Oscilloscope Confusion - Page 1
Forum section: electronics / testgear

author: FlyMario
author_rank: Contributor
timestamp: August 23, 2018, 02:29:33 pm
content: Hi, I have a Siglent SDS1052DL+ scope and really like it. But something about it puzzles me.I always expect that my probe line marker to be 0 volts. However, that is not the case. Perhaps I am just being a noob with my scope. I normally have used scopes for digital signals really and I am working on an analog project.I have been working on a Mechanical TV for a while now and have made a circuit to deal with the NBTV signal. I know where 0 volts should be... right before the negative voltage sync pulse.I have a picture I attached. Where I have the arrow should be around 0 volts. But you will notice it is nowhere near my 1 marker. I do nothing special with my scope. All I really care about is placing my signal where I want and set the frequency.Do any of you have any suggestions?Thanks,Pete

author: luismh
author_rank: Newbie
timestamp: August 23, 2018, 02:38:57 pm
content: Hi Pete,Try switching to "AC" coupling.Regards. Luis

author: rsjsouza
author_rank: Super Contributor
timestamp: August 23, 2018, 02:40:56 pm
content: Perhaps the channel is on DC coupling and there is a DC bias?(edit) Luis beat me to it!

author: FlyMario
author_rank: Contributor
timestamp: August 23, 2018, 02:48:11 pm
content: oh. Wow. I didn't even know my scope had that kind of a switch. I figured it automatically selected ac/dc. I remember old scopes having that.I will have to check that out.THanks!

author: FlyMario
author_rank: Contributor
timestamp: August 23, 2018, 02:54:44 pm
content: Well well... it is on my menu. Coupling. It is set to DC. LOLThanks folks!

author: StillTrying
author_rank: Super Contributor
timestamp: August 23, 2018, 06:13:33 pm
content: DC/AC coupling is always shown at the bottom left. CH1=500mV or CH1~500mV.Quote from: FlyMario on August 23, 2018, 02:29:33 pmIf shouldn't be off by 350mVHave you done the Self-Cal in Utilities.The rest of the thread - switching to AC coupling because you don't like where the 0V indicator is on a video signal makes no sense.

author: FlyMario
author_rank: Contributor
timestamp: August 23, 2018, 06:26:44 pm
content: Well, the signal is AC. Anything below 0v is a sync pulse for NBTV anything above 0v is video. My only issue was that 0 volts were not sitting on the probe marker.Sorry if I am confusing. I am not as skilled with electronics as many of you folks are I am sure.Pete

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 07:46:35 pm
content: Another little trap for the scope novice is Invert.

author: xavier60
author_rank: Super Contributor
timestamp: November 23, 2018, 12:59:36 am
content: My current DSO is a 20 year old HP 54645A. It has a menu setting that allows a choice of Left, Center or Right for the Trigger zero reference position. Most of the time it is set to the Left because I'm only interested in seeing waveform after the trigger event.The Trigger zero reference position stays put regardless of change in Time base setting.Does the SDS1052DL have the same capability?I have not been able to find a definite answer in the operator's manual.

author: tautech
author_rank: Super Contributor
timestamp: November 23, 2018, 01:11:59 am
content: Quote from: xavier60 on November 23, 2018, 12:59:36 amNo it doesn't. Trigger can be set to any horizontal position on the display but only with the Hor Pos control.So if it's not set to 0.00s timebase adjustments can shift it off screen, however with just a press of that control it's bought back to 0.00s again.Later X-E models have the ability to set trigger positions where you want them and they stay there.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 23, 2018, 06:17:39 am
content: Quote from: FlyMario on August 23, 2018, 06:26:44 pmPossibly your video source does not present blanking or black level as zero volts.(NTSC has "black" as zero volts, PAL has blanking level, hence the use of "or" above)In Broadcast analog TV, loss of the DC reference level was a common problem.(AC coupling in amplifiers, etc, tries to balance the signal around zero volts.)To get around this, "DC restoration" of various types was used, principally, the "Keyed Clamp".This took the line sync pulse, used it to create a narrower delayed pulse which was centred on the "back porch"(the bit of black or blanking level just after the sync pulse).*This pulse would "turn on" a clamp circuit which would conduct, pulling the level desired to be clamped to zero volts.*Usually-----sometimessync tiplevel was clamped to zero or even other voltages for special cases.Classic circuits were combination of resistors capacitors & diodes but later designs used FETs or even analog switch ICs.The capacitor was vital in all such circuits, as it would maintain the clamped voltage level, until the next clamp pulse came along around 64 microseconds later.Such circuits were ubiquitous in analog TV.