23 June 2013

Gaining Some Wiggle Room

For some time now, I've wanted an oscilloscope. An O-scope is a stand-alone testing device that allows the user to visualize and make measurements of voltage wave forms (oscillations) in alternating current (AC). A good multimeter can also measure AC voltage, but provides no information about the waveform. When you set a digital multimeter (DMM) to measure AC voltage (Vac), and put the probes into a typical American wall outlet, you should get a reading of 120 volts. But the DMM doesn't tell you that the voltage is an AC wave form that is (typically) oscillating at 60 cycles per second.

Well I've had a real, actual, crappy job for some time now, and I've put most of my compensation into investment accounts. Recently the desire for this kind of test equipment was rekindled in me. Sadly, I can't remember what the recent trigger was. Maybe I decided I wanted to fix something, and then forgot what that something was. Anyway, I watched eBay for a while and did some casual research on the models that I saw there. There are several brands of O-scopes, and each brand comes in a range of bandwidths. "Bandwidth" for oscilloscopes means the range of frequencies that can be *reliably* tested with that specific unit. For example, a 100 MHz O-scope is designed to look at anything from DC (zero frequency) to a signal that is 100 million cycles per second. [There's a big caveat implicit in my statement; the upper bandwidth limit is usually restricted by signal attenuation - that's a fancy engineering term that I'll get to later.]

After some time surveying what was typically available on eBay, I decided that I needed a Tektronics model. I know myself pretty well, and I can say with some certainty that I may have had good reasoning to back it up, but my choice of Tektronics was at least somewhat arbitrary. [I do that. The list of positive qualities helps soothe my ego later when I realize how arbitrary it was.] So I started watching for Tektronics O-scopes. Before long I realized that for the bid prices that I was seeing, it made more sense economically to spend the money for a high-bandwidth unit. The things that I intended to look at were nowhere near 100 MHz, but even the used 20 MHz units were going for around $140, while 100 MHz units were anywhere from $180 on up.

The first few items that I chose to bid on were snapped up by bid-snipers who jumped in at the last minutes of the auction, while I was at work. Karl hates bid-snipers. So, in my frustration, I looked around and decided that the best non-auction value that I saw was a Tektronics 2336 YA model that had a $180 "buy it now" price. The Tek 2336 is a 2-channel, 100 MHz 'scope. What strongly appealed to me is that it is designed to be a portable model made to certain MIL specs; it was built to be somewhat rugged (as far as that can be said about sensitive instruments). I have a history of my nice things getting damaged, vandalized, broken, or stolen, so a rugged 'scope pretty much has my name on it in neon lights.

I should also mention that one should be EXTREMELY cautious about buying an oscilloscope on eBay. Almost everything is sold "as-is", so the buyer should have some technical knowledge, and either some technical ability - or have a very good friend to advise and make any necessary repairs. A DOA scope is a very real possibility, and you should understand that before bidding. I have repaired my own LCD monitors and other appliances, so I had an unreasonable and unrealistic amount of optimism in accordance with my inflated ego. I got lucky, but that's a story for the next entry.

1 comment:

google_PEAK_OIL said...

The 2336 bears a striking resemblance to the 2213 I won in a VICA competition in 1985 and is still in my garage.
I have been an electronic technician for over 30 years. I would not bother today with a non digital storage scope. In order to make a high speed analog scope you must use costly precision high speed CRTs and associated electronics. Expensive to build, expensive to calibrate, and expensive to maintain. And it still wont remember anything.
A digital scope just needs a fast ADC and a chunk of fast memory. The rest of the system is just cheap commodity computer processor, storage, and display.
Even simpler, just build the ADC and ram into a USB dongle and let your pc or pad computer do the rest. Several inexpensive examples of this exist.
You will want to capture and extract data from non repetitive waveforms, and an analog scope won't be much help. They are obsolete.