Detailed Explanation of Various Interferences in RF System Design

As the core component of electronic devices such as communications, radar, and navigation systems, the design quality of RF systems directly determines their operational performance. Interference remains a persistent challenge throughout the RF system design process, where various types of interference can cause signal distortion, reduced signal-to-noise ratios, communication disruptions, and even equipment damage, severely impacting system functionality. Among these, stray interference, intermodulation interference, harmonic interference, and crosstalk interference are the four most common and significant types affecting RF systems. This article provides a detailed analysis of each interference type, clarifying their causes, characteristics, mechanisms, and key prevention measures during the design phase, offering valuable insights for RF system interference suppression design.

No.1 Spurious Interference

Spurious interference is one of the most common types of interference in RF systems. Its core characteristic is "spurious signals from non-target frequency bands intruding into the useful signal reception frequency band." Essentially, it results from the combined effects of non-ideal transmitter performance and insufficient receiver sensitivity. This issue is particularly prominent in scenarios involving multiple system coexistence and high-power transmission, making it a critical consideration in the multi-frequency band compatibility design of RF systems.

The primary source of spurious interference originates from transmitters. The core mechanism involves the generation of additional spurious signals (referred to as 'spurious radiation') outside the intended frequency band when transmitting high-power useful signals. This occurs due to device imperfections (e.g., amplifier nonlinearity, oscillator phase noise) and system design flaws. Although these spurious signals operate at frequencies outside the transmitter's operational range, they exhibit measurable power intensity.

The sources of spurious emissions can be categorized into three types: First, thermal noise generated and amplified by power amplifiers. As high-power components, RF power amplifiers inherently produce thermal noise during operation. This noise is amplified proportionally with the power gain, resulting in spurious emissions. Second, intermodulation products from the system. The nonlinear characteristics of components in RF systems generate intermodulation signals. When these signals fall within the non-operational frequency band, they become sources of spurious interference. Third, external spurious intrusions. Spurious radiation from other systems or devices within the received frequency range can also cause spurious interference.

The impact of spurious interference exhibits distinct "band overlap dependency": interference only occurs when the frequency of spurious signals falls within a radio frequency system's reception band and their amplitude exceeds a specific threshold. When interference occurs, it directly reduces the signal-to-noise ratio (SNR) of the affected system, causing useful signals to be overwhelmed by spurious signals. This manifests as communication quality degradation (e.g., signal distortion, increased bit error rate), decreased receiver sensitivity, and in severe cases, the receiver may fail to properly demodulate useful signals, resulting in communication interruption.

It should be noted that receivers in jammed systems typically cannot filter out high-amplitude stray signals within their own frequency bands, as their filters are designed for their specific operating bands and lack capability to suppress stray signals within the same band. Therefore, the primary focus for stray interference prevention must be placed at the transmitter end.

Key Points for Radio Frequency Design Prevention and Control

In radio frequency system design, the core of preventing and controlling spurious interference is "suppressing the generation of spurs and blocking the propagation of spurs". Specifically, three key measures can be taken:

1.  Transmitter filtering suppression: Install high-performance filters (such as cavity filters, SAW filters) at the output port of the transmitter to specifically filter out spurious radiation outside the frequency band of the transmitted signal, reduce the power intensity of spurious signals, and ensure that the amplitude of spurious signals is lower than the tolerance threshold of the interfered system.

2.  Calculate isolation levels rationally: Through interference analysis, determine the required isolation level to reduce stray interference's impact on the system to a reasonable range (i.e., without significantly reducing reception sensitivity). In multi-system coexistence designs (e.g., POI combiner schemes), adopt the maximum isolation level among systems as the engineering standard. Enhance inter-system isolation by increasing antenna spacing or implementing shielding structures to block stray signal propagation.

3.  Optimize device selection: Choose radio frequency devices with excellent spurious suppression performance (such as low-spur power amplifiers and low-phase noise oscillators) to reduce spurious generation caused by non-ideal characteristics of devices and suppress spurious interference from the source.

No.2 Intermodulation Interference

Intermodulation interference is a typical interference caused by nonlinear devices in RF systems. Its core mechanism involves the interaction of multiple signals within nonlinear devices, generating new interference signals. In RF systems with multi-channel sharing and coexistence of multiple signals (such as mobile communication systems and radar systems), the impact of intermodulation interference is particularly prominent. Moreover, this interference is highly concealed and challenging to mitigate.

