Tektronix 2440 slow to update waveform - Page 1
Forum section: electronics / testgear

author: sachleen
author_rank: Contributor
timestamp: September 18, 2014, 09:47:26 pm
content: In the smaller time bases (ns, or even ms) I see the waveform constantly updating as I probe different parts of my circuit or make changes to the display (vertical position or volts/div) but when I go out to something like 500ms/div or above, it takes forever (tens of seconds) to update. The waveform is just frozen on the screen.I can make changes to the horizontal position but not vertical. If I turn the volts/div knob, my measurements will update to reflect the new settings (if the pulse width was 1s at 1s/div and I change it to 500ms/div, the measurement will show a pulse width of 2s)It should still be triggering as normal since the only thing I change is the time base... and if I change the trigger to rolling, I'll see the new, and constantly changing, waveform appear from the right edge instantly (assuming i'm scrolled to the right in the horizontal position). Once I change it back to auto or normal and check the trigger level, the waveform will freeze to whatever it was at that point for a long time.I want to say the time it takes is proportional to the time base setting times the total number of divisions that can be displayed (including horizontal scroll) but that's just a guess, I haven't measured it.Is this normal/limitation of this scope? I haven't experienced this in the more modern scopes I've used at school.

author: veeev
author_rank: Newbie
timestamp: September 23, 2014, 02:04:18 pm
content: The 2440 fills its 1024 point capture buffer before it retriggers. That may take a while, but not so many seconds.Have you checked that the output is correct if you press TRIG STATUS (source, coupling, position, edge)?The TRIGD LED should be on after triggering. If READY is lit, it is waiting to trigger.Though this only happens if you choose normal trigger mode, not auto. Auto enables free-running mode after a second.I own a 2440, but never ran into the problems you described.BTW, I hope that the internal calibration is OK. Else, your trigger voltage may be off and other strange things happen on faster deflection times.

author: MarkL
author_rank: Unknown
timestamp: September 23, 2014, 03:03:28 pm
content: 500ms/div with 10 divisions will take 5 seconds to fill the capture memory, so you have at least that long to wait.Confirm that your trigger point is on the screen. You should see the "T" riding on the waveform. If you have any delay options set then your trigger point might be way off to the left, and the scope is just waiting extra time before doing the capture and showing you the waveform.Failing that, try resetting the scope to default settings with "PRGM --> INIT PANEL". Maybe there's something else unexpected that's set.

author: rf-loop
author_rank: Super Contributor
timestamp: September 23, 2014, 06:07:17 pm
content: Quote from: MarkL on September 23, 2014, 03:03:28 pmCapture memory is more than screen width in this model if I remember right.On the screen is 512 and whole capture is 1024 data points. (if trig is center then half screen over both sides) 50 sample points/div exactly.So with 500ms/div filling capture memory take 10.24sec.(Ref also 2440 Operator's manual)

author: MarkL
author_rank: Unknown
timestamp: September 23, 2014, 09:15:31 pm
content: Quote from: rf-loop on September 23, 2014, 06:07:17 pmAh, you're right. From the ops manual:QuoteI don't have this scope either, but I've been through the manual. Missed that point, though. The OP's observation of "10's of seconds" still seems too long even taking the off-screen points into account.

author: rf-loop
author_rank: Super Contributor
timestamp: September 24, 2014, 04:53:54 am
content: Quote from: MarkL on September 23, 2014, 09:15:31 pmYes, it is not 10's of seconds, it is 10.24s + rearm and waiting next trig event (if selected 500ms/div.)And for OP, this is same with even most modern and expensive scope. If capture lenght is 20div exactly and there is selected 500ms/div it takes 10s (+ waiting next trigger event and this it start waiting after it have captured pretrig part of capture buffer (there is three selection where is trigger position in capture memory (default is center)).If it take unexpected long time (related to settings and basic fundamentals how it works) check your trig settings. If scope is out of order (loosed its calibration or failed, it may be impossible or very difficult to adjust) welcome to Tek scope service calibration wonderland.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 10:17:31 am
content: As pointed out, the 2440 waveform acquisition record is 20 divisions so at 500 ms/div it takes 5 seconds to arm and fill the pre-trigger record and 5 seconds after the trigger before the display is updated to fill the post-trigger record at least with a 50% pretrigger.There is a further delay of about 2.5 seconds after arming (500 ms/div) but before triggering which I do not know the cause for. My guess is that this has to do with operating the digitizers in short pipeline mode.The 2440 continuously updates the ground level marker on the display even during the acquisition process so the vertical control should at least position that. It also supports a display roll mode at slow horizontal settings if you want to watch the acquisition continuously.