Question: It seems that most portable DSOs in the 200MHz/1GSPS class don't offer delayed trigger anymore which makes pulse-echo measurements a real pain. Why don't they?
After all it's just a simple counter that is needed. Same for hold-off which typically ain't there either :-(
Beats me but IIRC I used delayed trigger on a lower end TDS220 although its data sheet said that it could not do it. However, that scope had some other serious issues (noise) so I asked the client to obtain a used
2265. Old Faithful, of course, always has delayed trigger but sure enough the plastic clutch for that function fell apart right away. Designing a trigger delay into an analog scope is no small feat but for a DSO it should be really easy.
Is this a "when are you going to stop beating your mother" sort of question? Check out the Agilent 5000 series of scopes, listed as "new" on the Agilent web site today. Trigger delay range: pre- trigger, the greater of 1 screen or 125usec; post-trigger 1 second range minimum, up to 500 seconds for slower timebase settings.
On bottom-end scopes, especially ones with deep memory, trigger delay may well be dropped for a combination of reasons: the desire to differentiate the product line, and the lower need given lots of memory--acquire once, inspect in detail wherever you want. One of my DSOs doesn't have specific trigger delay, but does let you set up the display of any portion of the memory, as a "delayed trace".
Ok, but now we are talking $5k class. I meant the lower end, around $2K such as the TDS2024.
Memory is a problem on some. The Tek mid-range offers 2.5K, Agilent mid-range is 4K and mine (Instek) is 25K. This is only for single channel use. So a pulse-echo measurement at 1nsec slivers becomes impossible for anything above a few ten usec.
Leaving it out for differentiation reasons would be rather foolish because it causes (and has caused in my past) potential users to be hesitant and stick to their old analog scopes. Those offer delayed trigger in rather basic models which can and are bought via EBay. There has rarely been a client of mine where I didn't ask them to bid on a
2465, and none has ever regretted it. On the contrary, sometimes we discovered that we needed more of these "real" scopes because everyone suddenly wanted to have it.
Is it that it isn't there, or is no longer an explicit feature - you just scroll the display..?. I find it hard to believe that any digital scope doesn't have holdoff.... can you name specific models?
In my case it's the Instek GDS-2204 but when looking at competitor models such as the Tektronix TDS-2024 it seems that it is a similar situation there. John Larkin would know for sure because he's got one.
No hold-off. Here is the real puzzler: I've got the 4-ch model and the less expensive 2-ch model does contain delayed trigger (!). Snooping around in the firmware I saw that it's still in the control set but somehow not engaged, IOW I can access delayed trigger but not tie it into any of the channels which renders it useless. Plus it isn't available via the scope's buttons, only if I enter via the doggie door with HyperTerminal Wrote to Instek about it but no answer yet. Hopefully they'll realize that disconnecting delayed trigger on their most expensive flagship was not such a great idea, and maybe they'll fix that in the firmware.
Scrolling the display gets you a few usec on Tek, and a few tens of usec on the Instek. But even the 25K on mine will eventually run out, precisely at 25usec when using the highest sample rate. Ok, on the Hameg
2008 with its Megabyte buffer I could understand this strategy but not on the others. Even 25usec is nothing on pulse-echo jobs.
I do have to say that the advanced trigger features of these scopes are really great. For example, this Instek can trigger on stuff like "only when pulse exceeds x usec" and this feature goes to 10 seconds. But of course then it waits for the slope so I can't hitch a delay-trigger ride here. So they've got a long counter in there already and implementing a delay feature should almost be a no-brainer.
Maybe you need a firmware update. Our TDS-2024B has holdoff. What it doesn't have is A and B horizontal sweeping, with an adjustable time delay before the B trigger is armed, like our older TDS-3024B and TDS-3034B scopes. It's too bad, because the new TDS-2024B is very sexy with its two USB plus ethernet ports, etc. The older TDS-3034B does have the ethernet port.
So the 2024B still doesn't have a delayed trigger then? I've got the Instek GDS-2204, not the Tek, because I need the 25K of memory. Even better would have been the analog/digital Hameg 2008 comboscope but it's too large for carry-on if you have to fit some clothes in for overnight stays, plus I am not sure how that classic CRT in there would fare during really bad turbulence.
