What is a true time delay beamformer and when is it needed instead of a phase shifter array?
TTD vs Phase Shifter
Phase shifters introduce a constant phase at all frequencies. The beam angle for a phase-steered array is: sinθ = Δφ/(kd) = Δφ·λ/(2π·d). Since Δφ is constant and λ is frequency-dependent, the beam direction changes with frequency. This is beam squint. For narrowband signals (BW << f₀): the squint is negligible. For wideband signals: the squint can cause significant beam pointing error and gain reduction at the band edges.
| Parameter | Low Gain | Medium Gain | High Gain |
|---|---|---|---|
| Gain Range | 2-6 dBi | 6-15 dBi | 15-45 dBi |
| Beamwidth | 60-360° | 15-60° | 1-15° |
| Typical Types | Dipole, monopole, patch | Yagi, helical, horn | Parabolic, array, Cassegrain |
| Bandwidth | Narrow to wide | Moderate | Narrow to moderate |
| Complexity | Low | Medium | High |
Frequently Asked Questions
When can I use phase shifters?
When the product of fractional bandwidth and maximum scan angle is small: (Δf/f₀) × sinθmax < θ3dB/4. For a 1000-element array (θ3dB ≈ 3°) scanning to 60°: maximum fractional bandwidth for phase shifter operation is approximately 3/(4×0.866×57.3) ≈ 1.5%. For wider bandwidth: use TTD.
Can I combine TTD and phase shifters?
Yes. Sub-array-level TTD with element-level phase shifters is a common hybrid approach. TTD corrects the coarse time-delay error across the array (preventing beam squint), while the phase shifters provide fine beam steering within each sub-array. This reduces the number of expensive TTD elements by the sub-array size.
What about photonic TTD?
Photonic TTD uses optical fiber to create true time delays with very low loss and extremely wide bandwidth (DC to 100+ GHz). The RF signal modulates a laser, propagates through variable-length optical fiber, and is detected by a photodiode. Fiber TTD is used in some military wideband radar systems where no electronic technology can meet the bandwidth and delay accuracy requirements.