I got my oscilloscope today.

Maybe that was a bit of an understatement; I'll have to resort to gratuitous typography:

I GOT MY OSCILLOSCOPE TODAY!!!!

Those of you who are reading this blog may remember I made a post about two years ago about searching for the right oscilloscope for me. Since then, I changed jobs and have been getting situated in the world of applications engineering, working on motor control projects. I've been gradually working to fill in gaps in the infrastructure available to me and my coworkers, and one of those has been an oscilloscope I can use in my office. Our group's budget recently allowed us to fill in some of those gaps. So:

I got my oscilloscope today! And it's even the one I asked for!

No, wait — it's better than the one I asked for! Here's why:

The oscilloscope I asked for was an MSOX3024A. When I got a quote from a distributor, they said there was a promotion from Agilent, and they could offer me a lower price. Agilent has a deal ("Supercharge Your Bandwidth!") that runs until March 31. Most oscilloscope models come in series, with different bandwidth options. You want a less expensive scope? Pick the lowest bandwidth. You want something with a high bandwidth? It'll cost you. So the Agilent deal offers you the scope you want at the price of the next lower bandwidth. I would get a MSOX3024A (200MHz) at the normal price of an MSOX3014A (100MHz). Net savings: about $350. Yay! Quote obtained, forwarded on to manager, no problem, back to work.

And then I got this funny nagging hunch in the back of my mind. MSOX3024A at the normal price of the MSOX3014A... hmmm... hmmm... what about the MSOX3034A at the normal price of the MSOX3024A? I checked Agilent's list prices for the MSOX3000 series:

MSOX3014A 100MHz $5199 MSOX3024A 200MHz $5556 MSOX3034A 350MHz $9169 MSOX3054A 500MHz $11993 MSOX3104A 1GHz $15819

Goodness, I could save $3500! I called the distributor back and asked if the promotion applied to the MSOX3034A. Yep. Some quick discussions and pleading with my manager and we got the order in for the MSOX3034A. Woot!

So I got my oscilloscope today. (Did I mention that yet?) First action of the day: put the probes on the scope along with the little color coded bands to help you keep the channels from getting mixed up, and compensate the scope probes.

What? You've never compensated an oscilloscope probe before? It's easy. You connect the probe to the calibration signal post on the front panel. It's a precision square wave. Then you zoom up the vertical scale so you can see the transients. Here's the deal: The scope comes with BNC connectors, and most scopes these days allow you to configure the input impedance with one of two choices. If you are doing high frequency measurements, they're typically 50Ω characteristic impedance to avoid reflections. If you are doing lower frequency measurements on high-impedance nodes of a circuit, you don't want the oscilloscope loading down the input, so you pick the other option, which is usually 1MΩ input impedance. But that's not good enough for most applications, and usually you want a probe with a voltage divider so you can get the input impedance higher, and have a higher acceptable input voltage range. Typical scope probes are 10x probes, which mean they have a 10:1 voltage divider inside, and are therefore 10MΩ input impedance.

(Figure from Agilent's Application Note 1603: Eight Hints for Better Scope Probing )

Now, both the probe tip and the oscilloscope itself have parasitic input capacitance. If these aren't matched with the resistor divider, the probe will give you a transfer function that is frequency-dependent, and the waveforms you see on the scope will be distorted. So oscilloscope probe manufacturers usually put a variable capacitor in the probe, either near the probe tip or near the BNC end of the probe, so they can match the input network and make it close to frequency independent. If you connect the probe to a square wave, you can tune the variable capacitor so that the square wave has edges that are square, and not rounded off or with an overshoot. The variable capacitor has a slotted adjustment screw, and usually you want to use a nonconducting nonferrous screwdriver to prevent the signal from being altered while you're turning the screw. Scopes usually come with a little plastic mini-screwdriver you can use to adjust the capacitor screw.