I am quite sure Instek will do firmware updates in the future and hope they'll throw in a delayed trigger. It's just that I don't get it why they took it out. Just like I don't understand why some CAD systems fail to provide hierarchical sheet structures. Well, at least I brought it to their attention that their marketing brochure promised delayed trigger and the product then didn't have it. It can't be rocket science to put it back in since the next lower model has it.
Other than that it's a nice scope, especially regarding USB connectivity. One thing I want to try out when I find the time is operating it out of Excel via VBA since they published the complete command set for it. That would be cool. Their custom SW is ok for catching CSV files and the like but otherwise it is very spartan. Freezes a lot, too, but then again it is a beta version.
The usual reason is because the people implementing the tools don't actually end up *using* the tools combined with management that doesn't have a strong technical background. Hence, the only feedback path of what features are desired is customers and the competitions!
Yep, and I did let off a couple remarks to that effect, hoping that this might "accelerate" the firmware upgrade. If they are smart they'll turn around and use people like me as testers. That's what we do in medical devices all the time but somehow that concept catches on really slowly in electronics. It's taken over half a century by now ...
But, if it's a digital storage scope, why can't you just trigger it at the beginning of the test and "zoom in" to the part you're interested in seeing delayed?
Because the Instek runs out of memory after 25usec. Similarly priced HPs run out in under 5usec and the Teks run out even earlier :-(
The 2.5K of memory on a Tek-2000 series is IMHO really pathetic. I wanted to buy one but when I saw that I didn't. If you do pulse-echo stuff you can't use that.
I know, but then you are talking $5k and up. I mean, heck, even the old Tek 2465 has delayed trigger. Bidding ist pretty active on EBay and Tek is cutting into their own sales by not providing delayed trigger in "modern" scopes. Because people like me tell clients to bid on a 2465. And then they do. From a marketing point of view it makes absolutely no sense.
Le Wed, 15 Aug 2007 07:13:09 +0000, Joerg a écrit:
Right now, I have to do some really accurate compensation to a system's step response. From the 10ns scale, down to the 100ms scale. What would really be handy is a logarithmic sweep. That could easily be implemented with say a sampling rate scaling down by a two factor while time rises by a two factor. Well, with GPIB and some post-processing it works, but it's much longer and a bit... bulky.
Maybe the real question to ask: when is someone (almost certainly a new party) going to come out with a customizeable DSO "kit".
Basically, I'm thinking of something that is workable with the shipping "reference design" but gives you / "the community" lots of options to rewrite the firmware, FPGA code, etc. It's possible that this would limit performance (1 GSPS into an FPGA requires quite a bit of care), but there'd probably still be a market for a fully customizeable fractional-GSPS instrument.
That's how it is with the GDS-2202, the less expensive 2-ch sibling of mine. Beats me why they did that but I could live with external trigger in most cases. However, what really puzzles me is that they completely took it away for the 4-ch version.
The newer scopes such as this Instek allow data dumps via USB into the PC at an amazing clip. Not realtime but close. When I want to do something unusual with the data I use the CSV import into Excel. However, I am not an expert (yet) in VBA so I can't do any processing on-the-fly. But I'll learn it. Was hoping it was C and not Basic but, oh well, I guess that was Bill Gates' decision.
That's what I was thinking when the GDS-2204 arrived and I took it for a spin. Endless possibilities. But while Instek is quite open in accessing the data and the control codes there is no service manual with schematics and no firmware access. I bet that what you described could already be done with many new DSO, it's just that the manufacturers don't want us to.
They must have some blazing processor in there, the speed of the FFT blew me away. You can experiment with, say, a snubber network while watching the spectrum change. However, with this scope you have to get used to the bonbon-colored traces ;-)
If you really want to get ambitious, note that VBA (as provided in Word/Excel/etc.) will let you access COM objects which -- for the purpose of this discussion -- boils down to nothing more than a function call interface with a prescribed set of data types and calling conventions. COM objects can be programmed (implemented) using C++ (and plenty of them are), although it's much easier to write them using one of the ".Net" languages such as C#.
Realistically I think Gates' choice of VBA rather than C for Office makes sense... I know office workers who have absolutely zero formal programming background, and they can manage to modify and write various little utilities and macros in VBA, whereas I think they'd find the initial learning curve a lot steeper with C and wouldn't even bother with it.
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