Triggering digital oscilloscope for a slow sweep - Page 1
Forum section: electronics / testgear

author: SilverSolder
author_rank: Super Contributor
timestamp: October 13, 2020, 04:56:18 pm
content: Who would have thought this would be difficult, but I'm having trouble getting it to work:I want to run an up to 500 second sweep across the screen of the oscilloscope, triggered by an external device (via the External Trigger port). I'd like to be able to do that both as a single shot, or a repeating "Normal" trigger.The problem is that on the scope I'm using (an Agilent Megazoom 54622d), the trigger doesn't always trigger the start of a sweep....I understand that a digital scope likes to fill its internal buffer to a certain point before it will trigger. I work around that by setting the trigger point all the way over to the left edge of the screen, so the scope has no reason to fill any buffer (the user is not asking to see what happened before the trigger point). That works... a trigger input from the external device will start the sweep immediately...Except! - if I try to use anything other than the standard timebase settings like 2s/div, 5s/div, etc. - as soon as I use the vernier to choose a number like 2.2s/div the scope will no longer fire at the edge of the screen, but insists on waiting the full 500 seconds (or whatever the total sweep time is) for the next go-around of the trigger before showing anything.Waiting a few milliseconds for a buffer to fill is probably something we could live with, but 500 seconds is rather a long time to be staring at a blank screen!So my question is... - Is there a way to get a DSO to trigger immediately on receiving the trigger signal and just get on with the job, like an analog scope does - without all the fiddling around, which doesn't even work in some cases?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2020, 09:29:29 pm
content: I can't think of a time I have ever had a 500 second single sweep on a DSO.So, it's not the trigger but rather you just want the display to update live with the collection.Have you considered using a DMM and software? I guess I am trying to understand why you would use a scope?

author: SilverSolder
author_rank: Super Contributor
timestamp: October 14, 2020, 12:47:15 pm
content: I'm using the scope to display the (analog) output of a vintage spectrum analyzer.The SA can be swept at a rate of 4.5Hz/second at the narrowest resolution bandwidth, so a span of 2KHz takes 444.4 seconds!By using a scope to do this, I can average several sweeps to reduce noise and bring out hidden signals - almost like a modern spectrum analyzer. This actually works really well, but I'm struggling to understand why the scope triggers immediately on the first event when e.g. set to 20 seconds per division (no vernier), whereas if set to 22.3 seconds using the vernier, it refuses to react until the second trigger event... wasting 440 seconds!I'm beginning to suspect this is a bug in the firmware of the scope and that the way I am already doing it is as good as it is going to get? (I.e. live with the sweeps of 100, 200, and 500 seconds that do work immediately, or live with it taking longer when using the vernier. This was never going to be a fast measurement anyway, right?)