Transient Power Supply Modulation

The specific drop in the regulated supply voltage rail during the transient current surge, a direct indicator of the equivalent series resistance (ESR) of the power supply decoupling network. This momentary voltage depression amplitude-modulates the RF carrier, imprinting a unique signature.

• Measurement: Captured via high-speed differential probes at the power amplifier supply pin
• Key Metric: Depth of sag in millivolts and recovery time in microseconds
• Hardware Link: Reflects the impedance of voltage regulator, PCB traces, and decoupling capacitors

The high initial current drawn by the power amplifier and digital logic during the first microseconds of operation. The magnitude and shape of this surge are dictated by the power distribution network (PDN) impedance and the total decoupling capacitance.

• Typical Values: Can reach 2-5x steady-state current in modern GaN PAs
• Fingerprint Element: The exponential decay rate of the inrush current reveals the RC time constant of the supply
• Measurement: Non-invasive estimation via magnetic field probes or shunt resistors

The supply voltage fluctuation directly modulates the gain of the power amplifier, creating an unintentional amplitude modulation (AM) envelope superimposed on the RF burst. This parasitic AM is a deterministic function of the PDN impedance.

• Mechanism: PA gain is proportional to drain/collector voltage; sag causes gain compression
• Result: The transient envelope exhibits a characteristic dip or ripple before stabilizing
• Distinction: Differs from intentional ramp shaping; reveals uncalibrated analog physics

The interaction between decoupling capacitor inductance and capacitance creates a resonant tank circuit. When excited by the transient current step, this network produces a damped sinusoidal ringing on the supply rail that directly modulates the RF output.

• Resonant Frequency: Typically in the 1-50 MHz range, dependent on capacitor values and layout parasitics
• Q-Factor: The damping ratio of the ringing is a unique identifier of capacitor ESR and aging
• Signature Stability: Resonant frequency remains stable over device lifetime, making it a robust feature

A voltage spike on the internal ground reference of an integrated circuit caused by transient current inrush flowing through the parasitic inductance of bond wires and package pins. This ground potential shift appears as a common-mode signal that modulates the output.

• Physical Origin: L(di/dt) voltage drop across ~1-5 nH of bond wire inductance
• Signature: Appears as a high-frequency glitch at the very start of the burst
• Uniqueness: Highly dependent on package geometry and die attach quality, varying even within same wafer lots

The instantaneous self-heating of the power amplifier transistor junction during the high-current turn-on event causes a rapid thermal transient. This temperature shift alters electron mobility and threshold voltage, creating a secondary modulation effect that compounds with the voltage sag.

• Time Constant: Thermal time constants (microseconds to milliseconds) differ from electrical ones
• Compound Signature: The interaction between thermal gain droop and voltage sag creates a unique, non-linear envelope shape
• Modeling: Requires coupled electro-thermal simulation to accurately replicate