Introduction

Delay lines are an important application of superconductors and one in which they have a distinct advantage over other technologies. A delay line consists of a long transmission line, usually microstrip, stripline or coplanar line, which is deposited on to one or more substrates. This chapter is split into two main sections. Section 6.3 discusses delay lines which just delay the signal over a wide bandwidth; in principle the output should be a replica of the input signal but delayed by a certain time. However, the more interesting aspect of delay line technology is discussed in Section 6.5 and the following sections. Here, delay lines are described which perform a filtering or signal processing function and have applications both in electronic warfare and communications.

An idea of the capabilities of superconducting delay lines can be gained by considering the wide microstrip. Figure 6.1.1 shows the attenuation of a wide microstrip calculated from Equation (2.3.6), together with a number of other methods of delaying a signal. It can be seen that it is possible to obtain hundreds of nanoseconds of delay for only several decibels of loss for a superconducting wide microstrip at 10 GHz. Superconducting delay lines offer one of the lowest loss transmission media, even approaching the attenuation of the atmosphere. The transmission along optical fibre is also very low loss and in fact a whole technology related to signal processing using optical fibre has been developed.