scope selection

Erik Friesen

I have been perusing ebay's selection of tektronix scopes. I don't really have enough experience to know exactly what I need. I would like to be able to debug spi comms, so I assume I need at least a 100mhz scope with 4 channels. The thing I am unclear on, is whether I need a digital storage scope for this, or what. Any suggestions on the minimums spec's I'd need?

localroger

Scopes really aren't good for multichannel logic analysis because they don't switch back and forth between the channels at sampling rate. What you want the scope for is to make sure the signal isn't bent or misshapen by something like stray capacitance. You can verify that with a single-channel scope, though you might want a storage function so you can catch a bit of comms and see it without the jitter. For real analysis of something like SPI, once you've verified that the signals aren't mangled, you can use a digital input device like, oh, another Propeller to sample the signals on multiple channels and save them for display in all their synchronized glory.

Erik Friesen

The SPI thing is secondary, really. I just want something that holds the last waveforms since the last trigger. I actually own an old tube scope, and need to move up to something better. I know that a logic analyzer is what I need for SPI, but I don't have the budget and the space for it at this point.

At what rate do they switch back and forth between channels?

Know anything about the Tektronix 2465?

localroger

I think it depends on the scope. The one my father had in his lab back in the 1970's alternated; it wasn't capable of displaying two traces at the same time. It would be very hard to keep two deflection mechanisms from interfering with one another in a cathode ray tube. In any digital scope, of course, you can have multiple ADC's so that's not a consideration. Even the cheapest digital scopes tend to have pretty good triggering and multichannel options, but the cheap ones tend to have really low bandwidth.

kwinn

Erik, as localroger says, some older scopes displayed 2 traces at a time by alternating between the two signals. Display all of signal one for the sweep, then display all of signal 2 for a full sweep.
Some also had a chop mode that displayed both signals simultaneously by rapidly switching between them during a single sweep time.
A very small number of scopes had multiple electron guns and could display multiple signals simultaneously. Rare and expensive.

A decent Digital Storage scope can acquire multiple signals simultaneously and display them on a CRT or LCD display. The bandwidth will depend on the sampling rate and the cost will depend on the sampling rate, sample memory size, and number of channels.

Get the scope with at least 2 channels (4 is better) and the highest sampling rate you can afford. Signal speeds are only going up.

Erik Friesen

Take this one for example - http://cgi.ebay.com/TEKTRONIX-TDS-510A-OSCILLOSCOPE-4-CH-500MHz-500MS-s-/150524681447?pt=BI_Oscilloscopes&hash=item230bf85ce7

Will it do what I want? who knows :-) Is a person better off trying to find one with probes?

davejames

Erik - a very nice scope (http://www.teknetelectronics.com/DataSheet/TEKTRONIX/TEKTR_244573741.pdf)

We used them in the automotive test engineering group at Phillips Semi. As with all Tek scopes, it had great triggering. One thing to understand on this scope is that channels 3 and 4 have different characteristics than channels 1 and 2 (volts/div, impedance).

Like the others have suggested, an o'scope is not the best tool for debugging communication interfaces. For that, a logic analyzer is the better way to go:

http://usbee.com/sx.html
http://www.saleae.com/logic

I use the usbee SX for debugging my product and it works quite well (used to use a stand-alone Parallax Basic Stamp Logic Analyzer but I kept blowing up the inputs). The SX environment (as does the Saleae) includes an SPI decoder for easier interpretation of the signals.

Many Regards,

davejames

Erik - another nice scope you've picked.

I have a Tek 754C which is similar and you need to know that channels 3 and 4 on the 510 are 50 Ohm, so not a nice match to wimpy digital signals.

Regards,

Gadgetman

What you need isn't a Scope, but a logic analyzer.

Have you tried the one included in the Viewport package?

Erik Friesen

I have looked at a few of those 2465 scopes, davejames. I believe they are analog. Perhaps exposing my ignorance here, you can't hold and view a waveform with those, right?

Thanks on that information, that makes some difference, I had no idea that 3-4 inputs were different. What specific thing are they intended for?

I know that a logic analyzer is the way to go, but I still want the capability of viewing some analog digital mixes, like triggering by spi clock and viewing adc input, etc. A logic analyzer has its place.

I tend to shy away from computer based scopes, etc, although those logic analyzers referenced above look fine.

Erik Friesen

@gadgetman - I have tried the one written by jazzed.

Like I said, that comms thing is secondary. My next project involves some slow (noisy 2hz) waveforms that I wish to view. I want a scope that can give me some idea of the voltage levels, etc.

Don M

@Eric-

I can personally vouch for the Saleae Logic analyzer. I have had mine for several years now and have used it quite a bit for decoding various signals. Very inexpensive and works quite well.

For a scope I own a Tektronix TDS1012B DSO. This too has been quite useful for catching glitches and such. What I really like about it is the ability to save oscillograms to a usb drive and be able to print them out or email them. Many of the new digital scopes have this feature. The old scope that I still own but rarely ever use is a Hitachi V1100A 4 channel 100Mhz.

