Abstract :
This paper describes a passive time-transformation technique (`Stretch¿) that permits an exchange of signal time duration for bandwidth. It is not limited to electrical signals or to a particular portion of the spectrum. It is linear in the sense that the principle of superposition is applicable. Good waveform fidelity is preserved and signals that are resolvable at the input remain resolvable at the output, even though the bandwidth may be changed by a large factor. Signal/noise ratios are not affected by the transformation. The basic components of the system are: (1) the uniformly dispersive delay device in which envelope delay is a linear function of frequency and phase shift is a parabolic function of frequency, and (2) the heterodyne mixer in which the heterodyne signal is linearly modulated in frequency. By proper combinations of these non-linear phase and amplitude elements, systems can be devised to speed up, slow down, or reverse the time scale of the input signal. In much the same way that a transformer permits matching the impedance properties of a circuit, the technique permits matching of the bandwidth capabilities of the circuit. One of the most obvious applications is to enable the observation or transmission of extremely fast phenomena with economical, low-bandwidth equipment. Other applications include speed-up of signals to permit efficient time-division multiplexing and the generation of accurately controlled fast waveforms. Application of the technique to wide-bandwidth, wide-baseline directional systems will be discussed. Removal of most of the bandwidth takes place at the individual receiving sites. Because the phase and amplitude characteristics of the signals are preserved, remoting, combining, processing and recording of the information can be performed at practical bandwidths.
