Every RF chain is defined as much by what it rejects as by what it amplifies. Filters protect sensitive front ends from out-of-band jammers, keep transmitters legal, and clean up local oscillators. But "a filter" covers four very different technologies — LC, SAW, BAW, and cavity — and each owns a different corner of the frequency-versus-performance map.
The Four Technologies at a Glance
| Technology | Frequency Range | Typical Insertion Loss | Selectivity | Power Handling | Relative Cost |
|---|---|---|---|---|---|
| LC (lumped / distributed) | kHz – ~6 GHz | 0.1–1 dB | Moderate (rolls off gradually) | High (depends on parts) | $ |
| SAW | ~30 MHz – 2.7 GHz | 1–3 dB | Good (steep, close-in) | ~+20–30 dBm | $$ |
| BAW | ~1.5 – 8 GHz+ | 1–2.5 dB | Excellent, temperature-stable | ~+25–33 dBm | $$$ |
| Cavity / waveguide | ~300 MHz – 40 GHz+ | 0.1–0.5 dB | Very high (narrowband) | Very high (kW class) | $$$$ |
LC Filters: The Flexible Workhorse
Built from inductors and capacitors — lumped at low frequency, distributed (microstrip/stripline) at higher frequency — LC filters are fully custom. You choose the topology (Butterworth for flat passband, Chebyshev for steeper skirts, elliptic for the sharpest transitions), the cutoff, and the impedance. The limits are physical: self-resonant frequency of inductors erodes performance above a few GHz, and selectivity costs insertion loss. Use LC where requirements are unique, frequencies are sub-6 GHz, and volumes justify the design effort.
SAW: The Sub-3-GHz Standard
Surface acoustic wave filters convert the electrical signal into a mechanical wave traveling on a piezoelectric substrate — an elegant trick that delivers steep, close-in rejection cheaply at cellular, ISM, and GNSS bands. Watch for two practical constraints: insertion loss (plan 1.5–3 dB into the link budget) and temperature drift — standard SAW center frequencies shift downward as temperature rises, which matters at band edges. Better packages now mitigate this, but check the temperature spec, not just the nominal response.
BAW: Built for the Crowded Bands
Bulk acoustic wave filters trap the acoustic energy in a thin film stack, achieving much higher Q. That translates into steeper skirts, better rejection adjacent to the passband, and far less temperature drift — exactly what crowded mid-band LTE and 5G bands with tiny duplex gaps demand. BAW costs more than SAW and dominates from ~1.5 GHz upward where SAW Q collapses. If your receiver must survive a nearby transmitter in an adjacent band, BAW is usually the answer.
Cavity and Waveguide: When Nothing Else Survives
Mechanical resonant cavities offer the lowest loss and highest Q of all — and the highest power handling — at the price of physical size. Base station TX filtering, radar, satellite, and test equipment live here. Modern implementations add ceramic resonators to shrink the package; the physics (and the selectivity) remain unmatched.
Reading the Datasheet Like an RF Engineer
- Passband edges vs. ripple: a "2.4 GHz filter" is defined by its ripple band, not its center frequency alone.
- Rejection at specific offsets: ask "how many dB at 10 MHz out-of-band?" — the number that protects your LNA.
- Insertion loss plus flatness: loss costs range; ripple costs modulation accuracy (EVM).
- Power handling and linearity: filters in the TX path see real power; SAW/BAW parts degrade under sustained overdrive.
- ESD sensitivity: acoustic filters are piezoelectric structures — handle per MSL/ESD guidance like any sensitive IC.
JTDZ Tech supplies SAW, BAW, and LC filter components across sub-GHz, cellular, GNSS, and ISM bands with datasheets and traceable date codes. Send us your band plan and we will quote options within two business hours.
