What is the difference between a bandpass filter and a band reject or notch filter in terms of topology?
BPF vs BRF Topology
The fundamental building blocks are the series resonant circuit (low impedance at resonance, allowing signal to pass) and the parallel resonant circuit (high impedance at resonance, blocking signal flow). A bandpass filter arranges these to create a passband around the resonant frequency: series resonators in the signal path pass the desired band, while shunt parallel resonators short-circuit out-of-band signals to ground.
| Parameter | LC Lumped | Cavity | SAW/BAW |
|---|---|---|---|
| Q Factor | 50-200 | 1,000-20,000 | 500-2,000 |
| Frequency Range | DC-3 GHz | 0.1-40 GHz | 0.1-6 GHz |
| Insertion Loss | 1-6 dB | 0.2-2 dB | 1-4 dB |
| Size | Small (PCB) | Large (machined) | Very small (chip) |
| Tuning | Fixed or varactor | Mechanical screw | Fixed |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
When do I use a notch filter?
Use a notch filter to suppress a specific known interferer while passing the desired signal band. Common applications: suppressing a transmitter's carrier frequency in a nearby receiver, eliminating a specific spur or harmonic, and protecting a receiver from a co-located transmitter.
How narrow can a notch be?
Single-resonator notch filters achieve 0.1-1% bandwidth at 3 dB. Multi-resonator designs achieve wider notch bandwidths (5-20%). The minimum bandwidth is limited by the resonator Q: narrower notches require higher Q. At microwave frequencies, cavity or dielectric resonator-based notch filters achieve the narrowest bandwidths.
What is the insertion loss of a notch filter in the passband?
A well-designed notch filter has very low passband insertion loss (0.1-0.3 dB) because the signal path is nearly transparent at frequencies away from the notch. The loss occurs only near the notch edges where the resonators begin to interact with the signal. Wideband notch filters have slightly higher passband loss.