The generation of intermodulation interference must satisfy two core conditions: first, there exist two or more signals (interfering signals) with different frequencies; second, these signals act on nonlinear devices in the radio frequency system simultaneously. All types of devices in radio frequency systems (whether active or passive devices) inherently have certain nonlinear characteristics (ideal linear devices only exist in theory). When multiple signals are input into a nonlinear device simultaneously, the nonlinear transformation of the device will cause these signals to modulate and mix with each other, generating a series of signals with new frequencies (i.e., "intermodulation products").

When the frequencies of these intermodulation products are close to or overlap with the frequencies of useful signals, they will enter the passband of the receiver, interfere with the useful signals, and form intermodulation interference. In mobile communication systems, intermodulation interference is mainly divided into three categories: transmitter intermodulation (radiation of intermodulation products generated by nonlinear devices at the transmitting end), receiver intermodulation (intermodulation transformation of external multiple signals by nonlinear devices at the receiving end), and intermodulation caused by external effects (such as intermodulation caused by antenna mutual coupling and cable interference).

The order of intermodulation interference is determined by the frequency expression of intermodulation products. Common ones include the 3rd order, 5th order, 7th order, etc. Among them, the 3rd order intermodulation interference is the most serious and common type of intermodulation interference. This is because the lower the intermodulation order, the higher the power intensity of the generated intermodulation products, and the more obvious the interference to useful signals; the higher the order, the weaker the power of the intermodulation products, and their impact can be ignored. Therefore, in the design of radio frequency systems, the focus of prevention and control of intermodulation interference is mainly on the 3rd order intermodulation.

The specific generation process of third-order intermodulation: Suppose the frequencies of two input signals are F1 and F2 respectively (F1 > F2). When these two signals act on a nonlinear device simultaneously, two main third-order intermodulation products will be generated, with frequencies of 2F1 - F2 and 2F2 - F1 respectively. Since the frequencies of F1 and F2 are usually relatively close (such as adjacent channels in a radio frequency system), the frequency of the generated third-order intermodulation products will be close to those of F1 and F2, and they are very likely to fall into the receiving frequency band of useful signals, causing severe interference.

The impact of intermodulation interference has "signal dependence" and "nonlinear dependence": the intensity of interference is positively correlated with the signal power input to the nonlinear device. The greater the input signal power, the stronger the power of intermodulation products and the more serious the interference; at the same time, the higher the degree of nonlinearity of the device, the higher the intensity of intermodulation products and the more obvious the interference.

In a radio frequency system shared by multiple channels, intermodulation interference manifests in various forms. For example, intermodulation in mobile station receivers can cause co-channel interference; mutual coupling and intermodulation in base station transmitters can lead to mobile stations incorrectly stopping at channels and an increase in call drop rates. Ultimately, all these will result in a decline in system communication quality, a reduction in receiving sensitivity, and affect the normal networking operation of the system.

Key Points for Radio Frequency Design Prevention and Control

The core of preventing and controlling intermodulation interference is "reducing device nonlinearity and minimizing interference from coexisting multiple signals". Combined with the design scenarios of radio frequency systems, the specific measures are as follows:

1.  Selecting low nonlinearity components: Prioritize RF components with excellent linearity (e.g., high IP3 power amplifiers, low-distortion mixers). IP3 (third-order intermodulation point) is the key metric for evaluating component linearity. Higher IP3 values indicate lower nonlinearity, resulting in weaker third-order intermodulation power.

2.  Optimizing signal allocation and isolation: In systems where multiple signals coexist, rationally allocate signal frequencies to avoid simultaneous input of signals from adjacent channels into the same nonlinear device; increase the isolation between signal paths to reduce mutual coupling between different signals (such as using shielded cables and arranging signal links separately)

3.  Control the power of the input signal: Avoid excessively high signal power input to nonlinear devices to prevent the devices from entering a deep nonlinear region. The signal power can be reasonably controlled through an attenuator to balance linearity and power requirements.

4.  Install intermodulation suppression filters: Install targeted filters at the front end of the receiver to filter out third-order intermodulation products that may fall into the receiving frequency band, and further suppress intermodulation interference.

Quantitative Analysis of Intermodulation Interference (Adapting to Multi-Channel Radio Frequency Systems)

To optimize channel utilization, most radio frequency communication systems employ a multi-channel sharing networking approach (where M mobile stations share N channels, with M significantly exceeding N). Mobile stations select idle channels through base stations for communication. When the system allocates working channels at N equally spaced intervals, the overall intermodulation interference can be categorized into six types. During the design phase, targeted quantitative analysis must be conducted to ensure controllable interference.

