oscilloscope channels

[patroclus]

Hi
I opened a post a month or so ago, asking for some help in choosing an oscilloscope.
Fortunatelly, I'll get a Tektronix TDS 60MHz scope soon. I decided to pay more, and get more (more warranty on top of everything, and why not, a high quality scope compared with other "low-cost" ones, and also very small, which is something I really apreciate.
Now, I'm still not sure about channels. 2 or 4. I know someone asked something like this a few months ago, and no one argued in favor of 4 channels. Most of things I can think of now require two channels. Maybe sniping into a SPI bus will get and advantage with 3 channels (data in, data out and clk), and of course many digital applications, where usually lot of signals are present, get benefit (control signals, for example). No doubt a logic analyzer is most suitable (also the TDS has very short buffer). I own one, USB based. But of course, simple applications that can be handeld by the scope is preffered, as it's not always comfortable to have a PC around.
In analog world, 4 channels seems to be more confortable, as lets you look at 2 stages at the same time, but most of times I suppose it'd be just fine if you take 2 measures at a time. One before, and the other one later. The fact is that just $250 is the difference between a 2 channel and 4 channel TDS scope, with probes included, as far as I know.
How do you see it?
Thanks.

 

[patroclus]

Well, I know it's something very personal, and weather it is fine or not to spend an extra money on something, is someone's own decision... but, any opinions out there ?

 

[JimB]

My own scope has two channels, 98% of the time I only use one channel.
However, there has been a couple of times when I have been using both channels, the external trigger input, and wished that I could have displayed a third channel.

If you can afford it, go for it!

JimB

 

[patroclus]

Well, I read many times, that some engineers like 4 channels for some tasks. So far I'm not sure what do they mean by "some tasks". Of course, in digital world is so easy to need not 4, but 20 channels. Basically I opened this post to know opinion from first hand, of people who are really using their scopes. so, any experience is appreciated.
Thanks JimB.

I'm now thinking if it would be possible to take 4 or 5 digital lines, use a R2R ladder to generate analog output, and then record it using one channel. 4 channels could mean 2 analog + 10 digital. Just need a simple home made software that decodes the voltages into separated digital signals. Just a bit of a crazy idea, but nice.

 

[dknguyen]

No way. THe difference between 2 and 4 is $250?!!! I thought it was something like $1000?

I've never gotten a chance to use 4-channel oscopes because I've never seen one. But if you were working with brushless motor drivers (as in 3 phase brushless motors), I can see why it would be really nice to have 4 channels so that you can monitor all the phases and compare them all. It'd be a lot harder to do with 2 channels even if you took two separate readings two times.

 

[BeeBop]

Fortunatelly, I'll get a Tektronix TDS 60MHz scope soon. I decided to pay more, and get more (more warranty on top of everything, and why not, a high quality scope compared with other "low-cost" ones, and also very small, which is something I really apreciate.

Nice!!! I have always been rewarded when I went the extra bit on tool purchases. Usually, when I have 'skimped,' I have regretted it; often, when one purchases a cheap tool, one ends up re-purchasing the better tool in the end.
I have four channels. I most often use only two, but can think of a few times when I was very happy to have the third channel to use, and have seldom used the forth.

No way. THe difference between 2 and 4 is $250?!!! I thought it was something like $1000?

The price difference would depend on the brand, type and base price of the scope. Since mine was used, I think I got a fair deal, but yes, your mileage will vary...

 

[Hero999]

Is this a digital or analogue scope?

I imagine that more channels isn't so important with a digital scope, especially for a periodic waveform, since you can take different measurements with the same channels, save the images and overlay them on top of each other. It's a fiddle I suppose but handy if you want to compare, eight waveforms if you only have a four channel scope.

One thing I'd like to see on a scope is for it to be double insulated. This would save messing around with isolation transformers when testing line powered equipment.

Having solated inputs would also be a good ideaas it would alow allow each channel to have a different ground. You could do things like look at the waveforms on high side and low side driver transistors in SMPses at the same time.

