• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Yet another oscilloscope question

Status
Not open for further replies.

Andy1845c

Active Member
I've been watching oscilloscopes on ebay for a few weeks now. I'd like to get one to learn with, but also one that will do most of what I will ever want to do with one. What are some applications of 2 or more channels? I doubt I will need this at first, but everyone seems to recommend it. What about if I wanted to look at 3 phase mains or the output of a 3 phase generator or roto-phase? With 3 or 4 channels could look at all 3 phases and see if they were all uniform? Would they appear seperatly on the screen or can they be super imposed on top of eachother? Or would there be no reason to ever do this?
 

dknguyen

Well-Known Member
Most Helpful Member
2 channels is really really high up there on the list. It's used so you can compare the input and outputs simultaneously so you can make sure the output is correct based on the input...after all...how can you know if the output is correct if you have no idea what the input is? Anything beyond 2 channels is really REALLY expensive I believe. Are you really going to use the oscilloscope on 3-phase systems so often to warrant all those extra channels? You probably don't need it (I'm guessing), especially if it's a balanced system. You are not going to be able to get a 6 channel oscilloscope off of Ebay...you can barely get one from an electronics store.

The channels appear all at the same time on the screen (or whatever combination thereof that you wish). You can shift them up and down and left and right on the screen so you can have on above the other, or superimposed, or whatever you want.
 
Last edited:

Sceadwian

Banned
If you need more than two channels you can design your own custom external chopper circuit easily. You'd only need access to the scopes internal time base signal or provide your own trigger pulse.
 

kchriste

New Member
Forum Supporter
What are some applications of 2 or more channels
Checking the time delay through a circuit is one common use. Basically, 2 channels is usefull for taking a snapshot of one channel when it is triggered by a signal on the 2nd.
3 or more channels:
Don't forget that good scope probes are really expensive! :D I've never used the 4 channels on my scope because I couldn't justify buying two more probes for something I'll rarely do.
 

Andy1845c

Active Member
Thanks for the replys. I'll definitely try to get atleast 2 channels. Yeah, i've seen that the probes can be expensive. But even if I got lucky and got a 4 chennel scope, I wouldn't need more then the 2 channels for a while. But like dknguyen said, i'd seldom need that many channels.

What about calibration? Is this somthing that you can do yourself?
 

dknguyen

Well-Known Member
Most Helpful Member
It shoould come calibrated shouldn't it? I don't own an oscilloscope but that's what I assume. You'd only need it calibrated every few years (every year?) - probably something you should get a professional to do.
 

stevez

Active Member
Andy - I've found that 2 channels is incredibly valuable as I learn more about electronics. As already stated, you can look at two things at once separately or together. More channels would be nicer but if you are just beginning you'll have your hands full with 2 channels.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
dknguyen said:
It shoould come calibrated shouldn't it? I don't own an oscilloscope but that's what I assume. You'd only need it calibrated every few years (every year?) - probably something you should get a professional to do.
Why bother?, calibration isn't about accuracy, it's about having the paper trail to show it's been calibrated. Even if you buy a brand new scope you have to pay extra to have it calibrated and get a calibration certificate.
 

philba

New Member
you pay a lot for a calibrated scope.

I'd also look for an external trigger input so you can trigger on one signal and watch the other two. some of the cheaper scopes omit this.

Delayed sweep is a very nice feature. It allows you to trigger and then look at a signal that happens later. I could have used this several times recently.

A lot of scopes have an output signal that you can use to do a quick check/crude adjust with. My 2225 has .5V PP/1 KHz. You can also use it as a signal source for your circuits.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
philba said:
A lot of scopes have an output signal that you can use to do a quick check/crude adjust with. My 2225 has .5V PP/1 KHz. You can also use it as a signal source for your circuits.
Almost all do, it's intended for setting the x10 probes up - but as you say, it can come in handy if you want a 1KHz squarewave.

One of my scopes even has a preamp output on the back, so you can plug a frequency counter (or anything you want) in the back - the output is a normal sort of line level.
 

Andy1845c

Active Member
How does one calibrate one? I mean how does it work? Is the scope hooked to a known freq. and simply adjusted to show exactly that? Or is it far more complex? I just don't want to buy one thats way out of whack if its not somthing I can do on my own. I don't live in an area where I would be able to find a professional oscilloscope calibrater;) lol

I've got a bunch of junk for sale on ebay. Hopfully I will sell some of it so I can start actually trying to bid on one.
 

Sceadwian

Banned
Calibration is always a tricky business. But yeah that's usually how it's done. You feed it a known signal and adjust it to read properly. There's usually at least two calibration points.
 

Rolf

Member
Calibration.........

Andy1845c said:
How does one calibrate one? I mean how does it work? Is the scope hooked to a known freq. and simply adjusted to show exactly that? Or is it far more complex? I just don't want to buy one thats way out of whack if its not somthing I can do on my own. I don't live in an area where I would be able to find a professional oscilloscope calibrater;) lol

I've got a bunch of junk for sale on ebay. Hopfully I will sell some of it so I can start actually trying to bid on one.
Gross inaccuracies in o'scopes went out with the vacuum tube. If you are worried about it, get a Tektonics or HP scope, their service manuals tell you everything or at least they used too. But it takes a lot of calibrated expensive calibrated equipment to do it. A simple probe calibration is probably about the most important and it can be done with the 1Khz output on the front panel.
 

stevez

Active Member
Calibration and accuracy is certainly important but it may be less important in your situation. If the scope works ok you might be able to get by with what I'll call relative measurements.

In some situations you might check your scope against a 1 kHz square wave and find that the frequency is off. You might keep track of that error and apply some correction. You might also use a signal generator and a radio. You can zero beat a signal against an AM radio - tuned to a station of known frequency. That can become a point of reference. It's not perfect but it may be sufficient for your needs.

For voltage you could use a transformer to get yourself a 60 Hz sinewave (don't use an autotransformer). Measure the voltage with a DVM that works. Use resistors to create a voltage divider. You might purchase relatively high precision resistors but do the math first to see how much error results with less precise components.

See if you can find some local folks to work with too. Quite often someone in that group will have the right equipment - at least enough for you to see how much error might exist.
 

Dean Huster

Well-Known Member
No professional calibration services in the Twin Cities area?

Most Tektronix scopes that are specified with an accuracy of (for example) ±3% will usually come into the cal shop after a five-year period of no calibration and still have an actual accuracy of ±1%. As mentioned, quality scopes will hold their calibration well. If you want accuracy and precision in your measurements, an oscilloscope is not the ideal instrument to use unless you have some highly-specialized plug-ins for a Tektronix 7000-series lab-grade instrument. For DC and AC voltages and curent, a decent DMM is always better. The scope gives you a PICTURE that you can't get from a DMM.

I'd caution anyone from using a scope in a high-energy application, such as around 3-phase power systems, etc. Probe grounds are too-easily available and its too easy to do lots of damage to you and/or the scope. Specialized test instruments for industrial situstions like that are available if you need information on power factor or harmonics on the lines.

Dean
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Dean Huster said:
Most Tektronix scopes that are specified with an accuracy of (for example) ±3% will usually come into the cal shop after a five-year period of no calibration and still have an actual accuracy of ±1%. As mentioned, quality scopes will hold their calibration well.
Like I said above, scope calibration ISN'T about calibrating the scope, it's merely a paperwork exercise to give you a calibration certificate - from a hobbiest point of view it's almost certainly a complete waste of time (and MONEY!).
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top