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A DC Block to Measure Low Frequencies

Following the making of my DC-coupled low frequency, low noise amplifier board I discovered that a crucial piece of equipment was missing from my toolkit if I wanted to measure voltage ripple on power supply rails: a low corner frequency high pass filter, or DC-block.
Such device allows you to remove the DC component of any signal to only look at “high” frequencies… in short, it’s a capacitor. But surely you must now be wondering the same thing I wondered when I embarked on this project: why doesn’t this exist? Surely people must have had this need before, right?
I wish I had an answer, as when scouting the web I wasn’t able to find any commercially-available DC-block with a corner frequency lower than 100kHz… which while it may be enough for many applications, wasn’t low enough for mines (see a typical lab power supply output noise measurement below).

Skeptical that a DC-block only was a capacitor, I searched online and found this page from Rohde & Schwarz with the above schematics in it.
Looking at the above picture we can see:
- 1 non polarized series capacitor
- 2 PIN diodes at the output
These 2 diodes effectively limit the maximum voltage that can be output by the filter, as when applying a high voltage at the filter input the very same voltage would be seen at the filter output (due to the high capacitor value)… which your measurement instrument might not particularly appreciate.
But why PIN diodes, and what are they?”

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