# Test and Measurement World

## Test and Measurement World Terminology Section

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## Oscilloscope Bandwidth Vs Oscilloscope Rise Time - Difference between Oscilloscope Bandwidth and Oscilloscope Rise Time

This page compares Oscilloscope Bandwidth vs Oscilloscope Rise time and mentions difference between Oscilloscope Bandwidth and Oscilloscope Rise time. There are different parameters required to be studied before selecting oscilloscope for your need. The parameters are bandwidth, sample time, rise time, record length, capture rate, triggering, effective bits, sweep speed, gain accuracy etc.

### Oscilloscope Bandwidth

The term Oscilloscope Bandwidth refers to range of frequencies which can be measured accurately by the Oscilloscope. It is defined as frequency between two points which are 70.7% of amplitude of signal or -3dB on log scale.

As we know when frequency increases oscilloscope's capability to measure the signal accurately decreases. Hence in order to measure accurately, larger bandwidth is needed. If this is not taken care, then amplitude will be distorted or edges will be vanished. This results into loss of crucial details present in the signal under test.

In order to avoid this situation, oscilloscope bandwidth is choosen to be greater than 5 times the highest frequency component present in the signal under analysis. With this consideration, error in the measurement can be kept below +/- 2%. It is difficult to achieve this 5 times rule of thumb for high speed oscilloscopes. In this situation, higher oscilloscope bandwidth is achieved using advanced DSP techniques.

### Oscilloscope Rise Time

The rise time is very useful parameter for oscilloscope selection. Sufficient rise time is required to capture the rapid transitions of the signal under test very accurately.

Oscilloscope rise time can be calculated as per following equation:
Rise Time <= Fastest rise time x (1/5)
Rise time decreases useful frequency range of the oscilloscope.

Oscilloscope with faster rise time is ideal for measuring fast transitions of the signal. Different logic families can provide different rise times. For example, TTL provides 2 ns, CMOS 1.5 ns, GTL 1ns, LVDS 400 ps, ECL 100 ps and GaAs 40 ps. Hence GaAs can provide fastest rise time among all the logic families. GaAs support highest signal bandwidth of 8.75 MHz.

Oscilloscope Bandwidth and Oscilloscope Rise time are related as per following equation:
Bandwidth = K/Rise Time
Where K is between 0.35 and 0.45
For Oscilloscope bandwidth > 1 GHz, K is between 0.4 and 0.45
For Oscilloscope bandwidth < 1 GHz, K is about 0.35