Introduction

In this chapter refinements to pulse reflectometry are presented. First we remove the need for a dc tube between the source tube and the instrument under test. The main improvement this provides is a greater level of convenience but the elimination of the inevitable small discontinuity between the source tube and dc tube is another benefit. Next we show changes which increase the length of the time window in which the object reflections can be measured, allowing the measurement of longer instruments. Using a longer source tube means that there is more time to sample the object reflections before they suffer interference from the source reflections. A post-processing method for source reflection cancellation is also presented.

All experiments presented so far use a pulse-like excitation signal. The obvious advantage of this is to minimise the processing which must be done to determine the input impulse response. However, the corresponding disadvantage is that we are limiting the amount of energy input into the system. This chapter therefore also includes the use of pseudo-random noise signals called maximum length sequences (MLS) in pulse reflectometry. These signals share many properties with white noise, but one significant difference is that the phase response of the system can be extracted in addition to the frequency response.