What is the Djordjevic-Sarkar model for dielectric loss and when should I use it?
Wideband Dielectric Modeling
The simple transmission line model assumes constant εr and constant tan δ across frequency. This approximation is reasonable for narrowband circuits but causes errors for wideband designs and time-domain simulations. Real dielectric materials have εr and tan δ that vary with frequency due to multiple molecular relaxation mechanisms operating at different time scales.
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
- 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 is the constant-tan δ model adequate?
For narrowband designs (<10% bandwidth), the constant-tan δ model is adequate because εr and tan δ do not change significantly across the narrow frequency range. Also adequate for initial design estimates where 10-20% loss accuracy is sufficient.
How accurate is the Djordjevic-Sarkar model?
It matches measured dielectric behavior within 5-10% across 4+ decades of frequency (1 MHz to 100 GHz) for typical PCB materials. It is more accurate than the constant model for FR4-type materials whose tan δ doubles from 1 to 10 GHz.
Do commercial simulators support it?
Yes. Keysight ADS, Ansys HFSS, and Cadence (Sigrity) all support the Djordjevic-Sarkar model or equivalent wideband causal dielectric models. It is the recommended dielectric model for SI/PI analysis of high-speed digital designs on FR4 and similar materials.