 

[kjennejohn]

Hi. I can do most of my work OK with 2 channels. B/W display is OK, but color helps keep things straight, though. I've had a few moments where B/W traces overlapped and then went off in seperate directions, leaving me asking: which is which?

I have a 200MHz, four channel color LCD 'scope, a Tek TDS 2024B. No battery, though. I use it at work all the time. This allows me to track what's happening on the communications channels between two boards at the same time. Priceless.
I can track three digital signals and see what the rise//fall times of a glitch on a fouth trace look like, all at the same time, and see the timing between all these events. Even more priceless.
Then I dump the display to a USB thumb drive, save it to the design folder, write up a Word document with the displays in it, and pass it off to the engineer. No more scribbling my impression of the signals from a 2 channel scope, with crude measurements penciled in willy-nilly, for THIS boy! Priceless, indeed!
Still, at $2250, you have to sell the house to get one of these. Which I did.

For just under $400 you can get a 500 MHz, 32 channel, USB-driven logic analyzer.
https://www.pctestinstruments.com/
This covers those moments when even 4 channels won't cut it, which is where digital troubleshooting usually takes you. This allows you to set the triggering levels (-6 to +6 Volts); decode SPI and I2C traffic; and takes millions of samples in slow-changing circuits that make most analyzers unusable.

The moral: a 60MHz, two-channel B/W scope is a great starting point. But, no matter what you got, if you get serious about electronics, you'll always come to those moments where your equipment is inadequate for the job at hand. All you can do is try to figure what you'll need for equipment in the near future, then budget and buy something a little better than that. Grow into your projects, so to speak. Haunt Ebay for good, used items, hopefully with recent certification, from a technician who upgraded and is looking to sell off his earlier equipment. It happens!
Or, look to Ebay for new items from the Pacific Rim. Some are good, most are c*p. My rule(s) of thumb: if the manual reads well (no Chin-lish, thank you) and the specs are comparable to a name brand unit, then you're probably safe to buy it. Still, you'll want to go to the forums and ask around about other's experience with this stuff. A lot of this equipment is fine, and the prices are drop dead good.

And have fun doing it!
kenjj

 

[patroclus]

kjennejohn , I do own a logic analyzer. In fact, LogicPort, from the web site you mentioned.

I'm now about to go for a 4 channel color scope, probably TDS 2004B, the same scope you have, but 60 MHz. Yes, it's more expensive than 2ch B/W, but I'm going to buy it new, and it comes with 10 years warranty. I don't want to end up needing 4 channels or color in the future, and own a 2 channel B/W, which isn't free, by the way. It's a long term investment, not a cheap transitory tool, so I think it worths it.

Thank you very much to all of you

 

[patroclus]

I got the scope today. Four channels and color LCD, but only 60 MHz.
It looks great. Just a bit weak, plastic feeling, compared with my heavy old analog scope, that seems to be able to survive from a nuclear attack. But overall well constructed. It's so light, and so small. Just takes some more space on my bench than my portable multimeter. You can lift it with two fingers. It came with four 200 MHz probes, which look far better than my 60 MHz ones.

It's more than I was expecting to spend, but I think a long warranty and Tektronix quality deserves it.

If anyone ever considers buying one of these Tektronix oscilloscope, or trying to choose a low-cost digital oscilloscope, feel free to ask for anything about TDS series, advice, details, ... I'll be pleased to answer.

By the way dknguyen, did you get the TDS1001 ?? I tried to send you an email through the froum but you don't allow it.

 

[dknguyen]

No! I forgot to account for my car insurance in my budget so I'm about a bit more than a grand. I also bought a RC helicopter to mess around with which cost about the same. That means I only have enough money right now to maybe build all the crap intended for the helicopter. Travelling and oscopes will come back on the list as soon as I get a summer job.

