Basic lab equipment - Page 1
Forum section: electronics / general

author: Christopher
author_rank: Frequent Contributor
timestamp: April 12, 2012, 08:07:02 am
content: Multi: Already got an Extech ex330, going to be getting another £50 china one (UT71E or something) soon.Scope: Either RIGOL DS1052E (£250) with an upgrade to 100MHz, or the OWON 25MHz PDS5022S (£180) 25MHz but it looks easier to use and has a massive screen. Which would you suggest? I think 25Mhz should be more than enough, and I don't really want a CRT scope.Also function gens etc, I would like a basic one but don't want to spend LOADS, and I struggle to see how it can be used, I could probably make up a basic one?

author: amspire
author_rank: Super Contributor
timestamp: April 12, 2012, 08:19:20 am
content: Definitely not the PDS5022S. It only has 100mbit/sec sampling which means it is more of a 10MHz oscilloscope then a 25MHz oscilloscope.Also it only has a memory of 5000 samples instead of 1,000,000 in the Rigol long memory. A big memory means you can capture a waveform and then zoom in looking for a problem. The Rigol is a basic screen, but it is good enough, and the 10x faster sampling speed and 200x more sample memory more then offsets the extra cost.Richard.

author: rf-loop
author_rank: Super Contributor
timestamp: April 12, 2012, 10:40:41 am
content: Quote from: amspire on April 12, 2012, 08:19:20 amIf we look what Tektronix or Agilent talk 100MSa/s is well enough for 20-25MHz or even more. Oscilloscopes frequency range mean always sinewave bandwidth. And of course, all what ever waveforms are just sinewaves. There is not any other. If you look square or what ever, it is just couple of sinewaves. If oscilloscope BW is 25MHz it do not attenuate more than 3dB this frequency component in waveform under test.Hewlett Packard 100MHz oscilloscope HP54600B have 20MSa/s samplerate.Samplerate do NOT reduce Owon PDS5022 to under 25MHz oscilloscope. If someone think so it is better to learn more. Whole this samplerate and frequancy BW have so big hassle in allwhere in talking and lot of misunderstanding.I can tell it can not well 1kHz square wave if want look 1ns risetime edges. So can I now tell that it is not even 1kHz oscilloscope. No it is still 25MHz oscilloscope becouse this 25MHz means sine.If I take Tektronix analog 2465B 400MHz oscilloscope. It is just 400MHz sinevawe oskilloscoe and not just any more.It can capture signals what have up to 25MHz frequency components and even more becouse it do not have brick wall BW shape.Of course if look example square wave. 25MHz square wave looks very distorted sinewave. Of course becouse scope can show only mother frequency but not any harmonics what it need also show. If there is 25MHz square with around 2ns risetimes it need roughly 200MHz scope to show it even nearly right. And this need analog BW 200MHz and it need absolute minimum 400MSa/s. If analog BW or samplerate is not enough it show square but ristetimes are wrong.In Rigol if it is modified to 100MHz it have around 3-3.5ns risetime and what ever is samplerate it do not show more details.Owon PDS5022x have 14ns risetime and samplerate is not any limiting factor for this 25MHz frequency band.But example old HP 54600B 100MHz oscilloscope... it have enough analog bandwidth BUT samplerate is limiting factor and it drops this 100MHz repetitive oscilloscope to maximum 4-5MHz single shot oscilloscope. (4-5MHz real time)This Owon is still 25MHz real time scope.There is not any need sample more than 3 - 5 sample per cycle as long time if we use good sin x/x and as long as we talk sinewave response. And all oscilloscopes what I know with nearly 40years experience have specified sine BW.There are lot of basic oscilloscope fundamentals availabe just after some keyword and mouse click so I do not link these here.About this Owon.It have one poor thing. It is old! PDS5022S have old piece of crap DSTN display!Becouse this DSTN display, think twice before buy.Owon have made PDS5022T and it have real good 8" TFT.If you have posible to first look and try somewhere with own eyes this PDS5022S (dstn) model and if you still can accept this display ... well ... it ok entry level oscilloscope if frequency is not limiting.(but becouse this is DSTN display and manufacturer have done time ago same model with TFT, I think you can ask markable amount discount!)btw, it have not channel shared memory, both channels have individual 5k (in TFT model)I do not know how this scope sampling is arranged, do they have simultaneus sampling or do they have interleaving? (my quess is it have 2 x 100MSa/s simultaneous - just quess and I hope someone tell whoknowit.)But ower the world this Canadian - Chinese oscilloscope l is lot of used, specially in many many schools.

