Schwarzbeck DC-Block 500 DC-Blocking Capacitor

A lot of measurement setups require to decouple the device under test (DuT) from the measurement equipment DCwise. Measurement equipment for RF often provides an impedance of 50 Ω and does not provide galvanic isolation. Hence the measurement equipment would be damaged if direct current would be fed into the device.

To prevent damage to the measurement equipment an isolating capacitor is needed. It should let low frequency or high frequency signals pass through without attenuating them. Direct current should be blocked from the measurement equipment though.

The most important criterion for such a capacitor is to have an impedance as small as possible compared to the system impedance of the measurement device (mostly 50 Ω). Hence it has a low attenuation as well.

Its dielectric strength is also important.

The physical construction of capacitors leads to a compromise when it comes to the usable frequency range. The connecting wires limit the higher frequency range and the capacitance the lower frequency range.

Adapted to the requirements when measuring with a broadband artificial network (BAN) in the automotive division according to ISO 11452-7 the isolating capacitor "DC-Block 500" has been developed. It is built into a robust housing. It features a BNC socket and a BNC plug, hence it can easiliy be looped into an existing measurement setup. 

Exact impedance measurements at high frequencies are always difficult. Thus the insertion loss is measured instead within a 50 Ω system. A 5 Ω resistor would result into an attenuation of 0.42 dB. If the attenuation is smaller than 0.42 dB the resistance is smaller than 5 Ohm (Z < 5 Ω). An ideal capacitor would result in an attenuation of 0 dB.

Typical Insertion Loss