B&K Precision 4034 - Manual - Manual (Page 11)

11
2.8.1 Maintaining Pulse Fidelity
Due to the extremely fast pulse rise times obtained from the instrument, special consideration must be given to
preserve pulse fidelity. Even at low repetition rates, high frequency components are present in the output waveform.
Use high quality coaxial cables, attenuators and terminations.
Note: RG 58 type coaxial cable and typical BNC connectors exhibit impedance tolerances which may cause visible
reflections. For maximum fidelity, use short, high quality, 50
Ω coaxial cables.
When signal comparison measurements or time difference determinations are made, the two signals from the test
device should travel through coaxial cables with identical loss and time delay characteristics.
When making connections that are not in a 50
Ω environment, keep all lead lengths short, 1/4 inch or less.
2.8.2 Impedance Matching
A mismatch, or different impedance in a transmission line, generates a reflection back along the line to the source.
The amplitude and polarity of the reflection are determined by the load impedance in relation to the characteristic
impedance of the cable. If the load impedance is higher than the characteristic impedance of the line, the reflection
will be of the same polarity as the applied signal. If it is lower, the reflection will be of opposite polarity. These
reflections add or subtract from the amplitude of the incident pulse causing distortion and irregular pulse shapes.
Impedance-matching network that provides minimum attenuation
A simple resistive minimum attenuation impedance matching network that can be used to match the instrument
output into relatively low impedance is shown in the above figure. To match impedance with the illustrated network,
the following conditions must exist:
and
Therefore:
( ) R Z R
R Z R
Z
1 2 2
1 2 2
1
+
+ +
=
R
R Z R
Z R
1
1 1 2
1 2
+
+
+