Harmonic Distortion Suppression

The first line of defense against harmonic emissions. Low-pass, band-pass, and cavity filters are placed directly after the power amplifier to attenuate integer multiples of the carrier frequency.

• Cavity Filters: Provide the lowest insertion loss for high-power base stations but are physically large and narrowband.
• Surface Acoustic Wave (SAW) Filters: Offer excellent out-of-band rejection in a small form factor for handsets.
• Tunable Filters: Adapt to frequency changes, critical for software-defined radios operating across multiple bands.

Filtering alone is insufficient for wideband signals where harmonics overlap with the fundamental band.

The dominant active linearization technique. DPD injects an inverse nonlinearity into the baseband signal before the power amplifier, such that the cascaded response is linear.

• Memory Polynomial DPD: Compensates for both static nonlinearity and dynamic memory effects.
• Volterra Series DPD: A more general model capturing complex interactions but at higher computational cost.
• Neural Network DPD: Emerging approach using deep learning to model highly complex GaN amplifier behaviors.

DPD is essential for meeting ACLR specifications in 4G/5G infrastructure without excessive power back-off.

A legacy but still relevant hardware technique. The system splits the signal into two paths: a main amplifier path and an error amplifier path.

• Signal Cancellation Loop: Extracts a sample of the distortion by subtracting the input from the attenuated output.
• Error Injection Loop: Amplifies the isolated distortion and injects it 180 degrees out of phase at the output coupler.

Feed-forward offers ultra-wide bandwidth correction independent of signal modulation but suffers from poor power efficiency due to the auxiliary amplifier.

A highly efficient transmitter architecture that inherently suppresses distortion. The modulated signal is decomposed into envelope and phase components.

• Limiter: Strips the amplitude modulation, creating a constant-envelope phase signal for a highly efficient saturated amplifier.
• Envelope Modulator: Re-applies the amplitude information to the PA supply voltage.

EER, also known as the Kahn technique, allows the use of nonlinear but highly efficient switch-mode amplifiers (Class D/E/F) while maintaining linear output.

An intrinsic design technique applied at the transistor level. By presenting specific impedances at harmonic frequencies to the device's drain or collector, the generation of harmonic energy is suppressed at the source.

• Class-F/F⁻¹ PA: Uses resonant harmonic terminations to shape voltage and current waveforms, ideally achieving 100% efficiency with zero harmonic output.
• Class-J PA: Provides a continuous space of high-efficiency modes with less stringent harmonic termination requirements.

This technique is fundamental to GaN HEMT amplifier design for 5G massive MIMO arrays.

Circuit topologies that inherently cancel even-order harmonic distortion through symmetry.

• Push-Pull Amplifier: Two active devices operate 180 degrees out of phase. The fundamental components combine in phase, while even-order harmonics cancel in the output transformer or combiner.
• Balanced Amplifier: Uses quadrature couplers to split and recombine signals, improving input/output return loss and providing redundancy.

These architectures are highly effective at suppressing the second harmonic (H2), which is often the most problematic due to its proximity to the fundamental band.

The fundamental nonlinear mechanism that generates harmonic and intermodulation products. When multiple signals pass through a nonlinear device, IMD creates spurious frequency components at sums and differences of integer multiples of the original frequencies.

• Third-order IMD (IMD3) products fall closest to the carrier and are the most difficult to filter
• IMD products can land in-band (causing EVM degradation) or out-of-band (causing spectral regrowth)
• The intercept point (IP3) is a figure of merit used to characterize a device's linearity and predict IMD levels

The process of reducing adjacent channel leakage caused by power amplifier nonlinearity. Harmonic distortion suppression directly targets the out-of-band spectral components that constitute regrowth.

• ACLR (Adjacent Channel Leakage Ratio) is the primary compliance metric, typically requiring -45 dBc or better for 4G/5G
• Spectral regrowth is essentially the frequency-domain manifestation of time-domain harmonic and intermodulation distortion
• Mitigation combines digital predistortion for active cancellation and cavity filtering for passive attenuation

Passive suppression of harmonic content using radio frequency filters placed after the power amplifier. While essential, filtering alone is often insufficient for wideband signals.

• Low-pass filters attenuate all harmonics above the fundamental carrier frequency
• Band-pass filters provide channel-selective suppression but introduce insertion loss that reduces transmitted power
• Cavity resonators offer extremely high Q-factor filtering for narrowband harmonic suppression in high-power broadcast and radar systems
• Filtering becomes challenging when harmonics fall close to the fundamental, as in wideband multi-octave systems

An active linearization technique that pre-distorts the input signal with the inverse nonlinear characteristic of the power amplifier. This causes the cascaded DPD+PA system to behave linearly, inherently suppressing harmonic and intermodulation distortion.

• DPD operates in the digital baseband domain, allowing adaptive, software-defined correction
• Memory polynomial models capture both static nonlinearity and dynamic memory effects that contribute to harmonic generation
• Modern DPD systems achieve 20-30 dB of ACLR improvement, far exceeding what passive filtering alone can provide

A circuit-level technique where the power amplifier's output matching network is designed to present specific impedances at harmonic frequencies. This controls the harmonic energy generated at the transistor's intrinsic current source.

• Class-F amplifiers use short-circuit terminations at even harmonics and open-circuit terminations at odd harmonics to shape voltage and current waveforms for maximum efficiency
• Inverse Class-F reverses this termination strategy
• Harmonic load-pull measurements characterize the optimal harmonic impedances for simultaneous efficiency and linearity
• Proper harmonic termination can reduce the burden on external DPD and filtering systems

A legacy but still relevant active cancellation architecture that extracts the distortion component from the power amplifier output and injects it back out-of-phase to cancel the distortion at the system output.

• Uses two parallel signal paths: a main amplifier path and an error amplifier path
• The error path isolates the distortion by subtracting a scaled, delayed version of the input from the PA output
• Offers broadband distortion cancellation without the bandwidth limitations of DPD feedback loops
• Primarily used in multi-carrier base stations where extreme linearity is required and power efficiency is a secondary concern