An oscilloscope is the experimental physicist's most important tool, and has been since the 1930s. Essentially all physics experiments operate by transforming a physical effect into an electrical signal, and the oscilloscope is used to make plots of electrical signals vs time. The analog scopes we have in the lab are over 10 years old and obsolescent, but have all the functions needed for our experiments, and you need to first learn to use an analog scope to properly use a digital scope. At present, it is possible to make your PC into a high-performance digital scope using a relatively inexpensive plug-in card. However, for high-frequency and precision applications, it is still necessary to invest $5K or more in a good HP, Tektronix, or LeCroy GHz scope. Most of the controls and functions described are present in both analog and digital scopes. If you have not used a scope since freshman lab, start by reviewing the functions described below.

1. Connections and termination

Because scopes are used to view high frequency signals, we generally use 50-ohm coax cable for connections. Scopes are normally equipped with standard BNC coax connectors (BNC stands for "Berkeley Nucleonics Corp.", a company founded in the 1950s which still manufactures equipment used for nuclear physics research). Newer equipment uses the more compact LEMO connectors (LEMO is a Swiss company similar to BNC in origins), and RG-174 coax. Remember, coax is not just "wire", it is a waveguide, so you need to terminate coax cabling properly to avoid reflected signals. The usual procedure is to simply put a BNC tee connector on the scope, with the signal cable on one ear of the tee and a 50-ohm terminator on the other.

2. Bandwidth

The frequency rating of the scope indicates the minimum feature size the scope can display: for example, a 100 MHz scope can display pulses no smaller than 10 nsec wide, while a 1 GHz rating is needed to observe features as small as 1 nsec. Some scopes let you deliberately reduce the bandwidth to "smooth" noisy traces. Also, note that if you are working near the bandwidth limit of your scope, what you see on the display may be slower than reality. If f is the bandwidth rating of a scope in MHz, it would display an ideal pulse (perfectly vertical edge) with apparent risetime t_SCOPE = (350 / f ) nsec. In general, the displayed risetime for a pulse is approximately related to its true risetime by t²_DISP = t²_TRUE + t²_SCOPE .

3. Coupling

Input signals can be "DC coupled" or "AC coupled". The latter means that the scope is isolated from the source by a capacitor, so any DC level present is blocked and you see only the time-varying component of the signal. DC coupling means you can use the scope as an absolute voltmeter; it will show time-varying signals superposed on any DC levels present.

4. Alternate vs Chop

Useful scopes have "dual traces" allowing you to comparing one signal to another. Analog scopes have only one CRT beam, so the two channels can either be traced alternately, or the beam can switch from one to the other several times in mid-sweep (chopping). Most also have an option to add the two channels and display the sum as one trace. Digital scopes show stored signals on a computer display, so these controls are not relevant.

5. Time base and vertical scale

The horizontal scale is determined by the sweep rate, selected in terms of seconds per division (typically a 1 cm grid is overlaid on the CRT). The minimum time per division is limited by the bandwidth of the scope.

The vertical axis scale (volts per division) is determined by the amplification or attenuation imposed by the scope circuitry.

Position controls let you offset the origin of the V and t axes.

If you are looking at a signal that is very different in DC level from what you expect, the beam may seem to "disappear". The "beam finder" is a handy button that pulls the beam spot into the center of the screen however far away it might be, indicating the direction it came from so you can adjust your horizontal and vertical scales appropriately.

6. Triggering

In order to synchronize the time scales, both traces can be launched simultaneously, by a selected trigger event. Most scopes allow a variety of triggering modes.

"Normal" means the scope sweeps when the input signal reaches a selected level with the selected slope. The display is otherwise blank.

"Auto" means the scope is triggered either by the signal exceeding a user-set threshold, OR after some specified time has passed if no signal exists. This is the most commonly used mode. However, the presence of the default trace may obscure small or intermittent signals. Unless you are sure of the presence of a signal, it is better to use "normal" than "auto".

"Line" means the horizontal trace is launched every 1/60 sec, by a signal derived internally from the AC line voltage.

There are 2 related settings: trigger slope (+ means trigger on a rising edge, - means on a falling edge) and threshold level in volts (which can also be + or -). When the input signal passes the threshold level (in the selected direction, with the selected slope), the trace is launched. One can also trigger either trace A or B on the other channel's signal. This is very commonly used when you need to compare two signals' time relationship. The display then shows a plot of both signals with a common t=0, determined by the triggered channel. You can also delay one channel relative to the other, using built-in delay lines. This is needed when you want to trigger on the later of two signals being compared.