 

[Dean Huster]

All of which is why I generally recommend buying used scopes. An analog Tektronix 7904 (500MHz bandwidth) can have four channels easily (I can't remember if there's a 4-channel vertical preamp available in the 7K series as there is in the 5K series which would give you 8-channel capability). Or, if you don't like the analog channels, slip in a 7D01 or 7D02 logic analyzer for 16 channels. Most of this stuff is available for pennies on the dollar, is still high-quality, reliable, high-end and affordable.

You'd probably spend less on a fully loaded 7K system than you would on a 4-channel color TDS series.

Dean

 

[erosennin]

Hely, I wouldn't be calling a Tektronix "Just a bit weak, plastic feeling"... lol, I wish I could afford one... I'm living on a 5MHz 1 Channel Portable Velleman lol, it's better than no scope, but I'd love a colour Tek or LeCroy ahaha...

 

[patroclus]

Well, I need a digital signal oscilloscope. A logic analyzer is not always suitable, and worthless in analog world. An analog scope is just great most of the times. I own an old cheap second-hand 20 MHz dual trace, which is still working, with some minor issues. The problem here is that you are completely out of the game when trying to see transient, non repetitive events. This sometimes happends in digital systems, where you need to make analog measurements, of check analog circuits driven by some logic circuits.
I considered a four traces 100 MHz or higher scope, probably new, but as I wanted to make a long-term safe investment, I thought it was a better idea to go digital.

When I said "weak plastic feeling", I meant to say that the scope is built upon plastic, handle and legs too, and that the buttons feel a bit weak, compared to my old scope, made of metal and hard controls. The TDS seems, nevertheless, robust and well manufactured. I just had the same impression that I felt when I bought my new compact digital camera. Cannon, great specs, but feels weak and kind of delicate compared to old models, or actual reflex cameras.

So far, the scope looks terrific. I lived two years without scope, and two years with an old cheap 20 MHz scope, so this is a giant leap. Expensive, but so far worths it.

 

[Dean Huster]

Considering the the current crop of digital scopes appears to be nothing more than the front panel and the first three inches of depth of a traditional analog scope, it does seem a little weird if you're used to the typical 1980 scope, or worse yet, the typical 1970 scope!

If a Tek TDS220 were built using similar components to the Tek 454, you'd have at least 1.5 of that 3 inches as a pair of aluminum alloy castings. The tilt bail would outweigh the current TDS. The power supply would consume the entire volume of the instrument. It takes plastics and other componsites to keep the price and the weight down. And who would even consider buying a scope with the power of the TDS in a package the size and weight of the 454? Be thankful that Tek had the foresight to provide the security loop on the back!

Dean

 

[patroclus]

Yes, I know.
Just in case, when I said, "it looks terrific", I meant to say that it looks great.
So far I tested the 4 channel operation, at all sweep speeds and voltage divisions.
No matter if one or four channels are on. Full real time sampling in all cases.
The responsiveness is a bit slower than an analog scope.
When you adjust a trace position or division, the screen takes a bit of time to refresh.
Maybe it's me, that I'm used to CRT.
But I've got no previous experience with DSO, and I heards that TDS is quite fast,
compared to other cheaper digital scopes. You get used to this soon.
Color screen is handy. If the budget isn't just enough for that, I suppose B/W
is fine, as only two channels model are available in B/W. I like color because in analog
world, I rather superimpose the waves than having each of them in separate vertical
zones of screen. the different colors let me exactly how waves interact, and see when
two waves are in phase (XY is also a way to achive this, but in XT you just see a
different color showing from behind of the superimposed waves, when they don't match.
You can easily see lot of details in single shot aquisitions, and even in continous mode)

The USB is just great. Any pendrive can hold a massive ammount of waves, screenshot,
and settings.

Of course the main drawback is the cost, which for an average hobbyist is somekind
prohibited. But if I didn't have the cash, which was my case, I'd wait, or if I couldn't
which is my case now, but still don't want to spend this much, I'd go for a lower cheaper
model, like a B/W 60MHz dial trace, or even the 40MHz model if I wouldn't be demanding too
much. This should save around $500 and $750 from the color four channel 60MHz model.