Here’s a concise, technically balanced comparison of SAW (Surface Acoustic Wave) filters and Cavity (Resonator) filters — especially relevant to radio astronomy, RF front-ends, and communications receiver design.
🧩 1. Operating Principle
Feature
SAW Filter
Cavity Filter
Mechanism
Converts RF signal to acoustic waves on a piezoelectric substrate; uses surface wave interference for frequency selection.
Uses resonant cavities (metallic or dielectric) to establish standing EM waves at specific frequencies.
Type
Solid-state, planar device.
Mechanical/volumetric resonator.
⚙️ 2. Frequency Range
Aspect
SAW Filter
Cavity Filter
Typical range
30 MHz – ~3 GHz
300 MHz – >50 GHz
High-frequency use
Limited by acoustic wavelength and material properties.
Excellent for microwave and mm-wave frequencies.
📏 3. Size and Integration
Aspect
SAW
Cavity
Physical size
Very small (a few mm²).
Bulky; depends on resonant wavelength (often cm scale at 1 GHz).
Integration
Easy to integrate in PCB, surface-mount.
Usually external component or in waveguide housing.
🎯 4. Performance Characteristics
Parameter
SAW Filter
Cavity Filter
Insertion Loss
Moderate to high (2–6 dB typical).
Very low (0.2–1 dB typical).
Q factor
Low to medium (~10³).
Very high (~10⁴–10⁵).
Selectivity / Skirt steepness
Moderate; limited by acoustic dispersion.
Excellent; very steep skirts possible.
Power handling
Low (milliwatts).
High (watts to kilowatts).
Phase linearity
Poor to moderate.
Excellent, nearly linear group delay near passband center.
Temperature stability
Sensitive to temperature drift (piezoelectric substrate dependent).
Very stable with proper thermal control.
🔌 5. Cost and Application Context
Aspect
SAW
Cavity
Cost
Inexpensive in volume production.
Expensive (precision-machined).
Typical uses
Handheld radios, cellular, GPS, IF filtering, low-power front-ends.
Base stations, radar, satellite, radio astronomy front-ends.
Custom tuning
Difficult (requires re-fabrication).
Tunable with mechanical adjustments or varactors.
🧠 6. Summary
SAW Filter
Cavity Filter
✅ Pros
Compact, low-cost, easy to integrate; good for IF and moderate frequency RF; predictable response.
Extremely low loss, high selectivity and stability; excellent for narrowband, high-performance RF.
❌ Cons
Limited frequency range and power handling; moderate insertion loss; temperature drift.
Large, heavy, expensive; requires mechanical precision and alignment.
🛰️ In Radio Astronomy Context
For a 1420 MHz hydrogen-line receiver:
SAW filters are suitable for IF stages or preselecting a modest band (e.g. 2–10 MHz bandwidth).
Cavity filters are preferred for front-end image rejection or narrowband RFI mitigation, where phase linearity, low loss, and high dynamic range matter.