How do I design a suspended substrate stripline filter for millimeter wave frequencies?
Suspended Substrate Filter Design
At millimeter wave frequencies, conventional microstrip filters suffer from high dielectric loss (the substrate dominates loss above 20-30 GHz) and tight fabrication tolerances (trace widths of a few mils on thin substrates). Suspended substrate stripline addresses both issues by surrounding the substrate with air, reducing the effective dielectric constant and concentrating the electromagnetic field in the low-loss air regions.
| 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 |
Frequently Asked Questions
What Q improvement does suspension provide?
Typically 2-4× improvement over microstrip on the same substrate at the same frequency. At 40 GHz: microstrip on 5 mil alumina achieves Qu ≈ 200; SSS on the same substrate achieves Qu ≈ 500-700. The improvement comes primarily from reducing dielectric loss (air has essentially zero loss).
Is SSS difficult to manufacture?
The precision-machined channel adds cost compared to open microstrip. The substrate must be accurately positioned and supported without distortion. However, the wider trace dimensions compared to microstrip relax the photolithography requirements. Overall, SSS filters are moderately more expensive than microstrip but significantly cheaper than solid-metal waveguide filters.
Can I use SSS for other components?
Yes. SSS is used for couplers, power dividers, baluns, and matching networks at mmWave frequencies. The lower effective dielectric constant and higher Q benefit all passive circuit elements. SSS is particularly valuable for narrowband circuitsfilters, oscillator resonators) where Q directly determines performance.