Radio frequency harmonics

Harmonic concept: Harmonic refers to the signal whose frequency is an integer multiple of the fundamental frequency after the fundamental signal passes through the nonlinear element in a nonlinear circuit. In the field of radio frequency, the fundamental wave usually refers to the designed or specified operating frequency, and the harmonic is the second harmonic of the frequency, the third harmonic, etc.

Harmonic causes: RF harmonics are mainly derived from nonlinear components or devices in the circuit, such as power amplifiers, mixers, switching power supplies, etc. When the input signal strength exceeds a certain threshold, the output characteristics of these devices will deviate from the ideal linear state, resulting in the generation of harmonic components.

Harmonic effects:

System performance: Harmonics will occupy valuable spectrum resources, reduce the signal-to-noise ratio, increase the bit error rate, and affect the communication quality.

Electromagnetic compatibility: Harmonics may interfere with other devices operating in the same frequency band, causing electromagnetic compatibility problems and violating the requirements of FCC, CE and other regulations on emission limits.

Device interoperability: In an environment where multiple systems coexist, harmonics may cause mutual interference between devices, affecting system stability and reliability.

Test method for radio frequency harmonics

1, spectrum analyzer measurement:

Principle: The spectrum analyzer can display the amplitude distribution of the measured signal at different frequencies, and convert the time domain signal into the frequency domain signal by fast Fourier transform (FFT) of the RF signal, so as to visually display the amplitude, frequency and phase of each harmonic of the base wave.

Steps:

Connect the RF signal to the spectrum analyzer.

Set the appropriate center frequency, resolution bandwidth (RBW) and video bandwidth (VBW) to ensure that harmonics can be resolved and noise can be suppressed.

Observe the spectrogram and record the amplitude of each harmonic and its relative amplitude with respect to the fundamental wave (harmonic distortion coefficient).

For cases where an accurate measurement of total harmonic distortion (THD) is required, the ratio of the sum of the squares of all harmonic components to the square of the fundamental amplitude should also be measured and calculated.

2. Network Analyzer test:

Principle: The network analyzer measures the S-parameter (scattering parameter) in a specific frequency range and can evaluate the reflection and harmonic characteristics of the device.

Steps:

Connect the reception and measurement equipment with the network analyzer to set the appropriate frequency scanning range and step.

Perform S-parameter measurements, focusing on the S11 (reflection coefficient) and S21 (transmission coefficient) curves, to see if there are peaks that are integral multiples of the fundamental frequency, and these peaks correspond to harmonic components.

Calculate harmonic amplitude and THD.

3, special harmonic analyzer test:

Principle: Harmonic analyzers are designed to accurately measure the harmonic content of current or voltage signals with high precision and high dynamic range.

Steps:

Connect the signal to the harmonic analyzer.

Set measurement parameters, such as measurement range, sampling rate, harmonic order, etc.

At the beginning of measurement, the instrument will automatically calculate and display the amplitude, phase and THD parameters of each harmonic.

4. Software-assisted testing:

Principle: Use data acquisition card with computer software for real-time or offline data analysis, and realize FFT or other harmonic analysis algorithms through software.

Steps:

The RF signal sample is obtained by the data acquisition card.

Using software (such as MATLAB, LabVIEW, etc.) for signal processing, perform FFT calculation and draw spectrum diagram.

The spectrogram is analyzed to identify and quantify harmonic components.

Principle of radio frequency harmonic suppression

Rf harmonic suppression is mainly achieved through the following strategies:

A. Linearization technology:

Principle: By improving the working conditions or design of nonlinear devices (such as power amplifiers, mixers, etc.), the output characteristics are closer to ideal linearity, thus reducing harmonic generation.

How:

Predistortion technology: Add a predistortion signal opposite to the nonlinear characteristics in the input signal to offset the harmonics generated by nonlinearity.

Feedback technology: Using a negative feedback mechanism, the output is monitored in real time and the input is adjusted to keep the system working in an approximate linear region.

b, filtering technology:

Principle: By designing a suitable filter, only the fundamental wave is allowed to pass through, and the harmonic component is suppressed or greatly attenuated.

