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High Pass Filter Board – Isolation Milling

high pass filter board

Milling a high pass filter board with your desktop CNC is a good beginner’s project. Here is how I made mine.

My first project with isolation milling is complete and working. It is a pretty simple circuit, just a high pass filter board to reduce local medium wave overload on my software defined radios. I mounted everything in a 3D printed box with some BNC connectors, shown above.

I wanted to find out two things in particular. First, could I manage to mill a circuit board properly with my Genmitsu 3018 desktop CNC? Would my trace isolation be wide enough to easily solder through hole components without solder overflow. Turns out that I could with a bit of learning on adjusting cutting depth. See my previous article on the build process.

Second, was the medium wave overload happening in the active antenna amplifier or the front end of my SDR? My filter is placed between the amplifier output and the receiver input. Even without any shielding, my high pass filter board provides 35-40 dB of attenuation below 1.7 MHz. Unfortunately, the medium wave interference still shows up on 160 meters. This tells me that the overload is in the antenna amplifier, not an ADC artifact in the receiver. Oh well.

My problem is living too close to local MW transmitters. My good news is that the filter reduces signal strengths enough so that I can listen to broadcast band without adding 30 dB of attenuation in the receiver front end.

The next experiment might be to add a balanced, low impedance high pass filter board between the loop and its amplifier.

High Pass Filter Board Circuit

It’s amazing how much attenuation you can achieve with a few capacitors and inductors. I designed my circuit using an online LC filter design tool. My choice of a 5th order Elliptic Filter only requires five capacitors and two inductors.

However, to get the right values, I needed to pair some capacitors and inductors in parallel using available parts.

Placing this circuit in a shielded box would likely provide better attenuation, but I did not add shielding in this prototype. Later in the year I will experiment with metallic paint for shielding a plastic box.

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