RF

Group Delay

Group Delay is an indicator that represents how much phase delay of wave occurs over frequency while it is going through a component.  As a save goes through any component, a certain degree of phase delay happens and in addition the degree of the phase delay in most case varies depending on the frequency of the wave.

In a device which operate within narrow frequency range, the Group Delay may not matter much, but in most wide band device like wideband Filters, it is important to keep the Group Delay within a certain value.

Large Group Delay may not cause much issue in terms of energy (power) transfer across the device, but it would cause serious signal distortion and this becomes more serious issue in a wireless digital communication which is using higher order modulation and even more problem in the system which uses OFDM.

Probably the definition in mathematical form would look clearer since the mathematical expression is very simple as shown below. Be aware that 'Delay' and 'Group Delay' is different.

We can think of a couple of possible Group Delay patterns as follows. Horizontal axis represents frequency and vertical axis represents phase (Delay just with a negative sign to make it fit to the definition of Group Delay in mathematical form).

In following case, you see some None-Zero phase (delay) value but the value is same across all frequency range. It means 'phase change over frequency change' is zero and in turn, it means Group Delay is zero. This is a kind of imaginary case, you would not see this kind of plot in real device.

In following case, you see Phase continually changes as frequency changes. But the changing pattern is linear so the 'phase change over frequency' is a constant. It means Group Delay is constant.

Following case would be the most realistic case. Phase keep chainging over Frequency and the rate of phase change over frequency (Group Delay) also changes over frequency.