DSO vs Analogue scope -- what am I doing wrong here?  - Page 1
Forum section: electronics / testgear

author: torch
author_rank: Frequent Contributor
timestamp: January 04, 2016, 03:50:42 am
content: I recently acquired a second scope to compliment my venerable Rigol DS1052E (now DS1102EDU): a 40+ year old Tektronix 475. After some minor repairs, cleaning, and calibrating, I think it's working pretty good. This is a screen shot of the signal from a 10MHz OCXO (mixing A and B triggering):Recently I was tinkering with a circuit and wanted to check Vcc for ripple. This is what the Rigol showed initially:But it was pretty jittery. In an attempt to stabilize the displayed waveform, I started adjusting the trigger level, causing this inverted spike to suddenly appear (voltage reduced so it will fit on the screen):So I decided to look at it with the Tek. Here it is at the full 200MHz bandwidth (time base and voltage the same as the first Rigol screenshot):and with the bandwidth limited to the Rigol's 100MHz (time and voltage unchanged):But despite any and all trigger level adjustments, A and/or B, in either direction, positive- or negative-going, hold off, AC, DC, Line, HF reject, etc. etc. etc., I can't find that spike. I even tried swapping the probes between the scopes but the spike stays with the Rigol.I thought it might be some weird aliasing, but selecting Peak Detection on the Rigol makes no difference.Any ideas what might be going on here?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 04, 2016, 03:56:37 am
content: I'm wondering if the Tek is just missing that spike somehow. Can you do the Rigol shot again but at a slower timebase, so we can see several instances of the spike on the screen?ETA: and move the horizontal trigger position over to the left so there's more room for a train of spikes and whatever is between them.

author: Ian.M
author_rank: Super Contributor
timestamp: January 04, 2016, 04:29:59 am
content: Its very difficult to view a narrow spike with a low repetition rate on an analog scope. If the timebase is fast enough to resolve the spike, the amount of energy dumped into the phosphor by one pass of the beam is extremely small so the trace is very faint. A darkened room will help, but it may simply be beyond the capabilities of your scope. It should be possible to detect it by the spike triggering the timebase causing the triggered indicator light to flicker even if the spike cannot be resolved, and with care and experience its magnitude can be estimated from the trigger level setting.

author: w2aew
author_rank: Super Contributor
timestamp: January 04, 2016, 04:34:00 am
content: Quote from: Ian.M on January 04, 2016, 04:29:59 amAlso make sure the scope is on Normal Trigger (not Auto), adjust the trig level down until the normal wiggly trace that you're seeing goes away, turn up the brightness and watch with a hood or in a darkened room for the infrequent event. Even if you don't see it, a flash on the TRIGGERED LED may tell you that something happened...

author: ADT123
author_rank: Regular Contributor
timestamp: January 04, 2016, 07:54:28 am
content: Quote from: Ian.M on January 04, 2016, 04:29:59 amI think that sums up really well why however much we like them, the analogue scope has had its day and is no match for a DSO. Most circuits we measure have some digital content so intermittent events are more likely than in days gone by and are more likely to cause problems.Moderns digital scopes have deep memory, persistence displays, fast waveform updates (as well as being digital so able to capture one off events) and a whole host of features such as serial decoding, FFTs etc.Its great that so many people restore and love these instruments but the argument for needing / using an analogue scope for real work gets weaker as time goes on, they do however on these cold winter mornings have one benefit - they give off lots of heat so do help heat the lab.(If I said the above even a couple of years ago I would be shot down in flames - interesting to see if that's still true).

author: miguelvp
author_rank: Super Contributor
timestamp: January 04, 2016, 08:12:57 am
content: QuoteOnly expensive DSOs have that.My Rigol DS2000 has a spec of 50,000 wfrms/s, I can get up to 48,000 wfrms/s, If and only If I let the automatic memory depth to be 700 points, so to achieve 48,000 wfrms/s you have to be looking at a 5ns per division and the voltage and other things seem to matter as well.If I manually select the minimal selection of 14K points I get 5,000 wfrms/s, and if I want anything with deeper memory then forget about it.https://www.eevblog.com/forum/testgear/first-personal-'scope-purchase/msg681691/#msg681691But as I state on that post, if you know what you are looking for you can set the right trigger and catch what you are looking for. But the problem is, that you have to know what you are looking for.What DSO that has fast waveforms are you talking about, maybe those are actually faster than my Rigol (which would not surprise me).Edit: my analog tek 7613 with a 7A26 dual trace plug-in and a 7B53A time base module could achieve 609,190 wfrms/s.

