How do I buy and test an oscilloscope?

Question

I am looking to buy an oscilloscope that will last me a few years. Application is everything but RF: digital and analog with as wide a frequency range as possible/affordable. I would like to buy used, but I would prefer Craigslist in the NYC area to Ebay, so I can check it out before I pay money. Yes, I know Ebay has escrow and if the pickings are too slim on Craigslist I'll go that route. I need something that will allow me to do repair and testing on mixed (A/D) systems in the medium-to-near term. My questions are:

1. What is a good entry level-price range? Barring any definite statement, how much will I spend per, say, 500Hz additional frequency range?

2. What features should I look for? Again, what is the price point on each feature?

3. What is a reasonable minimum I should expect to spend?

4. If buying used, what tests should I perform before paying?

Answer 1

Price points always vary - especially with used scopes. You're likely to find a lot of old Tektronix two-channel analog scopes. If you're lucky they will include probes.

1. Bandwidth - You should look for 50-100MHz. Bandwidth tends to increase in multiples of 100 after that point, then multiples of 1GHz after the 1GHz mark. You might find a 150MHz or 200MHz scope on an odd day. Again, can't really give you a price point, but the knee of the price curve is in the low 100's of MHz. Beyond that it tends to get pricey.

2. Features - two channel at least. If it's analog you basically get no really cool features (like measurements, saving waveforms, etc). It will be only for viewing waveforms and you'll be figuring periods and such by counting tick marks on the lines on the screen. Once you get into digital scopes the world starts to change a lot. I typically like digital scopes more and look at least for measurements (evaluate period, Vpp, etc automatically) and cursors (graphically mark points on a graph and evaluate time deltas, voltage deltas, etc). Other features that are nifty are USB ports to save waveforms to thumb drives, network connectivity, printer connectivity, logic analyzers and many others. Generally, once you make the jump to digital you see a significant price increase (a few hundred dollars at least) and get many of these features essentially for free.

3. 0-$400 for an analog one (hopefully in working condition) and digital ones can start around upper $100's of dollars and, well, sky's the limit really. I should mention at this point that you can get a fairly decent digital 50MHz Rigol Oscilloscope for about $500 or so brand spankin new. It's by no means a Tektronix but it is very serviceable and has many of the nifty features I mentioned above.

4. Some people on eBay will say things like 'powers on'. This means... it turns on. And potentially nothing else. True, some people are idiots and don't know a working oscilloscope from the Dalai Lama but I wouldn't count on this. You'll want to make sure that it both powers on, passes self-test (many digital oscilloscopes have this and will notify the user on startup if the self-test passed) and that the test signal works on all channels. The test signal is usually a 1KHz square wave or something similar. It's built in to the scope and there's a contact point for it on the front of the scope. If each channel can read the signal it means that if there's a problem with the scope it's going to be in something not immediately obvious - like its bandwidth is declining with age, or it powers itself off after 3 hours of use and won't turn on again until it cools down, etc.

The only other piece of advice I can give you is to watch out for probes not being included. Generally they will say but even if you see a picture with probes in it don't assume they're included with the scope. Ask.

Answer 2

It's not exactly what you are looking for, but...looking for instruments, I've seen that there are USB Digital acquisition (DAQ) cards, with 8+ channels, starting from 100-200$...probably they won't be as comfortable as a scope can be, but I've seen a National Instrument DAQ and I can assure that when you have the data to your PC the features are limitless.

The main drawbacks I think are the need to connect to the PC everytime and eventually configure it, and the slower response to the screen.

EDIT: I forgot an important drawback: most of these DAQ systems don't have amplifiers, they are just fast and often accurate ADCs, so you have to stick to the voltage scale or build your own amplifier.

It's not a real solution, just consider if it can work for you.

Take a look at this, this, or this to give some examples.

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I've seen also that there are mini-oscilloscopes that you can attach to an iPhone, or maybe other devices. They are cheap and probably low-quality solutions, but they may give a better insight of certain signals than a handheld multimeter.

Answer 3

I have a Tektronix 466 analog oscilloscope that I purchased used and love. Since you are in a major metropolitan area, and will presumably pickup the unit directly from the seller, you can save the cost of shipping, which can be substantial.

By carefully shopping around you should be able to find a used analog scope for well under $100 in very good shape. I am not necessarily recommending the particular model I have, it was just what was available to me locally at the time. Mine is a 100 MHz dual channel model.

These older Tektronix analog scopes are considered some of the finest scopes ever made, and sell for an extremely small fraction of their original cost.

Virtually all new scopes will be digital scopes. A low end Rigol will set you back just over $300 while a top end model could reach $10,000.

As to the tests to perform; the best thing would be if you knew someone familiar with the particular scope you are considering who would go with you to check it out.

Answer 4

Here is a good ebay guide: http://reviews.ebay.com/Buying-an-Oscilloscope-on-ebay?ugid=10000000001568756

You'll probably have to get a function generator to check if the oscilloscope is calibrated before purchasing, which can make things a little difficult for whatever situation you are in -- meeting at a public place with limited power outlets.. Good luck and yes, be sure to check the thing is working before giving away your $$...