Control Chip
How a Control Chip Steers a Phased-Array Channel
Inside a phased-array transmit/receive module, the control chip is the part that decides where the beam points. Each radiating element must apply a precise phase delay and amplitude weight so that the wavefronts from all elements add constructively in the intended direction. Rather than mechanically steering the antenna, the array sets these weights electronically, and the control chip is the silicon that does it. A digital command word arrives over a serial bus, the chip latches it into state memory, and the on-die phase shifter and attenuator switch their cells to the commanded values within a few hundred nanoseconds.
The phase shifter is typically a switched-network design: cascaded sections of high-pass and low-pass filters, or switched delay lines, each selected by a FET switch to add a binary-weighted phase increment. The digital step attenuator works the same way, switching resistive pi or tee pads in and out to build the commanded attenuation from binary-weighted bits. Because both blocks are built from passive networks and FET switches, they consume almost no DC power and are reciprocal, so the same phase shifter and attenuator cells set amplitude and phase identically whether the signal flows toward the antenna on transmit or toward the receiver on receive. The T/R switch, not a reversal of the control cells, is what selects the active path.
Integration is the reason designers prefer one control chip over discrete parts. Combining the phase shifter, attenuator, T/R switch, and logic onto a single GaAs pHEMT or SiGe BiCMOS die shrinks the module footprint, reduces interconnect loss between blocks, and lets a single serial interface program the entire channel. In dense arrays at 28 GHz and above, one core-chip die may serve four radiating elements, with beam-state tables stored in on-chip memory so the array can hop between preloaded beam positions in a single clock cycle.
Phase and Amplitude State Equations
ΔφLSB = 360° / 2N (6-bit → 5.625°, 5-bit → 11.25°)
Beam-Steering Phase per Element:
φn = (2π / λ) × n × d × sin(θscan)
Attenuator Dynamic Range:
Amax = (2N − 1) × LSBdB (6-bit, 0.5 dB LSB → 31.5 dB)
Switch Figure of Merit:
FoM = Ron × Coff ≈ 250 fs (GaAs pHEMT)
Where N = bit count, λ = wavelength, d = element spacing, θscan = scan angle, Ron = on-state resistance, Coff = off-state capacitance. Example: a 6-bit chip resolves the 360° circle into 64 phase states with 5.625° granularity.
Control Chip Function Comparison
| Function | Typical Resolution | Insertion Loss (X-band) | Process | Role in Module |
|---|---|---|---|---|
| Digital phase shifter | 5 to 6 bit (11.25° / 5.625°) | 4 to 7 dB | GaAs pHEMT / SiGe | Beam pointing |
| Digital step attenuator | 5 to 6 bit (0.5 dB LSB) | 3 to 6 dB | GaAs pHEMT / SOI | Amplitude taper |
| T/R switch (SPDT) | 1 bit | 0.8 to 1.5 dB | GaAs / GaN | Transmit / receive routing |
| Core chip (combined) | 6-bit phase + 6-bit atten | 8 to 12 dB | SiGe BiCMOS / GaAs | Full channel control |
| Serial control logic | SPI / parallel, on-die memory | n/a | CMOS / BiCMOS | State loading and storage |
Frequently Asked Questions
What is the difference between a control chip and a core chip?
The terms overlap. A control chip is any MMIC whose job is to set amplitude and phase rather than amplify or generate signal, so a standalone phase shifter, digital step attenuator, or SPDT switch each qualifies. A core chip is a higher-integration control chip that combines a 5 or 6-bit phase shifter, a digital attenuator, T/R switching, and serial logic on one die. Every core chip is a control chip, but a single-function control chip is not a core chip.
What semiconductor processes are used for RF control chips?
GaAs pHEMT dominates X-band through Ka-band thanks to low on-resistance switches (Ron × Coff ≈ 250 fs) and low-loss cells. SiGe BiCMOS is favored at 28 and 39 GHz 5G mmWave because it integrates control logic and beam-state memory on-die at lower cost for large arrays. GaN HEMT serves high-power switches and limiters, while SOI CMOS covers cost-sensitive sub-6 GHz switch and attenuator products.
How many phase and amplitude states does a typical control chip provide?
A 6-bit phase shifter gives 64 states at a 5.625° LSB across the full 360° range; a 5-bit version gives 32 states at 11.25°. A 6-bit digital step attenuator provides 31.5 dB in 0.5 dB steps. Good X-band parts hold RMS phase error under 4° and RMS amplitude error under 0.4 dB. States load over a serial peripheral interface with update times in the tens to hundreds of nanoseconds.