112 kHz pulse train? This indicates a major interruption in the train. Are you sure you don't mean 1 usec? Jun 3 '16 at 15:05
• @WhatRoughBeast It is the output of a µC that indicates how often the µC finish a loop. An interrupt in the pulse train indicates a hang up, I mean something is making that the µC get delayed at the end of the loop. Jun 3 '16 at 15:26
• "I need to know if those pulse never last more than 1mS, no matter if stay high or low." - do you mean both if pulse width or time between pulses exceeds 1ms? What time period is 'never'? Jun 3 '16 at 19:12

Just in case your DSO doesn't have a pulse width trigger setting consider the following.

A square wave at 112 kHz with 50:50 duty cycle has a mark or space duration of about 4.5 us. Looking for a mark or space of 1 ms is easy if you feed the signal to the scope via a low pass filter and trigger on amplitude. The low pass filter can be set arbitrarily between 1 kHz and 10 kHz and you will definitely get a much bigger amplitude when the pulse length extends this much.

Here's a picture that explains: -

(source: evalidate.in)

So you get the trigger and you can measure the length on the scope because you said it was a DSO.

Unless your particular scope has the ability to trigger on a pulse greater than a particular length, you have to do this externally.

You could do this in hardware with a retriggerable one-shot. Incoming pulses trigger the one-shot. Additional logic sets the trigger output high whenever the one-shot output is low, but the incoming pulse is still high.

Or, if you want to detect long pulses of either polarity, then arrange for the one-shot to be triggered by both edges of the incoming pulse. If the one-shot output ever goes low, then a long interval has been detected.

In either case, this trigger signal is connected to the scope's external trigger input, and of course the scope has to be set up to trigger on that.

• @Eugene: I just checked the low end scope on my bench at this site, and it indeed has this capability. I'm pretty sure I've not seen it in other scopes I use regularly, but am not where I can check now. I'll remove the "most don't" comment. Jun 3 '16 at 15:06