author: ADT123
author_rank: Regular Contributor
timestamp: January 04, 2016, 11:31:47 am
content: Quote from: miguelvp on January 04, 2016, 08:12:57 amSome analogue scopes could update as fast as 1 million waveforms per second (http://materias.fi.uba.ar/6644/info/osciloscopios/basico/intro%20to%20analog%20scopes.pdf) whilst others were limited to the 50/60Hz mains cycle.I guess its the same with digital scopes - some of the earlier deep memory (1Meg or more) scopes took seconds per screen update so you had to turn the memory down to get a responsive display but over time this has greatly improved. I would regard the 50,000 waverforms / sec on your Rigol to be fast for a low cost scope. Some of the more expensive digital scopes (Tek, R&S etc) can do 1 million per second so match the fastest analogue scopes in terms of headline specs.The headline spec is of course not the whole story and risks getting off topic. All scopes will only get fast waveform update rates when collecting for short periods of time (100us/div timebase gives a theoretical max of 1000 waveforms / second) but assuming you have a fast timebase you then have to consider the input signal frequency (waiting for the trigger to occur), the trigger re-arm time, sampling rate, memory depth and the speed the hardware can process the memory. Good luck to anyone who wants to try to compare lots of different scopes in real world settings!

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: January 04, 2016, 12:31:31 pm
content: Quote from: ADT123 on January 04, 2016, 11:31:47 amActually, the waveform rate of most of the faster analog scopes was much closer to 600k waveforms/s (mostly due to their trigger circuits). I'm not aware of any analog scope which actually reached 1M waveforms/s or anything close to that.BTW, that Iwatsu scope (LeCroy just rebadged them) you linked above isn't even a true analog scope, it's actually a digital scope which uses a scan converter tube instead of an ADC chipset.QuoteIt depends. Some of the old Teks were pretty slow (as were early B-brand scopes) but even most very early DSOs could update their screen several times per second.However, what people often forget is that to fill a certain size of sample memory at a certain sample rate takes a certain amount of time - and that time is independent of the waveform update rate (which specifies how often a scope can re-acquire a waveform per second, i.e. it's only on analog scopes where the waveform update rate is the same the screen update rate, this is not the case for DSOs!) or any other scope property aside from the sample rate. For example, to fill 1Mpts of sample memory at 1GSa/s takes the same time (1ms) on say a 18yr old scope as it does on a modern day fast one, and that means on both scopes the display will not be updated any faster than 1ms (actually, since the update rate of most DSO displays is 60Hz it will never be updated faster than every ~16ms anyways, no matter what). Many modern scopes come with large sample memories (i.e. over >10Mpts), and especially on low end scopes which have a low sample rate it can take a considerable amount of time to fill the large memory, which to the user makes the scope look and feel slow.QuoteI agree with that 50k wfms/s is perfectly fine for a bottom-of-the-barrel scope. There are not many scopes that reach 1M wfms/s (there's the Keysight DSOX3k/4k, the LeCroy WaveRunner Xi/Xi-A,6zi and the R&S RTO; Tek doesn't even have anything anything that's even close to that as far as I know), and the majority of more expensive scopes are way below 1M waveforms/s (or even the 600k waveforms/s you can find on analog scopes) because the amounts of data to process is substantially higher than for a low end scopes which offer pretty low sample rates (<5GSa/s). Plus outside the low-end/entry-level update rates are less of an issue because these scopes tend to offer other, more efficient ways to capture glitches than to wait until they appear on the screen.QuoteThat's certainly true.

author: dom0
author_rank: Super Contributor
timestamp: January 04, 2016, 12:45:43 pm
content: What does the Rigols trigger frequency counter say about the repetition rate of the spike?

