About DC Block

RF circuit is composed of active and passive device.
Active device generate a power and passive device does not require the power. Amplifer (AMP) is the one of the representative products in active devices.

AMP amplifies the input signal while AMP get DC power and consume a current. if the DC power from the AMP flowing into an equipment that is supposed to get the AC signal only, the circuit will be damaged or out of order. In the worst case, high valued measuring equipment such as network analyzer will be damaged. Therefore, power which is supposed to be supplied to AMP should supply only to AMP and it should not be flowed into any device in RF circuit.

AC signal is input to AMP which is supplied DC at the same time output amplified AC signal as intended. When you monitor the input and output in AMP, Vcc Voltage +5V is biased and AC signal is created as +5V. Overall, it is + signal. Then if DC block is used proper at this situation, DC +5V is blocked out and AC signal is passed so that the amplified signal does not be affected and the circuit could be protected.

At this procedure, Capacitor blocks the DC power, so DC block is named as its functional work. As you have been seen example of AMP as above, DC block can be used any circuit which should block the DC out and pass the AC.

DC block should verify insertion loss(S21) and return loss(S11, V.S.W.R.) form DC to the working frequency. As a circuit drawing in the AMP above, if the DC Block has a big return loss, the signal can¡¯t be flowed to the AMP, returned back from DC block. Amp may not get the signal. It is preferred that insertion loss will be as close as 0dB. If the DC block would have a bad insertion loss, the little survived signal through DC block will be flowed to the AMP.

There are three basic forms of DC blocks. Inner DC block have a capacitor with the center conductor, outer DC block have a capacitor with the outer conductor, and inner/outer DC block have capacitors with both the inner and outer conductors. The DC block is classified by connector type, voltage and maximum frequency range.