Negative Group Delays without Distortion in an Electronic Filter

A negative group velocity occurs for an electromagnetic signal when the group refractive index of the medium, $N_{g}$, is less than zero. The group velocity, described as $v_{g}$ = ($c$/$N_{g})$, can be faster than $c$ or even negative when $N_{g}<$1. All experimental reports of this phenomenon have involved media for which the group velocity was frequency dependent, i.e. $v_{g}=v_{g}$(\textit{$\omega $}). As a result, signals are necessarily distorted because different frequency components travel with different velocities. An interesting circumstance would occur in a medium if the group velocity was negative and \textit{constant }over a certain range of frequencies. Any signal containing only frequency components within this range could experience a negative group velocity, yet not be distorted in the process. The group \textit{velocity} in optics has an analogous effect in electronic circuits called the group \textit{delay}. We present observations of constant negative group delays for electronic signals using a specially tailored amplifier design. The design allows a wide range of frequencies to experience a constant group delay, resulting in negatively delayed signals, without distortion.