Noob with a scope: Rigol DS1102CD & digital waveforms - Page 1
Forum section: electronics / beginners

author: casper.bang
author_rank: Frequent Contributor
timestamp: March 29, 2013, 04:44:01 pm
content: Hey there,I borrowed a small Rigol scope from work so I could try and learn a bit about these during Easter. The scope is the Rigol DS1102CD which is 6-7 years old and appears to be the generation before the popular hobby scope DS1052 line. It claims to do 100MHz with 400Ms/s (2½ times lower than what Dave recommends?).Anyway; even if these scopes only have 8 bit of resolution (within an arbitrary range I assume?) and the screen is limited to 234 vertical pixels, I still find a digital signal to be quite noisy when I hook up a 5V pl2303 chipset-based rs232 interface (see 0x55_0x00_0xff@8N1_pl2303@5V_#1.bwingmp) even when I zoom in (0x55_0x00_0xff@8N1_pl2303@5V_#2.bmp) showing undershoot, overshoot and a whopping 800mv ripple when the. signal goes high.Now, it obviously comes down to the quality of the signal too, because if I hook up a 3.3V ftdi chipset-based rs232 interface (0x55_0x00_0xff@8N1_ftdi@3.3V_#1.bmp and 0x55_0x00_0xff@8N1_ftdi@3.3V_#2.bmp) the signal looks somewhat cleaner but zooming in also shows overshoot and undershoot.What do you guys thing; do I simply have a bad China pl2303 adaptor on my hands here? Would it typically help with a load on (1K resistor)? Are there filtering/tricks that can be particular handy when wanting to see digital waveforms as clear as possible on a small Rigol?Thanks,/CasperPS: Damn those fans Rigol puts in these small scopes are loud, it deters me somewhat from wanting to to buy a DS1052!

author: Christopher
author_rank: Frequent Contributor
timestamp: March 29, 2013, 04:48:36 pm
content: Signal in the last pic seems fine. Does the scope have a square wave test point?

author: casper.bang
author_rank: Frequent Contributor
timestamp: March 29, 2013, 05:00:18 pm
content: Sure; I forgot about the test signal. I have attached #1 zoomed out (150mV ripple?!) and #2 zoomed in (60mV ripple). I guess the scope checks out then, if this behavior is within the norm of a low budget scope.Bummer, I find Linux kernel support for the pl2303 chip is more widespread than ftdi so had hoped to be using the pl2303, but it looks inferior when it comes to signal quality.

author: hammil
author_rank: Regular Contributor
timestamp: March 29, 2013, 05:12:54 pm
content: You only need an 8-bit ADC because the screen is under 256 pixels high. The analog frontend will do the job of attenuating and clamping the input signal to keep within the limits of the ADC. The range is determined by the vertical axis controlsTo test the noise floor of the scope, simply short the input pin to the connector with a small piece of wire. It should be well within the lowest vertical division. On my 1052 it's about 1mv peak to peak. Connect a battery to it and test again if you want to confirm it works with higher voltage inputs. Once you've done that, any other noise sources can be attributed to your probes and cables, or the DUT itself.One thing to remember: cheap power supplies can and will inject common-mode noise into the scope (I've seen 1000mv before). Disconnect stuff like laptop power supplies before doing this kind of test.If you have a noisy signal and there's nothing you can do about it, try turning on averages (Acquire menu) or limiting the bandwidth (Channel menu).

author: Christopher
author_rank: Frequent Contributor
timestamp: March 29, 2013, 05:17:09 pm
content: Signal quality of digital signals doesn't really matter that much, as long as it's between 2-5V then it's a TTL high. Obviously in an ideal world we would want it to be 5v all the time, but +-100mV really doesn't matter. The test signal on the scope is just there for setting the compensation of the scope so doesn't need to be all that accurate.If it bothers you that much pop a couple ceramic caps on the power input to the prolific board, 0,1u and a 10u or 100u in parallel, but really that pl board seems fine to me.

author: casper.bang
author_rank: Frequent Contributor
timestamp: March 29, 2013, 05:33:29 pm
content: I've attached a screendump of a shorted BNC with AC coupling showing a little bit of noise, as well as a more noisy dump of the scope measuring a 1.5V battery in DC coupling with probes. I guess I just find it odd to have so much noise, but it calms me to hear the Rigol 1052 has less!

author: casper.bang
author_rank: Frequent Contributor
timestamp: March 29, 2013, 08:40:06 pm
content: Quote from: hammil on March 29, 2013, 05:12:54 pmNot able to find any bandwidth limiting options, but changing memory depth from Long (50kSa) to Normal (100kSa) got rid of most of the overshooting and undershooting. Oddly enough I am not able to use the very last menu item "Sa Rate".

author: vk6zgo
author_rank: Super Contributor
timestamp: March 30, 2013, 04:52:22 am
content: I'm wondering if you are seeing 100/120 Hz hum,& the 'scope is interpreting it as a thickening of the top of the signal.An analog 'scope will do something similar.If so,it looks like the regulation of your supply is a bit suspect,as there is no thickening of the bottom part of the signal trace.

author: hammil
author_rank: Regular Contributor
timestamp: March 30, 2013, 09:30:28 am
content: Yeah, bad regulation seems pretty likely here.That's... a /lot/ of noise on what should be pure DC waveforms. Have you thoroughly checked your lab for common-mode noise sources? What about possible sources of radiated noise, like really high-power electrical stuff or transmitters of some kind?Either that, or the scope may be a little dicky. It is several years old, after all.

author: casper.bang
author_rank: Frequent Contributor
timestamp: March 30, 2013, 10:41:09 am
content: Quote from: vk6zgo on March 30, 2013, 04:52:22 amThe high signal stays bad quality, even if I remove all switching supplies in the vicinity and measure the signal from a laptop running off a battery.Quote from: hammil on March 30, 2013, 09:30:28 amI am pretty sure I did hear one of the engineers talk about him not liking this Rigol because of its noise. However, having no prior experience with a DSO and no other scope to compare with, I had to resort to you guys.So the conclusion must be that my pl2303 adapter is quite noisy and the scope is as well, making for some pretty ugly high signals. Happy to note though, that this is not normal on a Rigol 1052! Thanks for the feedback guys; I learned a lot.