FAQs from Rohde & Schwarz

RT-ZSxx – more noise with active probes?


I have compared passive and active probes. It seems that the noise with active probes is higher. What is the reason and what can I do to reduce the noise?


The effect always happens if there are some leads between the probe and the DUT. These leads act as an antenna. Active probes are more sensitive and have a higher bandwidth. These "features" amplifies that effect.

Passive probes may also show that - it depends how the probe is connected. In addition passive probe have a higher load at higher frequencies. Higher frequency components are suppressed which also reduces the noise.

To reduce the effect, there are some possibilities:

  • Use the acquire mode "high resolution" or "average".
  • Reduce the bandwidth in the channel menu.
  • The leads should be as short as possible.
  • If possible connect the probe as shown in the drawing:
RT-ZSxx – more noise with active probes