# TDR Vertical Units and Scaling Four vertical unit types are available in both ZTDR™ and XTDR™ software from HYPERLABS: voltage (mV), normalized voltage (Norm), impedance (Ohm), and reflection coefficient (Rho).

With the exception of impedance (see below), the general shape of the waveform doesn't change between unit types. However, it is still useful to know why these units are used and how they are derived.

### Voltage (mV)

The default vertical unit in HYPERLABS software is voltage in mV.

All of our TDR instruments generate a 250 mV pulse with a fast rise time. The software then analyzes the reflected voltage.

This unit has a range of -250 to +250 mV, with 0 mV being the baseline equivalent of a 50 Ohm transmission line.

### Normalized Voltage

A related unit of measure is Normalized Voltage, abbreviated to "Norm" in ZTDR™ and XTDR™ software.

As the name indicates, Norm is a ratio that normalizes 0 mV (or 50 Ohm) to 1.0. The range of this unit in our software is 0.0 (perfect short) to 2.0 (indicating a perfect open).

### Impedance (Ohm)

One of the most common applications of TDR technology is impedance characterization of transmission lines in cables and circuit boards. For these applications, Ohm is the simplest unit of measurement to understand.

Output in impedance is more nonlinear than mV, Norm, or Rho. This is because impedance (Z) is derived from the following formula: Z = 50 * (1 + Rho) / (1 - Rho).

Impedance is shown as a positive numeric value. The most frequently used transmission line in the test and measurement is 50 Ohm coax. Many differential lines have 100 Ohm differential, 50 Ohm single-ended.

The range of this unit in HYPLERABS software is 0 Ohm (short circuit) to 500 Ohm (open circuit).

### Reflection Coefficient (Rho)

Rho is a ratio of the reflected signal over the incident signal (250 mV).

In Rho, 0.0 is the baseline of a 50 Ohm transmission line. A value of -1.0 indicates a perfect short circuit, while 1.0 is a perfect open.