author: T4P
author_rank: Super Contributor
timestamp: April 12, 2012, 10:56:40 am
content: Didn't dave talk about the nyquist-shannon sampling theory before ?A 100MSa scope is basically a 10MHz GENERAL wave scope . Nothing more .Those low MSa scopes of the past are thepastthey were low back when digital scopes weren't good at all .

author: Christopher
author_rank: Frequent Contributor
timestamp: April 12, 2012, 11:18:20 am
content: I'm only a beginner, not going to be doing any highspeed stuff (Just pissing about mainly), so I need a cheap scope that's worth the money.

author: nctnico
author_rank: Super Contributor
timestamp: April 12, 2012, 11:48:25 am
content: Quote from: Dave.S on April 12, 2012, 10:56:40 amSorry, but this is just plain wrong. It is perfectly possible to use low sampling rates in order to capture high frequency signals. This is called equivalent time sampling. Older scopes that can show signals up to many GHz have sampling frequencies around 200kHz! What these oscilloscopes do is making sure the sample is taken at a certain time from the trigger event. Tektronix wrote an interesting article on this technique:http://www.cbtricks.com/miscellaneous/tech_publications/scope/sampling.pdfThe disadvantage of equivalent time sampling is that the signal may not change (much) so its only usefull for repetitive signals. It is better to have an oscilloscope where the samplerate is at least 3 times the bandwidth so you can take single shots from any signal.Regarding screen sizes: I wouldn't buy a scope with a 320x240 screen. The resolution is way too low. 640x480 is the minimum.

author: T4P
author_rank: Super Contributor
timestamp: April 12, 2012, 01:35:21 pm
content: That's why RTS is important , do you debug a faulty circuit that only produces repititive signals ?

author: amspire
author_rank: Super Contributor
timestamp: April 12, 2012, 01:47:20 pm
content: Quote from: nctnico on April 12, 2012, 11:48:25 amIt is low but totally useable. I would go for a 320x240 screen on a 1GHz sampling speed scope over a 640x480 screen on a 100Mhz scope any day. If you actually use a Rigol DS1052E, it is perfectly fine.Using the scope in sampling mode is OK for repetitive analog signals, but now we have so much digital stuff, the real time sampling is just worth much more then any pretty screen. Digital is often not repetitive so you need single shot, and often a big sampling memory.If you cannot afford a 1GHz sampling scope, get a $50 second hand analog scope for now which is better for most repetitive waveforms then a digital scope, and get a good digital scope when you can afford it.Richard.

author: Unknown
author_rank: Unknown
timestamp: April 12, 2012, 04:27:47 pm
content: Quote from: rf-loop on April 12, 2012, 10:40:41 amDid you also consider the front-end roll-off and interpolation? sample rate / 2 should be enough with a perfect brick wall filter and perfect sinc(x) reconstruction. In the real world, both filters and interpolation are finite, so extra margin is required for proper reconstruction. Knowing Owon's firmware skills, the interpolation is probably completely useless, and if the front-end response of the SDS series matches their design practices, than the front-end roll-off is probably also very shallow.Quote from: rf-loop on April 12, 2012, 10:40:41 amBut they don't specify the single shot bandwidth as 100 MHz, I'm sure. Single shot bandwidth is the important figure in many applications, unless your signal is perfectly repetitive. This generation of scopes gave digital scopes a bad name. People kept an analog scope on hand to check if the digital scope was 'lying', ie. aliasing, again.Quote from: rf-loop on April 12, 2012, 10:40:41 amThis makes the reconstruction harder / less accurate and has the potential to introduce aliasing.Quote from: rf-loop on April 12, 2012, 10:40:41 amAssuming there are no frequency components beyond the Nyquist frequency, which may not be the case with real-world signals if the roll-off is shallow.I seem to remember you arguing for acquisition quality over usability regarding the SDS series vs. Rigol/Hantek. Would you agree about the Rigol DS1052E being a superior choice in this case for the same reason?

author: Christopher
author_rank: Frequent Contributor
timestamp: April 12, 2012, 04:33:57 pm
content: I've also found this:http://www.dealextreme.com/p/utd2102cel-7-0-lcd-100mhz-digital-oscilloscope-115119Best of both, large screen and 100MHz. I'll probably go with the Rigol or get a cheapo analog scope, BUT this is very hard.

author: Unknown
author_rank: Unknown
timestamp: April 12, 2012, 04:41:44 pm
content: There have been some negative posts about Uni-T scopes on this forum, I don't remember the exact problems. Should be easy to find them with a Google search.

author: T4P
author_rank: Super Contributor
timestamp: April 12, 2012, 05:37:15 pm
content: Quote from: Christopher on April 12, 2012, 04:33:57 pmShort sampling memory and it costs about a Owon SDS7102T without the VGA option .Why i mention this ? 8" 800x480 and 10mpts .