DHO804 compensation signal rise time - Page 1
Forum section: electronics / testgear

author: ICPete
author_rank: Newbie
timestamp: January 04, 2025, 11:44:39 pm
content: I've been lurking on this forum for a while, helping me decide what new 'scope to buy.I just received my new DHO804 from Rigol.First thing I did was connect one probe (set to 10X) to the compensator signal terminal on the front, and the probe's ground clip to the ground terminal.Hit "Auto" and selected to display two cycles of the waveform (1 KHz square wave, 3V amplitude).It looks good until you decrease the time base; the risetime is several microseconds.Is this normal?I would have thought the comp signal would have a much faster rise time than that.Here are screenshots with different time base settings:And measuring the rise time (10% to 90%):Connected a second probe to Channel 2; same result (traces offset vertically for clarity):

author: Martin72
author_rank: Super Contributor
timestamp: January 05, 2025, 12:05:21 am
content: Hi,QuoteAmong other things, this also has something to do with the bandwidth of the scope.Your DHO804 has a declared bandwidth of 70Mhz, in “real” it is more, around 120Mhz.This coincides with the measured risetime.

author: TurboTom
author_rank: Super Contributor
timestamp: January 05, 2025, 12:15:31 am
content: This is microseconds, not nanoseconds...so far away from the bandwith limit.I guess for probe calibration, this simply is fast enough, and possible EMI may also have been a reason to keep the slopes rather "benign".

author: ICPete
author_rank: Newbie
timestamp: January 05, 2025, 12:23:50 am
content: Correct; 3 microseconds rise time, perhaps consistent with 150 KHz bandwidth! Not nanoseconds, which would make sense to me.And I checked all the menu settings I could find to see if there were some low-pass filtering applied by default. But no. There is a 20 MHz low-pass option, but turning that on or off makes no difference to this observed waveform.I just want to make sure the 'scope isn't defective due to this point. Kind of hoping someone else with a DHO800 series could try the same thing, to verify.I looked all over the User Manual and Datasheet, but no mention of the specified rise time of the comp waveform.BTW, I also adjusted the compensation of the first probe, but it was already pretty near perfect as received. And that adjustment didn't impact the visible "round shoulder" seen in my trace pix.Thanks for your replies.

author: TurboTom
author_rank: Super Contributor
timestamp: January 05, 2025, 12:32:02 am
content: @ICPete: Don't worry, my DHO800 shows the same 3µs rise time as yours. I'm pretty sure the rise time is like this deliberately, you definitely don't want an LVC gate output directly driving the compensation pad - this would make a pretty good "comb generator", transmiting up into low the gigahertz range... If you need someting like this, it's pretty easy to DIY oneCheers,Tom

author: Martin72
author_rank: Super Contributor
timestamp: January 05, 2025, 12:40:36 am
content: Quote from: TurboTom on January 05, 2025, 12:15:31 amSry, it´s late...

author: ICPete
author_rank: Newbie
timestamp: January 05, 2025, 01:25:17 am
content: Thank you Tom!That's a relief.I understand they wouldn't want to generate any more EMI than necessary.

author: Fungus
author_rank: Super Contributor
timestamp: January 05, 2025, 04:58:58 am
content: Quote from: ICPete on January 04, 2025, 11:44:39 pmYes.