author: torch
author_rank: Frequent Contributor
timestamp: January 04, 2016, 01:29:26 pm
content: Quote from: alsetalokin4017 on January 04, 2016, 03:56:37 amQuote from: Ian.MOk, I think the two of you are on to something. Yes, the trigger light flashes and yes, turning up the intensity to max in a darkened room makes something visible. And the pulses seem to be very intermittent and far apart. First the Rigol on a slow time base - note that it took a couple of tries to catch two pulses on the same screen and if I slow down the time base any further, they get so narrow the scope stops catching them:Now a couple of shots from the 475 in a darkened room. I'm guessing the first is more distinct because it repeated enough to charge the phosphor more?:And while it does not give a nice neat readout of the trigger level, the 475 does offer a view of the triggering signal at the push of a button. (I imagine one could calculate the trigger voltage from that, but I haven't got that far in the manual yet)Now, for the sake of comparison, when we were all experimenting with the Rigol hack, I build a Jim Williams style fast-pulse generator. Here is the output on the Rigol:And using the same settings, cable and 50 ohm terminator, on the Tek 475. I didn't even need to darken the room:

author: idpromnut
author_rank: Unknown
timestamp: January 04, 2016, 02:58:45 pm
content: Quote from: torch on January 04, 2016, 01:29:26 pm(SNIP)This should be obvious, but the pulse generator will output a repetitive signal which the analog scope can more easily trigger on. So it is not the pulse per-se that the Tek is having trouble with in your original tests, but rather the erratic occurrence of it.

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2016, 03:14:00 pm
content: Quote from: idpromnut on January 04, 2016, 02:58:45 pmEr, no.An analogue scope will trigger as well or as badly as a digital scope, whether it is erratic or completely regular. In this case I would expect to use DC trigger level and normal triggering; using AC trigger level and/or auto triggering will give "suboptimal" results.A brighter trace almost certainly means merely that there are more trigger events/second.With an analogue scope the key issue is the number of triggers per second (more=>brighter) and sweep speed (slower=>brighter). X-Y pen plotters excluded - and yes, with imagination and understanding it is possible to use such a plotter to record this signal. (I first witnessed it 40 years ago)

author: EV
author_rank: Frequent Contributor
timestamp: January 04, 2016, 03:31:29 pm
content: Quote from: torch on January 04, 2016, 01:29:26 pmThere is some matching problem because the pulse is ringing so much.

author: torch
author_rank: Frequent Contributor
timestamp: January 04, 2016, 03:52:05 pm
content: Quote from: EV on January 04, 2016, 03:31:29 pmHey, I never claimed to be anygoodat building it!Some of the issue is likely the termination. The 475 does not have an internal 50 ohm termination and I don't have an in-line terminator, so I used a BNC tee and end terminator. I realize this is not ideal, but in this case my idea was to look at how the 475 dealt with a fast pulse more than to ensure it saw a perfect pulse, so the ringing is probably a bonus.I think I have tracked down the source of the pulse. This is powering a 556 timer driving 2 pairs of alternating LEDs at two different flash rates. Each time one of the 556 sides changes state (ie: drives a new diode) I get the momentary pulse on the power supply. As the two rates differ, together they produce an interference pattern and so the pulse appears pretty erratic.And boy, isthatever easier to discern on the DSO. The flashing chopped pulse trains on the analogue Tek at such slow time bases are enough to trigger a migraine or epileptic seizure!

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2016, 04:17:40 pm
content: Quote from: torch on January 04, 2016, 03:52:05 pmSuch things are indeed much easier to diagnose with a storage scope, either an analogue storage scope or a digital storage scope.But analogue storage scopes always were very expensive, and have been completely replaced by digital storage scopes. I presume, but I wonder if there are still a few places where they remain the best, just as in the valves-vs-transistors battle.

author: dom0
author_rank: Super Contributor
timestamp: January 04, 2016, 04:22:10 pm
content: Analog storage scopes do a better job at noise filtering than the cheapo chinese DSOs with their super-slow averaging filters. A proper DSO excels at that of course.

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2016, 04:25:49 pm
content: Quote from: dom0 on January 04, 2016, 04:22:10 pmIs that relevant for a single-shot event? You can't average across multiple events, and who cares whether it takes 1ms or 100ms to be displayed.

author: dom0
author_rank: Super Contributor
timestamp: January 04, 2016, 04:30:34 pm
content: Quote

author: torch
author_rank: Frequent Contributor
timestamp: January 04, 2016, 05:55:09 pm
content: Quote from: torch on January 04, 2016, 03:52:05 pmFor those that are curious, here's a short video clip comparing the two displays:http://picturehosting.verhey.org/tektronix/waveforms/rigolvstek_conv.html