How should a DC blocking capacitor be inserted between the 42941A probe and a DUT when a live DC voltage exists on the device?

It is recommended to insert a SMA DC blocking adapter between the furnished pin probe and 3.5-mm connector at the tip of the 42941A as shown below:

Select SMA DC blocking adapter, which has appropriate frequency bandwidth, from among the adapters available from RF connector suppliers. The capacitor inside the SMA DC blocking adapter determines the low frequency limit of measurable frequency range. The value of DC blocking capacitor should satisfy the following equation:

For example, when C = 1 µF, the low frequency limit is approx 32 kHz. When the impedance of DUT is lower than 5 Ω, the C value should be proportionately increased inversely to the DUT’s impedance. If appropriate DC blocking adapter is unavailable, it is recommended to fabricate a custom adapter as shown below:

Note: The self-resonant frequency of the DC blocking capacitor must be at least 3 times higher than the maximum measurement frequency.

Open/Short/Load compensation should be performed at the tip of Pin Probe (or other probe adapter) as shown below:

Note: The impedance of DC blocking capacitor is eliminated from the DUT measurement values by performing Short compensation. At high frequencies (MHz region), Load compensation is required for minimizing measurement errors due to the electrical length of the DC blocking adapter.

Learn more about Keysight DC Blocks