1.  Intermodulation interference formed by mobile station receivers: When a base station transmits signals on multiple channels simultaneously, the nonlinear characteristics of the front-end circuit in the receiving part of the mobile station will generate intermodulation products. This type of interference belongs to co-channel interference, and it is necessary to control the intermodulation suppression capability of the base station's transmitted signals.
2. Intermodulation interference caused by the base station receiver: When two or more mobile stations transmit simultaneously in the vicinity of the base station, it can induce intermodulation interference in the receiver. This interference is independent of the interference signal strength (EI) and the useful signal strength (EC), necessitating an improvement in the linearity of the base station receiver.
3.  Intermodulation interference caused by transmitter coupling: Intermodulation products are generated by signal coupling among multiple base station transmitters. These products cause co-channel interference for the mobile station in the same system and spurious radiation for adjacent systems. The intermodulation products must be controlled to be at least 60dB below the carrier power (or less than 25μW).
4. Intermodulation interference caused by mutual coupling of mobile station transmitters: The intermodulation products generated by mutual coupling of multiple mobile station transmitters near the base station may interfere with the base station's reception of signals from mobile stations at the edge of the service area. This is an instantaneous random interference, and it is necessary to optimize the layout of mobile station antennas and improve the isolation.
5.  Intermodulation interference caused by the coupling between the mobile station and the base station transmitter: The intermodulation products generated by the coupling between the two will invade the receiver of the mobile station, affecting its reception performance. It is necessary to optimize the system networking spacing to reduce signal coupling.
6. Intermodulation interference caused by the mutual coupling between the base station and the mobile station transmitter: The intermodulation products generated by their mutual coupling can interfere with the base station receiver's reception of useful signals, necessitating an enhancement in the filtering and suppression capability of the base station receiver.

In engineering design, the first three types of interference should be prioritized with detailed quantitative analysis to ensure the interference level remains below the system's tolerance threshold.

No.3 Harmonic Interference

Harmonic interference, caused by signal harmonic radiation, is a common RF system disturbance alongside spurious and intermodulation interference. The key distinction lies in its source: harmonics from the original signal rather than external spurious or intermodulation products. This phenomenon stems directly from the nonlinear characteristics of RF components, making it particularly prominent in high-power systems like radar transmitters and large-scale communication base stations.

Harmonics are signals with frequencies that are integer multiples of the useful signal frequency. Specifically, the second harmonic (2f0) corresponds to twice the fundamental frequency (f0), the third harmonic (3f0) to three times, and so forth. In theory, an RF transmitter should output a single-frequency fundamental signal. However, in practice, RF components—particularly active devices like power amplifiers, oscillators, and mixers—exhibit nonlinear characteristics. When the fundamental signal passes through these nonlinear components, it undergoes distortion, generating a series of harmonic signals.

The essence of harmonic interference lies in "the emission and intrusion of harmonic signals": When harmonic signals generated by a transmitter are not effectively suppressed, they propagate as radiation. If these signals' frequencies fall within the receiving frequency bands of other RF systems or their own system, they create harmonic interference. Additionally, the nonlinear components in receivers may convert external fundamental signals into harmonic signals, leading to internal harmonic interference.

It should be noted that harmonic power intensity attenuates with increasing order—second harmonic power is the strongest, followed by third harmonic, while higher-order harmonics exhibit weaker power. Therefore, in RF systems, the primary sources of harmonic interference are second and third harmonics, with higher-order harmonics being negligible.

Core Difference from Spurious and Intermodulation Interference

In RF system design, harmonic interference, spurious signals, and intermodulation interference are often confused. The key differences among these three types lie in their interference sources and generation mechanisms. The following detailed comparison helps accurately identify interference types during design:

1.  The interference sources are different: the interference source of harmonic interference is the harmonic of the useful signal itself (2f0, 3f0, etc.); the interference source of stray interference is the stray radiation of the transmitter (non-harmonic type) and the external stray intrusion; the interference source of intermodulation interference is the intermodulation product produced by the interaction of multiple signals.

2.  The mechanisms of generation are different: harmonic interference is caused by a single fundamental signal passing through nonlinear devices, generating harmonics at integer multiples of its frequency; spurious interference results from spurious signals intruding into the received frequency band; intermodulation interference arises when multiple signals modulate each other in nonlinear devices, producing new frequency signals.

