Editing Time-domain reflectometer (section)

=== Incident signal ===
TDRs use different incident signals. Some TDRs transmit a [[pulse]] along the conductor; the resolution of such instruments is often the width of the pulse. Narrow pulses can offer good resolution, but they have high frequency signal components that are attenuated in long cables.  The shape of the pulse is often a half cycle sinusoid.<ref>1983 Tektronix Catalog, pages 140–141, the 1503 uses "1/2-sine-shaped pulses" and has a 3-foot resolution and a range of 50,000 feet.</ref> For longer cables, wider pulse widths are used.

Fast [[rise time]] steps are also used.  Instead of looking for the reflection of a complete pulse, the instrument is concerned with the rising edge, which can be very fast.<ref>1983 Tektronix Catalog, pages 140–141, the 1502 uses a step (system rise time less than 140&nbsp;ps), has a resolution of 0.6&nbsp;inch and a range of 2,000&nbsp;feet.</ref>  A 1970s technology TDR used steps with a rise time of 25&nbsp;ps.<ref>1983 Tektronix Catalog, page 289, S-52 pulse generator has a 25-ps risetime.</ref><ref>{{citation |title=S-6 Sampling Head |series=Instruction Manual |date=September 1982 |publisher=Tektronix |location=Beaverton, OR}}  First printing is 1982, but copyright notice includes 1971.</ref><ref>{{citation |title=7S12 TDR/Sampler |series=Instruction Manual |date=November 1971 |publisher=Tektronix |location=Beaverton, OR}}</ref>

Still other TDRs transmit complex signals and detect reflections with correlation techniques. See [[spread-spectrum time-domain reflectometry]].