FEDEVEL
Platform forum

Shiro , 01-30-2026, 09:28 AM

Hello, I am trying to understand this schematic.

In the design, the engineer placed a 100 pF capacitor (likely a high-Q, RF-grade capacitor) in series between the SMA connector and the RF switch as a DC-block.

However, at 70 MHz, this capacitor has a reactance of about 22.7 Ω, which is not negligible in a 50 Ω system. Wouldn’t this degrade the impedance match and cause noticeable insertion loss at the lower end of the band?

I am confused about how to properly choose a DC-blocking capacitor for a wideband RX front-end. If I choose a small capacitor, its reactance becomes large at low frequencies. If I choose a larger capacitor (e.g., 1 nF, 10 nF, 100 nF), its self-resonant frequency becomes lower, and the capacitor starts behaving inductively at higher frequencies.

How is this trade-off typically handled in wideband RF receiver designs, and why would a designer intentionally choose a value like 100 pF in this location? For example, in the HackRF SDR's schematic, we can see the engineer put a 100nF capacitor.

Shiro , 01-30-2026, 11:14 AM

Oh, the results are really interesting. I think the 100 pF capacitor preforms worst than the 1 and 10 nF capacitors. I think it's heavily dependent on the capacitor material and manufacturer.

Shiro , 01-30-2026, 11:14 AM

Red -> 100pF
Pink -> 10000pF
Purple -> 1000 pF

Shiro , 01-30-2026, 11:14 AM

Shiro , 01-30-2026, 12:45 PM