Method: Passive filter (such as LC filter, cavity filter, etc.) : The use of inductors, capacitors and other components to form a frequency selection circuit, a specific frequency (such as harmonic frequency) to present high impedance, prevent its passage.

Active filter: The use of integrated operational amplifiers and other active devices to achieve more flexible frequency selection and higher suppression effect.

c, power synthesis and distribution technology:

Principle: The use of multiple parallel, circuit breaker or splitter technology, disperse the power load of a single device, reduce the degree of nonlinearity of its operating point.

Method: Doherty power amplifier: Through the co-operation of the main amplifier and the auxiliary amplifier, the efficiency is improved and the harmonics are reduced.

d, digital signal processing (DSP) technology:

Principle: The signal is processed in the digital domain, and the harmonics are eliminated or suppressed by algorithms (such as digital pre-distortion, digital filtering, etc.).

Method: Digital predistortion algorithm: The signal is preprocessed in the digital baseband or IF stage, and the distortion contrary to the nonlinear characteristics of the hardware is added to offset the harmonics generated by the hardware.

Digital filter algorithm: At the digital receiving end, the digital filter is used to post-process the signal and filter out the harmonic components.

Radio-frequency spurious

1. Spurious concept: Spurious refers to the irregular frequency components produced by various unexpected factors in addition to the main signal (including the base wave and its harmonics) in an RF system. These components may be distributed near or away from the main signal frequency band.

2. Spurious causes:

   • Internal sources: including harmonic stray caused by nonlinear distortion, sideband stray caused by oscillator phase noise, stray caused by parasitic oscillation, stray caused by power ripple, etc.

   • External sources: strays caused by coupling, crosstalk, antenna reflection, intermodulation, etc.

3. Stray effects:

   • System performance: Spurious will also occupy spectrum resources, reduce receiver sensitivity, increase bit error rate, and affect communication quality.

   • Electromagnetic compatibility: The stray may cause interference to other devices, especially those operating in adjacent frequency bands, which may cause electromagnetic compatibility issues and violate relevant regulatory requirements.

   • Equipment safety: Some specific frequency band stray may have a potential impact on human health, such as high RF energy may cause thermal effects or non-thermal biological effects.

Rf harmonics and spurious suppression strategies

1. Design optimization: The use of high linear RF devices, reasonable layout of the circuit to reduce mutual inductance and mutual capacitance, the use of low noise oscillators, optimize the power supply design, etc., from the source to reduce the generation of harmonics and stray.

2. Filter technology: The use of LC filters, surface acoustic wave filters, microstrip filters and other hardware filters, or software filters in digital signal processing, effectively suppress harmonics and stray in specific frequency bands.

3. Shielding and grounding: Prevent external stray interference through good shielding measures and ensure good grounding to reduce internal stray coupling.

4. Test and calibration: Perform rigorous RF system testing and calibration, including harmonic and stray measurement, to ensure that the system meets the relevant regulatory requirements and the performance indicators of the application.

Rf harmonics and spurious regulatory and regulatory requirements

Countries and regions have strict regulations on the harmonic and stray emission of radio frequency equipment, such as the FCC Part 15 of the United States, the RED Directive of the European Union, and China's "Radio transmission equipment Model approval Management Measures". Manufacturers need to ensure that their products comply with the relevant regulations during the design, production and sales stages, or they may face the risk of fines, recalls and even market bans.

Harmonics and strays in RF are key factors affecting system performance, electromagnetic compatibility and device interoperability. Understanding its concept, causes, effects and countermeasures helps engineers take effective measures to suppress it at the design stage, ensure efficient and stable operation of RF systems, and meet increasingly stringent regulatory requirements. At the same time, for industry regulators, testing and certification bodies and users, understanding harmonics and stray knowledge also helps to improve the ability to evaluate the performance of RF equipment, compliance inspection and use and maintenance.