FAQs from Rohde & Schwarz

Measure Low Frequency Signals with DC Offset


I want to measure noise at a very low frequency (20 Hz to 100 kHz).

The signal has a DC offset, but at the input of the analyzer only 0 V DC are allowed with DC coupling.

The DC blocks starts at 7 kHz.

How can I measure these signals without destroying my device?


At these frequencies, an external block capacitor can be used. But a rather high capacity is required to ensure the low cut-off frequency, which will cause high charging currents. In order to protect the RF input, two antiparallel diodes (for example BAR64) as shown in the following circuit diagram are necessary.

These diodes will limit the DC voltage caused by charging the capacitor to a level of around 10 dBm.

The capacitor can be an electrolytic capacitor or two antiserial coupled capacitors with twice the capacity.

Measure Low Frequency Signals with DC Offset