A high-frequency (HF) notch filter is a specialized band-stop circuit engineered to severely attenuate a narrow, specific slice of the RF or microwave spectrum while allowing frequencies above and below that notch to pass with minimal insertion loss. If you are building a software-defined radio (SDR), designing a 5G front-end, or debugging EMI failures on a high-speed digital board, a broadband low-pass filter won't save you when a single, massive interferer is desensitizing your receiver. You need a surgical tool that removes the offending signal without destroying the adjacent spectrum.
What an HF Notch Filter Actually Changes in Your Circuit
When you insert an HF notch filter into a signal path, it changes the noise floor and dynamic range requirements of your downstream components. By carving out a 40 dB to 60 dB hole at a specific frequency, you prevent a strong local interferer from driving your low-noise amplifier (LNA) into compression or causing intermodulation distortion (IMD) in your mixer.
Topology Comparison: Choosing the Right HF Notch Filter
Not all notch filters are created equal. The topology you choose dictates your insertion loss, physical footprint, and cost. Below is a data-dense comparison of the four primary topologies you will encounter when sourcing or designing RF front-ends.
| Topology | Frequency Range | Typical Q-Factor | Passband Insertion Loss | Best Application |
|---|---|---|---|---|
| Lumped LC (Discrete) | 10 MHz – 500 MHz | 30 – 80 | 0.5 dB – 1.5 dB | Prototyping, ham radio, custom SDR front-ends |
| SAW (Surface Acoustic Wave) | 10 MHz – 2.5 GHz | 100 – 500 | 1.5 dB – 3.0 dB | Consumer WiFi/Bluetooth coexistence, GPS L1 |
| BAW (Bulk Acoustic Wave) | 1.5 GHz – 6.0 GHz | 500 – 1500 | 1.0 dB – 2.5 dB | 5G/LTE band edge notching, tight spectral masks |
| Coaxial Cavity / Stub | 300 MHz – 18 GHz | 500 – 2000+ | < 0.5 dB | Cellular base stations, radar, high-power TX |
For hobbyists and bench engineers, lumped LC is the most accessible, though it suffers from component parasitics above 500 MHz. For commercial product design above 1 GHz, engineers almost exclusively rely on Qorvo's overview of SAW and BAW filter technologies or similar integrated acoustic solutions, as discrete inductors simply cannot maintain a high enough Q-factor at microwave frequencies.
Worked Example: Designing a 100 MHz LC Notch for an SDR
Let’s say you are using an RTL-SDR or HackRF to monitor 144 MHz ham band traffic, but a local commercial FM broadcast tower transmitting at exactly 100.0 MHz is overloading your front-end. You need a parallel LC resonant tank placed in shunt (from the signal line to ground) to short out the 100 MHz energy.
Formula: $f_r = \frac{1}{2\pi\sqrt{LC}}$
First, select a standard, high-quality capacitor. At HF, you must use C0G/NP0 ceramic dielectrics; X7R or Y5V capacitors exhibit severe voltage coefficients and high equivalent series resistance (ESR) that will ruin your Q-factor. Let’s choose C = 100 pF.
Rearranging the formula to solve for Inductance (L):
$L = \frac{1}{(2\pi f_r)^2 C}$
Plugging in the numbers:
$L = \frac{1}{(2 \times \pi \times 100 \times 10^6)^2 \times 100 \times 10^{-12}}$
$L = \frac{1}{3.9478 \times 10^{17} \times 10^{-10}}$
L = 25.33 nH
The Real-World Catch: 25.33 nH is not a standard E24 inductor value. You have two choices: use a 27 nH standard inductor and swap the capacitor to 91 pF (which shifts the notch to roughly 101.8 MHz), or use a 22 nH fixed inductor in series with a 5 pF ceramic trimmer capacitor to tune the circuit exactly to 100.0 MHz on the bench. When winding your own air-core inductor for this, use 18 AWG enameled copper wire to minimize skin effect losses at 100 MHz.
Where You Meet HF Notch Filters in Practice
You will encounter these circuits in several critical RF and high-speed digital scenarios:
- SDR and Ham Radio Front-Ends: As shown in the example above, FM broadcast notch filters (often covering 88-108 MHz broadly or targeting a single strong station) are mandatory for SDRs operating in urban environments to prevent receiver desensitization.
- 5G and WiFi Coexistence: Modern smartphones have 2.4 GHz WiFi and 2.5 GHz LTE/5G antennas located millimeters apart. BAW notch filters are used in the 5G receive path to notch out the 2.4 GHz WiFi transmit energy, preventing the phone's own WiFi from deafening its cellular modem.
- EMI Pre-Compliance Testing: When a high-speed digital clock (e.g., a 125 MHz Ethernet PHY clock) generates a harmonic that exceeds FCC Class B radiated emission limits at 375 MHz, engineers will sometimes design a 375 MHz notch filter into the PCB trace routing or add a discrete LC trap near the connector to pass FCC certification.
For off-the-shelf RF solutions, consulting a Mini-Circuits filter selection guide is an excellent way to find pre-packaged, 50-ohm matched notch filters (often labeled as band-stop or trap filters) if you do not want to design the LC matching network from scratch.
Frequently Asked Questions
Can I cascade multiple HF notch filters to block a wider band?
Yes, but with diminishing returns. Cascading two identical LC notch filters will increase the depth of the attenuation (e.g., from -40 dB to -80 dB) and slightly widen the skirts, but it will also double your passband insertion loss. If you need to block a wide band (like the entire 88-108 MHz FM band), you should abandon the notch topology and use a high-pass filter with a cutoff at 115 MHz instead.
Why does my homemade LC notch filter have a high insertion loss in the passband?
Passband insertion loss in a shunt notch filter is usually caused by the Equivalent Series Resistance (ESR) of your inductor and capacitor, or by parasitic capacitance to the ground plane. At HF, a standard off-the-shelf ferrite-core inductor will have high core losses. Switch to an air-core coil or a high-Q ceramic chip inductor, and ensure your PCB layout uses a controlled 50-ohm microstrip with the filter components placed as close to the signal via as possible.
