文档介绍：Group Delay
Ron Hranac
The Basics
Consider the 6 MHz spectrum occupied by an analog TV channel or digitally modulated signal, the 5-42 MHz upstream spectrum, or any specified bandwidth or passband as the equivalent of a bandpass filter.
Bandpass filter equivalent
The Basics
A signal takes a certain amount of time to pass through a filter
The transit time through the filter is a function of the filter’s velocity of propagation (also called velocity factor)
Velocity of propagation is the speed that an ic signal travels through some medium, usually expressed as a percentage of the speed of light in a vacuum
Transit time and velocity of propagation
The Basics
In many instances the velocity of propagation through a filter varies with frequency
The velocity of propagation may be greater in the center of the filter’s passband, but slower near the band edges
-3 dB point
Passband center
Velocity of propagation versus frequency
The Basics
The finite time required for a signal to pass through a filter—or any device for that matter—is called delay
Absolute delay is the delay a signal experiences passing through the device at some reference frequency
Delay and absolute delay
The Basics
If delay through a filter is plotted on a graph of frequency (x-axis) versus time delay (y-axis), the plot often has a parabola- or bathtub-like shape
Frequency
Time delay
0
1
2
3
4
5
Delay versus frequency
The Basics
The upper trace shows magnitude versus frequency: the filter’s bandpass characteristics. The x-axis is frequency, the y-axis is amplitude.
The lower trace shows group delay versus frequency. The x-axis is frequency, the y-axis is time. Note the bathtub-like shape of the curve.
Network analyzer plot of Ch. T8 bandpass filter
Graphic courtesy of Holtzman, Inc.
The Basics
If propagation or transit time through a device is the same at all frequencies, phase is said to be linear with respect to frequency
If phase changes uniformly with frequency, an output