Performance Terms
The terms described in this section may come up in your discussions about oscilloscope performance. Understanding these terms will help you evaluate and compare your oscilloscope with other models.
Bandwidth
The bandwidth specification tells you the frequency range the oscilloscope accurately measures.
As signal frequency increases, the capability of the oscilloscope to accurately respond decreases. By convention, the bandwidth tells you the frequency at which the displayed signal reduces to 70.7% of the applied sine wave signal. (This 70.7% point is referred to as the "-3 dB point," a term based on a logarithmic scale.)
Rise Time
Rise time is another way of describing the useful frequency range of an oscilloscope. Rise time may be a more appropriate performance consideration when you expect to measure pulses and steps. An oscilloscope cannot accurately display pulses with rise times faster than the specified rise time of the oscilloscope.
Vertical Sensitivity
The vertical sensitivity indicates how much the vertical amplifier can amplify a weak signal. Vertical sensitivity is usually given in millivolts (mV) per division. The smallest voltage a general purpose oscilloscope can detect is typically about 2 mV per vertical screen division.
Sweep Speed
For analog oscilloscopes, this specification indicates how fast the trace can sweep across the screen, allowing you to see fine details. The fastest sweep speed of an oscilloscope is usually given in nanoseconds/div.
Gain Accuracy
The gain accuracy indicates how accurately the vertical system attenuates or amplifies a signal. This is usually listed as a percentage error.
Time Base or Horizontal Accuracy
The time base or horizontal accuracy indicates how accurately the horizontal system displays the timing of a signal. This is usually listed as a percentage error.
Sample Rate
On digital oscilloscopes, the sampling rate indicates how many samples per second the ADC (and therefore the oscilloscope) can acquire. Maximum sample rates are usually given in megasamples per second (MS/s). The faster the oscilloscope can sample, the more accurately it can represent fine details in a fast signal. The minimum sample rate may also be important if you need to look at slowly changing signals over long periods of time. Typically, the sample rate changes with changes made to the sec/div control to maintain a constant number of waveform points in the waveform record.
ADC Resolution (Or Vertical Resolution)
The resolution, in bits, of the ADC (and therefore the digital oscilloscope) indicates how precisely it can turn input voltages into digital values. Calculation techniques can improve the effective resolution.
Record Length
The record length of a digital oscilloscope indicates how many waveform points the oscilloscope is able to acquire for one waveform record. Some digital oscilloscopes let you adjust the record length. The maximum record length depends on the amount of memory in your oscilloscope. Since the oscilloscope can only store a finite number of waveform points, there is a trade-off between record detail and record length. You can acquire either a detailed picture of a signal for a short period of time (the oscilloscope "fills up" on waveform points quickly) or a less detailed picture for a longer period of time. Some oscilloscopes let you add more memory to increase the record length for special applications. |
