oscilloscope questions

First of all I wasn't sure where to post this, so I chose general electronics. Ok now I got that out of the way… I was considering an oscilloscope, and I wonder as I am looking at them, what I should be looking for, number of channels?, sampling rate?, bandwidth? I would like to pay as little as possible, I was looking online and found one for 38000 dollars or some ridiculous price lol. Yeah not for me. If I could keep it less then $200 dollars that would be nice. I guess a little information too would help with the guidance here. I am currently going to analyze analog input, output from a guitar to start with, however I may at some time in future (hard to say right now) may be using a digital signal as well. As you can see I am rather confused about what to buy. I saw a portable at dx electronics… it is just in my price range too, any more then that may be a bust. Here is the link. http://dx.com/p/ds-203-3-0-lcd-pocket-mini-oscilloscope-65990#.Uv0hp_2FVSU I have seen a post about portables, and the controversy regarding the ability of these things. (which really just confuses me more), since I have not a lot of clue about what I am using it for, thus making it difficult for what I need. I will say this, it will be for Arduino stuff. Well a little guidance for this purchase would be really appreciated. Post a link or two with what you think is a decent starter oscilloscope, and perhaps a bit of info, of why that one is good. Thanks in advance.

Understand that the bandwidth you need is not just based on the signal you are looking at, it is also based on any bad stuff that the signal may contain.

So if you have, for instance, an audio amp that you are troubleshooting, and you try using a computer audio card as an oscilloscope, the bandwidth is going to be limited to absolutely no more than 20kHz. That means if you have, for instance, clipping of signals at the high end of the audio band, the lowpass filter in the audio card will smooth those sharp edges out and hide the clipping.

Or if the amplifier is bursting into oscillation well outside the audio band due to a bad ground or bad component, it can damage the amp, cause excess heating, distortion, and reduced audio. But not show up on a sound card oscilloscope.

Same goes for RFI your amp may be picking up.

As for digital signals, a square wave is not just the fundamental frequency. It consists of the fundamental plus the odd harmonics in diminishing levels. Going merely to the 5th harmonic barely looks like a rounded off square wave. It really takes up to the 9th harmonic or better to display it more accurately.

However, if it is NOT 50% duty cycle, then the frequency components are based on the shorter part of the pulse.

There is also rise time and fall time to take into account. If your scope's bandwidth (and therefore rise time) is not at least 2x the rise time of your signal, then what appears on the scope will look a lot slower than what it really is.

Add to that, if you put a scope on it, you are troubleshooting. So you are looking for things like large, fast spikes and glitches that will have frequency components much higher than the digital signals themselves.

Note that benchtop scopes normally have a sample rate of at least 10x the bandwidth.

The scope you listed has a bandwidth somewhere between 3MHz and 8MHz, depending on whom you believe. I consider that barely adequate for audio, and digital up to 100kHz. The software, as it turns out, is really beta, and the state of development is such that if that is the latest hardware version, the latest cool software add-ons don't work with it.

My advice would be to save up some more money and get something like a Rigol or Hantek, 1Gsps 100MHz dual trace scope. For now, you might see if you can find a cheaper analog scope locally.

It may be better than nothing, but if it was me I'd look for regular bench-top oscilloscope. Anything in the true* MHz range should be adequate for audio. I don't think I've ever seen a "real" oscilloscope that was single-trace, but 99% of the time I'm only using one probe & one trace. (I use a 'scope at work, but I don't own one personally.)

I think some of the cheaper handheld 'scopes (and USB 'scopes) may have issues with refresh times... i.e. It may be able to capture a 1MHz wave, but then there may be a delay before it captures the next wave.

If you get an analog 'scope, beware that audio is not perfectly repetitive and a normal audio waveform (voice or music) generally looks like a mess. The same is true when looking at non-repetitive digital signals (such as data or address lines). A digital 'scope can capture and hold a single-sweep so it looks more like what you see in an audio editor. And at low audio frequencies (and associated low horizontal sweep rates), an analog display will flicker.

  • I've got a 100MHz Tektronix digital 'scope in front of me, and it has a sample rate of 1.25GS/s, so it shouldn't have any problems getting to 100MHz. (We currently don't manufacturer anything clocked that fast.)

polymorph: The scope you listed has a bandwidth somewhere between 3MHz and 8MHz, depending on whom you believe. I consider that barely adequate for audio, and digital up to 100kHz.

It's plenty for audio, and it has a special digital input for digital signals well into the MHz range.

polymorph: My advice would be to save up some more money and get something like a Rigol or Hantek, 1Gsps 100MHz dual trace scope. For now, you might see if you can find a cheaper analog scope locally.

Or look for a second-hand scope. Those old green-screen 'scopes can be had for very little money and they're brilliant for audio work.

For audio-only work, as well as repetitive digital signal work - a second-hand analog oscilloscope will work fine; you should be able to easily find an old dual-channel scope between 50-100 MHz for around $200.00 easily - if you bide your time, look for the deals, and be patient. Actually - you can find some nice stuff at a local hamfest, so be sure to check those out first (also Craigslist!).

The downside to purchasing such a scope (besides no storage capability - that said, if you really look, you can sometimes find old dual-channel DSO scopes going for little as well - I picked up a dual-channel 100 MHz Fluke Combiscope DSO for around $300.00 a couple of years ago) is mainly the fact that they aren't very portable (but they are luggable!), they can be a bit fragile, and when you purchase used, you better know exactly how to test it out before you buy, lest you find yourself with a pig-in-a-poke! In fact, I would never purchase a scope used without being able to test it out first-hand myself; if the seller won't let you, walk away.

Brands to look for (if you decide to go down this route): Tektronix, Agilent/HP, and Fluke; $200 should be able to get you a dual-channel 50 or 60 MHz analog scope fairly easily.

Otherwise, unless you need it right this minute - I would save my money and purchase a new Rigol DSO or such (part of me wishes I had gone down this route now, but what's done is done!).

A lot of good information here, Thanks to everyone who gave me their two bits. I appreciate it. I do not need it right away so perhaps I will just save for one, at a later date. I will keep looking though in case I find a good deal.