Mohr CT100 - Reflection Coefficient Measurements; Return Loss Measurements

Operating Instructions
Reflection coefficient measurements
The reflection coefficient is the ratio of the signal reflected back from a point of interest to the
signal going into it. It is also is a measure of the impedance change at that point in a cable.
Reflection coefficient is designated by the Greek letter ρ and is sometimes written as rho.
The CT100 lets you measure ρ three different ways:
The first method is the absolute reflection coefficient, which describes the total reflection to the
point of the active cursor compared to the input signal from the pulser/sampler at the beginning
of the cable.
The vertical scale reading at the bottom of the screen in mρ per division is always based off of
the absolute reflection coefficient.
The second and third methods rely on the relative reflection coefficient, which is a measure of
the signal reflected between two points in the cable. For the CT100 this is between the
passive cursor and the active cursor. The CT100 has two ways to calculate the relative
reflection coefficient:
1) In Classic mode, the measurement is related to the input signal at the fault. This mode is
the standard, most accurate way to measure the relative reflection coefficient and is the
default mode.
2) In Tektronix 1502C mode, while the measurement is relative to the passive cursor, it is
scaled to the input signal at the beginning of the cable. This mode was included so that the
CT100 could display reflection measurements that match those measured by the
Tektronix® 1502C. However, it is less accurate in longer cables and after multiple faults,
as it does not take into account intervening signal losses.
Return loss measurements
Return loss is another way of measuring impedance change in a cable. Return loss is given in
decibels (dB) and is always calculated using the relative reflection coefficient. Return loss is
related to reflection coefficient ρ by the formula:
Return Loss=−20⋅log
10
∣  ∣
The larger the fraction of energy in the reflected signal, the lower the numerical return loss
value, so that an open or a short that returns 100% of the signal has a return loss of 0 dB.
Mohr CT100 / CT100HF Operator's Manual 39