Limpkin’s Blog discusses making a 7Hz low pass filter to measure voltage ripple on power supply rails.

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?

The schematic used is below:

– 1x bi-polar 50V 470uF electrolytic capacitor (yes, that’s a thing!)
– 2x 13 Watts (!) RF PIN diodes
– 1x 10k input bleed resistor
I initially went for different PIN diodes…. but they exploded when I first applied 50V at the DC-block input! Thinking about it, 470uF at 50V is around 0.5 Joules so that energy needed to be dissipated somewhere.

Check out the final build and measuring its effectiveness in the post here.