DC Block and matching Network

1. Seldom do we use DC-blocking capacitors to alter raw impedance. That is, we don’t regard them as part of matching networks. Therefore, there are 3 common combinations: The first one is (DC-blocking capacitor + pi-type) The second one is (DC-blocking capacitor + T-type) The third one is (DC-blocking capacitor + L-type) 1 / 4

2. Q. Does a DC-blocking capacitor affect impedance? A. Certainly. But we don’t want it to do so. Thus, we often choose the capacitor whose self-resonant frequency is close to the operating frequency. For example, if the operating frequency in your application is approximately 1000 MHz, you ought to choose 100 pF since its self-resonant frequency is close to 1000 MHz most. As mentioned earlier, we don’t wantit to affect impedance. And as shown below, within the frequency range from 100 MHz to 5500 MHz, the 100 pF capacitor is close to 50 ohm most in 1000 MHz. To put it another way, the 100 pF capacitor deviates from raw impedance minimally under 1000 MHz. 2 / 4

3. Q. Can I regard a DC blocking-capacitor as part of matching network to save PCB layout space and BOM cost ? A. This approach is workable as well. Actually, you’re able to regard (DC- blocking capacitor + L-type) as T-type. It’s also workable when you wanna make a DC-blocking capacitor become part of matching network. Nevertheless, there is one limitation. 3 / 4

4. As shown above, once there MUST be a series capacitor in the L-type matching network, the shunt component is usually an inductor. So, with that, if the raw impedance is within forbidden region, it’s impossible for us to move the point to the center where 50 Ω lies. Namely, making a DC-blocking capacitor become part of matching network reduces the flexibility of impedance tuning since the series component MUST be a capacitor. Thus, this approach is workable, but not so suitable. If possible, it’s better NOT to make a DC-blocking capacitor become part of matching network. 4 / 4