High-Pass Filter
Understanding High-Pass Filters
High-pass filters are used less frequently than low-pass and bandpass filters but are essential for blocking low-frequency interference and defining band edges. Waveguide is a natural high-pass filter since it has a cutoff frequency.
| Filter Type | Q Factor | Frequency Range | Size |
|---|---|---|---|
| LC Lumped | 50-200 | DC-3 GHz | Small (PCB) |
| Cavity | 1,000-20,000 | 0.1-40 GHz | Large |
| SAW | 500-2,000 | 0.1-3 GHz | Very small |
| BAW/FBAR | 1,000-3,000 | 0.5-6 GHz | Chip-scale |
HPF Topologies
- Lumped-element: Series capacitors and shunt inductors. Works well below 3 GHz.
- Microstrip: Series coupling gaps (capacitors) and shunt stubs (inductors). Practical to 30+ GHz.
- Waveguide: Naturally high-pass. The cutoff frequency is set by waveguide dimensions.
HPF Applications
- DC blocking: Prevent DC from flowing through RF path while passing RF signals.
- Sub-harmonic rejection: Remove unwanted sub-harmonic mixing products.
- Band definition: Define the lower frequency edge of a wideband system.
Frequently Asked Questions
What is a high-pass filter?
An HPF passes signals above its cutoff frequency and attenuates lower frequencies. Used for DC blocking, sub-harmonic rejection, and defining the lower edge of wideband systems. Implemented with lumped elements, microstrip, or waveguide.
Is a DC block a high-pass filter?
Yes. A DC block (series capacitor) is the simplest high-pass filter. It blocks DC while passing RF. At very low frequencies, the capacitor reactance is high, so it also attenuates low-frequency signals near DC.
Why is waveguide inherently high-pass?
Waveguide has a cutoff frequency determined by its dimensions. Below cutoff, waves cannot propagate (evanescent mode). Above cutoff, propagation occurs. This is why waveguide works only above a minimum frequency determined by its size.