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.