-SCS Pulse Card (PGU and PMU) User's Manual Section 2:
4200A-PMU-900-01 Rev. B March 2023 2-3
Cable length
Use the shortest possible cable length to achieve the highest frequency output, the best pulse shape,
and the best results. Here are reasons to avoid using longer cable lengths:
• Longer cable lengths have longer reflection times, which can slow down transmission times.
• Longer cables may have impedance mismatches, which can cause distortions.
• Higher capacitance in longer cables causes higher capacitive charging effects during the pulse
transitions (see PMU capacitive charging/discharging effects (on page 3-35
)).
Only use the white SMA coaxial cables that are supplied with the PMU and RPM. These are 50 Ω
cables that match the internal 50 Ω resistance of the PMU. The PMU is supplied with 6.5 ft (2 m) SMA
cables and the RPM is supplied with 8 in. (20 cm) SMA cables. Always use the 8 in. (20 cm) SMA
cables with the RPM.
High frequency connections
Use these connection guidelines for high-speed testing (pulse width <1 μs).
• Use cables and connectors optimized for high frequency (at least 150 MHz). The SMA coaxial
cables supplied with the PMU and RPM are rated for high frequency.
• Probe manipulators must be rated at least 150 MHz.
• Properly connect the shields of the coaxial cables and minimize the loop area of the shield
connections (see Shield connections (on page 2-2
)).
• Minimize cable length (see Cable length (on page 2-3)).
• Use a signal path that matches the impedance of the instrument (50 Ω). The SMA cables
supplied with the PMU and RPM are 50 Ω.
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