Supply Modulator

The maximum frequency component of the envelope signal that the modulator can reproduce without significant attenuation or phase distortion. This specification must exceed the signal's instantaneous bandwidth to prevent envelope-bandwidth mismatch.

• Typically 3-5x the RF signal bandwidth for 5G NR signals
• Insufficient bandwidth causes clipping distortion and spectral regrowth
• Directly determines the maximum supported carrier aggregation configuration

The maximum rate of change of the modulator's output voltage, measured in volts per microsecond. The slew rate must accommodate the steepest rising edge of the envelope waveform to prevent tracking errors.

• Insufficient slew rate introduces slew-induced distortion at signal peaks
• Critical for high-PAPR signals like OFDM
• GaN-based modulators achieve superior slew rates compared to silicon

A measure of the modulator's ability to suppress ripple and noise from its input power rail from appearing at the output. Poor PSRR allows switching ripple artifacts to intermodulate with the RF carrier.

• High PSRR prevents spurious emissions in the transmitter output
• Critical at the switching frequency and its harmonics
• Degrades receiver desensitization if not adequately suppressed

The ratio of output power delivered to the PA to the total DC input power consumed by the modulator itself. This directly impacts the overall ET system power added efficiency (PAE).

• Linear-assisted switching architectures achieve 85-95% efficiency
• Losses include conduction losses, switching losses, and quiescent power
• A low-efficiency modulator can negate the efficiency gains of envelope tracking

The fidelity with which the modulator's output follows the target shaping function voltage. Nonlinearity in the modulator itself introduces ET modulator nonlinearity that compounds with PA distortion.

• Includes DC offset, gain error, and integral nonlinearity (INL)
• Modulator nonlinearity must be captured in the ET-DPD joint model
• Closed-loop architectures can correct for residual modulator errors

The broadband noise floor at the modulator's output, typically measured in nV/√Hz. This noise directly modulates the PA supply and appears as amplitude modulation (AM) noise on the RF carrier.

• Excessive noise degrades error vector magnitude (EVM)
• Critical for receive band noise to prevent receiver desensitization
• Switching modulators require careful filtering to meet noise specifications

A dynamic power supply technique where the supply modulator's output voltage tracks the instantaneous envelope of the RF signal. Unlike average power tracking, which adjusts voltage on a slot-by-slot basis, ET operates at the carrier bandwidth timescale, dramatically improving power amplifier efficiency during low-amplitude signal moments. The supply modulator is the physical actuator that makes ET possible, converting a baseband-derived envelope reference into a high-current, high-bandwidth supply rail.

The precise time-synchronization of the RF signal path and the envelope tracking supply voltage path at the power amplifier's transistor drain. A mismatch of even hundreds of picoseconds causes the supply voltage to be misaligned with the RF envelope, resulting in severe spectral regrowth and degraded error vector magnitude (EVM). Alignment is typically achieved through digital delay buffers in the baseband processor or FPGA fabric.

A fundamental limitation where the required bandwidth of the dynamic supply voltage exceeds the tracking capability of the supply modulator. The envelope of a wideband signal (e.g., 100 MHz 5G NR carrier) may contain frequency components 3-5x the signal bandwidth. If the modulator's slew rate and small-signal bandwidth are insufficient, the supply voltage clips or slews, introducing nonlinear distortion that the DPD must compensate for.

A deterministic mapping function, often implemented as a look-up table (LUT), that translates the instantaneous baseband signal magnitude into a target supply voltage for the power amplifier. The shaping function defines the operating trajectory on the PA's iso-gain contours, balancing efficiency against linearity. It is the critical interface between the digital baseband and the supply modulator's analog input.

The maximum rate of change of the supply modulator's output voltage, typically specified in V/µs. For wideband signals, the modulator must reproduce fast-rising envelope peaks without introducing tracking errors. Insufficient slew rate causes the supply voltage to lag behind the RF envelope, creating a dynamic voltage error that manifests as AM-AM and AM-PM distortion at the PA output.

Residual high-frequency voltage ripple at the output of a switching supply modulator, caused by the pulse-width modulation (PWM) or hysteretic control loop. This ripple can intermodulate with the RF carrier, creating spurious emissions at offsets equal to the switching frequency and its harmonics. Mitigation requires output filtering, multi-phase interleaving, or hybrid linear-assisted switching architectures.