Tracy Allen

ransom notes:
-- That TDS510 you pointed out includes option 1M, which is extended sample memory. That is a valuable resource, because it allows you to scan in detail through a longer period of time before and after the trigger.
-- It also includes option 1F, but I don't know what that is. Look through the manual or 'scope specs to see what options are available. Often those are "extras" that may or may not be included, or relevant. Look for math function libraries to do averaging, fourier transform and stuff like that, and special triggering smart triggering options and i/o functions. I'm not sure how Tektronix handles its feature bundling.
-- Be sure you can acquire or download a manual. Looking at either analog or digital signals will at first be tricky with a high bandwidth 'scope. It takes a while to get used to any 'scope and a digital 'scope has a whole different feel than an analog 'scope.
-- Ebay 'scopes often don't come with probes--something ($$) to look for--you can also find them alone on ebay but be sure they include the probe tips.
-- Also consider LeCroy 'scopes such as this ebay offer in the same price range (2Mbytes sample depth per channel).
-- These older 'scopes are great for general signal analysis, but they don't include color or the modern features that show thinks like RS232 or I2C decoding on screen. (maybe real soon for the prop scope!?)

davejames

Erik Friesen wrote: »

I have looked at a few of those 2465 scopes, davejames. I believe they are analog. Perhaps exposing my ignorance here, you can't hold and view a waveform with those, right?

Thanks on that information, that makes some difference, I had no idea that 3-4 inputs were different. What specific thing are they intended for?

I know that a logic analyzer is the way to go, but I still want the capability of viewing some analog digital mixes, like triggering by spi clock and viewing adc input, etc. A logic analyzer has its place.

I tend to shy away from computer based scopes, etc, although those logic analyzers referenced above look fine.

Hi Erik,

There were analog scopes that could hold an image ("storage scope"), but I think users today would find the image unacceptable after being exposed to digital storage scopes. Not sure without digging into the specs if the 2465 had storage ability and my memory fails me anyway (used them 15 years back).

RE: channels 3 & 4...typically when there's a low impedance noted, it means extended frequency bandwidth. Having reduced voltage ranges could have been a matter of price-to-function to keep the costs down. At Phillips, we found the reduced voltage ranges to be a hassle also.

I agree with the tendency to shy away from computer-based scopes/analyzers (mainly due to them being a bit delicate/fragile)...but at some point, the price/function becomes reasonable in regards to the need.

In combination, I use an "old" (1998) LG 20MHz, two channel o'scope for audio, the TDS 754 for critical timing views of logic, the USBee SX for viewing/debugging general logic operation, and a Parallax BSLA for monitoring the BS2.

Regards,

Erik Friesen

Do you consider lecroy in the same class as tektronix?

Another question, do both use proprietary software and protocols for 232 communications?

Its kind of like rebuilding a motor. You start with a carburetor rebuild and end up doing the whole thing. For just $200 more you can go to the next step.

I really like the usb option, but to get a comparable scope is about twice the money.

Don M

LeCroy makes a very good scope. I have played with some at trade shows, etc. They are priced a little more reasonable than the Tek models. If I were looking for a new one I would very heavily consider the LeCroy.

davejames

Erik Friesen wrote: »

Do you consider lecroy in the same class as tektronix?

Another question, do both use proprietary software and protocols for 232 communications?

Its kind of like rebuilding a motor. You start with a carburetor rebuild and end up doing the whole thing. For just $200 more you can go to the next step.

I really like the usb option, but to get a comparable scope is about twice the money.

Hi Erik,

When I first encountered the LeCroy scope, the department engineers treated it with such a level of respect that caused me to believe it was a superior product to an equivalent ability Tek. So, I have a *perception* that it's a level higher product.

I briefly used a LeCroy while teaching a class in Singapore some years back and it worked flawlessly; great triggering, great picture (nice and crisp).

I have no information concerning the related communication protocols.

The USB-based products offer quite a bit for the money. Since all the horsepower is contained in your computer (that's already been purchased), the scope/analyzer functionality is contained in front-end circuitry thereby cutting the cost.

If you're desiring the capability of viewing analog and digital signals, you might consider the USBee product line. The combo unit isn't cheap...but it'll be cheaper than buying the individual scope/analyzer equipment. Treat them kindly (no static and/or over-voltage conditions) and they'll work work well for quite a while.

Regards,

Erik Friesen

davejames wrote: »

Hi Erik,
The USB-based products offer quite a bit for the money. Since all the horsepower is contained in your computer (that's already been purchased), the scope/analyzer functionality is contained in front-end circuitry thereby cutting the cost.

I should clarify. What I mean is a scope with a usb host capability to operate a thumb drive and printer is about twice the money.

davejames

...oh - gotcha!

DJ

Erik Friesen

What about these ? - http://www.attenelectronics.com/products/rf_microwave/ADS1000.htm

My gut says there is a reason they are cheap. Any known reasons?

Just make sure you don't type it on a chinese keyboard. :-)

davejames

...for me, they're an unknown.

What's the asking price?

DJ

Tracy Allen

The features look good in terms of sample rate and depth, triggering modes, averaging, and so on. The devil may lurk in the details of the implementation. I wouldn't want to jump to conclusions and dismiss it as a cheap knock-off, but I wouldn't want to put down any money without a test drive. How much is it? Is there any support?

Tektronix and LeCroy and others have booths at trade shows. Tek 'scopes are always impressive, but the LeCroys make my jaw drop. Of course they are showing off their most recent entries $$$$ and up. I do have an older LeCroy 9354AL that I bought on ebay, my main workhorse 'scope at present.

Erik Friesen

I read somewhere about some chinese scopes adc being overclocked 250%.

The ads1102c is about $400 shipped to your door.

Erik Friesen

I picked up a fluke pm3382 on ebay. It is analog or digital. Everything appears to work ok, so we'll see how it works out.

NewGuy2

I've used the Ant-8 (now probably the Ant-16) as well as the "USBee" logic analyzers. Reasonably affordable, they're USB-based multi-channel logic analyzers, and work very well.

As has been said, an analog oscilloscope requires a repeating signal, so has limited use in digital electronics. A digital scope can take a 'snapshot' of a signal -- but getting 8 or 16 signals at the same time is MUCH more useful when troubleshooting CPU-based electronics.