Designing a High-Pass RF Filter for 5 GHz Applications: A Step-by-Step Guide

Ever tried to listen to a radio station and heard a hiss that just wouldn’t go away? That hiss is often low‑frequency noise that a high‑pass filter can block. In today’s 5 GHz world—think Wi‑Fi 6E, automotive radar, and satellite links—getting that filter right can be the difference between a clean signal and a garbled mess. Let’s walk through a practical design, a practical step‑by‑step guide, that you can build in the lab or simulate on a laptop.

Why a High-Pass Filter at 5 GHz?

A high‑pass filter (HPF) lets frequencies above a chosen cut‑off pass while attenuating everything below. At 5 GHz we usually want to reject:

• DC and low‑frequency drift from mixers or power supplies.
• Spurious signals from nearby bands (2.4 GHz Wi‑Fi, Bluetooth, etc.).

The result is a cleaner front‑end for amplifiers, mixers, or antennas. In my first graduate lab, I built a crude HPF with a single inductor and a stray capacitor. The filter worked, but the insertion loss was so high I could barely hear the signal. That taught me the value of a systematic approach—something I share on RF Filter Lab every week.

Step 1: Define the Specifications

Before you pick parts, write down what you need. Typical specs for a 5 GHz HPF are:

These numbers give us a target shape for the filter response. If you need tighter ripple or deeper stop‑band, you’ll end up with more sections and a slightly larger board.

Step 2: Choose the Filter Topology

For a simple HPF at microwave frequencies, two topologies dominate:

1. L‑section (single series inductor, shunt capacitor). Easy to build, but limited stop‑band performance.
2. Chebyshev or Butterworth multi‑section (cascade of LC sections). Better control of ripple and attenuation.

Because we want ≤ 0.5 dB ripple and at least 30 dB stop‑band, a 3‑section Chebyshev HPF is a good compromise. It gives us the sharp roll‑off we need without making the layout too crowded.

Step 3: Calculate the Component Values

The classic design equations for a Chebyshev HPF start with a normalized prototype. I use the tables in Pozar’s Microwave Engineering and then scale them to our 5 GHz target.

1. Normalize the prototype to a 1 rad/s cut‑off and 1 Ω source/load.
2. Scale frequency: f_c = 4.5 GHz → scaling factor = f_c / 1 rad/s.
3. Scale impedance: 50 Ω source/load → multiply all reactances by 50.

The resulting values (rounded to standard parts) are:

These are small values, so you’ll need high‑Q surface‑mount inductors (e.g., 0402 or 0201) and NP0 or C0G capacitors for low loss. In my lab I prefer the 0402 size because it gives a little more mechanical stability while still fitting on a compact board.

Step 4: Simulate the Design

Before you solder anything, run a quick simulation. I use Keysight ADS or the free RF Toolbox in MATLAB. Set up a 50 Ω source and load, insert the three LC sections, and sweep from 1 GHz to 8 GHz.

Key things to check:

• −3 dB point should sit near 4.5 GHz.
• Ripple in the pass band stays under 0.5 dB.
• Attenuation at 3 GHz is at least 30 dB.

For deeper insight into testing and tuning high‑pass filters, refer to our dedicated guide. If the ripple is too high, you can tweak the capacitor values by a few femtofarads. If the stop‑band isn’t deep enough, add a fourth section—just remember each extra part adds loss and layout complexity.

Step 5: Layout Considerations

At 5 GHz even a short trace can act like an inductor. Keep these tips in mind:

• Use microstrip on a low‑loss substrate (Rogers RO4350B works well).
• Maintain 50 Ω line width throughout the filter. A calculator can tell you the exact width for your board thickness.
• Place ground vias close to each component to create a solid return path.
• Avoid right‑angle bends; use mitered or curved traces to reduce reflections.

When I first tried a layout with 90‑degree bends, the return loss jumped by 2 dB. A quick redesign with gentle curves brought the performance back on target.

Step 6: Build and Test

Order the parts, assemble the board, and then measure with a vector network analyzer (VNA). Connect the VNA ports to the filter’s input and output, and record S‑parameters.

• S21 shows insertion loss and pass‑band shape.
• S11 tells you how well the filter matches 50 Ω.

If you see a dip in S21 around the cut‑off, double‑check solder joints and component orientation. Small parasitic inductances from the pads can shift the cut‑off by a few hundred megahertz.

Step 7: Tune if Needed

Sometimes the real board behaves a bit differently from the simulation. A practical trick is to add a tiny trimmer capacitor (e.g., 0.1 pF) in parallel with the first shunt capacitor. By adjusting it you can fine‑tune the cut‑off without redesigning the whole board. I keep a small stash of these trimmers in my lab drawer—they’ve saved me more than one deadline.

Step 8: Document the Design

Finally, write down what you did. Include:

• Schematic (PDF or PNG).
• Layout files (Gerber).
• Measured S‑parameters (CSV).

Sharing this on RF Filter Lab helps others avoid the same pitfalls, and it builds a knowledge base we can all rely on.

Designing a high‑pass filter for 5 GHz may sound intimidating, but breaking it into clear steps makes it manageable. Define the specs, pick a suitable topology, calculate component values, simulate, lay out carefully, test, and fine‑tune. Follow this recipe and you’ll have a clean, low‑loss filter ready for any modern RF system.