As a core device for testing and verification of communication systems, the harmonic distortion of the output signal of a communication analog signal source directly affects the signal transmission quality and the accuracy of system performance evaluation. Harmonic distortion originates from the nonlinear characteristics of the circuit, causing the output signal to contain harmonic components that are integer multiples of the fundamental frequency, thus compromising signal purity. To reduce harmonic distortion, comprehensive optimization from multiple dimensions is required, including circuit topology optimization, component selection, feedback mechanism design, filtering techniques, power supply design, electromagnetic compatibility, and layout and routing.
Circuit topology is a key factor affecting harmonic distortion. Traditional nonlinear topologies easily lead to signal distortion, while balanced circuits, through differential signal transmission and symmetrical design, can effectively suppress even-order harmonics. For example, balanced amplifiers, through a dual-ended input/output structure, allow even-order harmonics to cancel each other out internally, thereby reducing total harmonic distortion. Furthermore, employing low-distortion modulation techniques, such as linear frequency modulation or quadrature modulation, can reduce nonlinear distortion introduced during modulation and improve signal fidelity.
Component selection is crucial for harmonic distortion control. As the core component of a signal source, the linearity of a power amplifier directly affects the quality of the output signal. Using field-effect transistors (FETs) or heterojunction bipolar transistors (HBTs) with excellent linearity can reduce nonlinear distortion. Simultaneously, high-linearity mixers can reduce harmonic generation during frequency conversion, while low-loss filters can effectively suppress out-of-band harmonics and prevent harmonic backflow into the signal source. Component parameter matching is also crucial; for example, impedance matching can reduce signal reflection and lower the risk of nonlinear distortion.
Feedback technology is an effective means of reducing harmonic distortion. Negative feedback feeds a portion of the output signal back to the input, compares it with the original signal, and adjusts circuit parameters to suppress nonlinear distortion. Voltage feedback and current feedback can compensate for different distortion mechanisms, while feedforward technology detects the distortion signal and generates a reverse compensation signal to achieve higher-precision distortion correction. Digital predistortion technology, combined with digital signal processing algorithms, can pre-compensate for nonlinearity in the signal, further reducing harmonic distortion.
Filtering technology is a key element in suppressing harmonics. Employing a multi-stage filtering structure, such as LC or π-type filtering, at the signal source output can effectively filter out high-order harmonics. Filter design must balance in-band flatness and out-of-band rejection to avoid introducing new distortion. For broadband signal sources, tunable filters or adaptive filtering algorithms can be used to dynamically adjust filtering characteristics to adapt to different frequency band requirements.
Power supply design is equally important for harmonic distortion control. Power supply noise can couple into the signal source through power pins, introducing additional distortion. Using low-noise regulators and multi-stage filtering circuits can reduce power supply ripple and noise. Simultaneously, a well-planned power distribution network ensures consistent path impedance from the power supply to each load, reducing voltage drop and noise coupling. For high-precision signal sources, isolated power supplies or linear regulators can be used to further improve power supply purity.
Electromagnetic compatibility (EMC) design can prevent external interference from introducing harmonic distortion. Proper wiring layout reduces cross-interference between high-frequency and low-frequency signals; shielding measures and filtering components suppress the impact of external electromagnetic radiation on the signal source. Furthermore, ground plane design must prioritize low impedance and high stability to avoid ground loop interference that could lead to signal distortion.
Layout and routing are the last line of defense for harmonic distortion control. In PCB design, high-frequency signal paths should be separated from low-frequency signal paths to reduce mutual interference; sensitive signals and power signals should be kept at appropriate distances to prevent harmonics generated by power devices from interfering with other components. Simultaneously, a well-planned ground layout reduces ground loop interference and lowers harmonic distortion. By comprehensively optimizing circuit topology, component selection, feedback mechanisms, filtering techniques, power supply design, electromagnetic compatibility, and layout and routing, the output harmonic distortion of communication analog signal sources can be significantly reduced, improving signal quality and system performance.