3.  The frequency characteristics are different: the frequency of harmonic interference is an integer multiple of the fundamental frequency, with a clear pattern; the frequency of spurious interference has no fixed rule and is randomly distributed in non-operating frequency bands; the frequency of intermodulation interference is a combination of multiple input signal frequencies (e.g., 2F1-F2), which is related to the input signal frequencies.

The influence of harmonic interference has the characteristics of "frequency integer multiple dependence" and "power correlation": whether the interference occurs or not depends on whether the harmonic frequency overlaps with the receiving frequency band of a certain RF system; the intensity of the interference depends on the power of the harmonic signal (the lower the harmonic order, the stronger the power, the more serious the interference).

The specific effects are similar to but more targeted than spurious interference: If harmonic signals fall within a system's own frequency band, they degrade the system's signal-to-noise ratio, resulting in reduced reception sensitivity and signal distortion. If they fall into another RF system's frequency band, they interfere with other systems, causing cross-system communication anomalies. For example, if an RF transmitter operates at 900MHz fundamental frequency with a 1800MHz second harmonic, nearby systems operating at 1800MHz may experience interference from this harmonic, leading to increased bit error rates and communication disruptions.

Key Points of RF Design for Prevention and Control

The core of harmonic interference prevention lies in "suppressing harmonic generation and filtering harmonic radiation". Based on the RF system design process, the specific measures are as follows:

1.  Select low-harmonic components: Prioritize RF components with superior harmonic suppression capabilities, particularly power amplifiers (PAs) as the primary source of harmonic generation. Adopting PAs with high harmonic rejection ratio (HR) can effectively reduce harmonic signal generation at the source.
2. Harmonic filter installation: A harmonic filter is installed at the transmitter output to selectively eliminate second and third harmonics (e.g., for a 900MHz fundamental frequency, a filter capable of filtering 1800MHz and 2700MHz signals is selected), ensuring the harmonic signal power remains below the interference tolerance threshold.
3.  Optimize the working state of RF components by setting their operational parameters to avoid entering the deep nonlinear region (e.g., maintaining the control amplifier's voltage and current within the linear range), thereby reducing nonlinear distortion and minimizing harmonic generation.
4. The design of shielding and isolation is adopted: the harmonic radiation source of the transmitter is shielded (such as using a metal shielding box) to reduce the propagation of harmonic signals; meanwhile, the isolation degree with other RF systems is improved to avoid the intrusion of harmonic signals into other systems.

No.4 Blocking Interference

Blocking interference refers to "receiver performance degradation caused by strong interference signals", characterized by "the interference signal not falling within the target system's frequency band, yet its high power level prevents normal operation". Unlike the first three interference types, this phenomenon occurs when the interference signal's frequency remains outside the target system's reception band, but its extreme power intensity qualifies as "non-band-overlapping interference". Such interference is particularly prone to occur in dense radio frequency environments, including urban base station clusters and radar station vicinity.

The root cause of blocking interference lies in the limited input dynamic range of the low-noise amplifier (LNA) at the receiver's front end. As the core component of RF receivers, the LNA is designed to amplify weak useful signals with a gain level tailored to the signal's strength. It operates within a defined dynamic range: when input power remains within this range, the LNA functions linearly to properly amplify signals; however, exceeding this range may drive the LNA into nonlinear operation or even saturation.

The mechanism of jamming: When a high-power interference signal (operating outside the receiver's frequency range) enters the front-end circuit, the receiver's filters attempt to suppress it. However, due to the signal's excessive strength, some interference still reaches the low-noise amplifier. This pushes the amplifier into its nonlinear region, causing its gain to plummet dramatically. In severe cases, it may completely suppress the amplification of weak useful signals, preventing the receiver from properly demodulating them and resulting in a jamming phenomenon.

The influence of blocking interference has the characteristics of "power dependence" and "non-band overlap", and the core features are three points:

1.  The interference signal frequency is not in the receiving frequency band, the receiver filter can not completely suppress the strong power interference signal, part of the interference signal will invade the low noise amplifier;

2.  The interference intensity is positively correlated with the power of the interference signal. The stronger the power of the interference signal, the more obvious the suppression effect on the low-noise amplifier, and the more serious the blocking interference.

3.  The main consequences of interference are a sharp drop in receiver sensitivity and loss of useful signal amplification capability, which manifests as severe degradation of communication quality, signal interruption, and even potential damage to low-noise amplifiers (in extreme high-power interference scenarios).

Key Points of RF Design for Prevention and Control

The core of blocking interference prevention lies in "enhancing the receiver's anti-blocking capability and reducing strong interference signal intrusion". The specific measures align with RF receiver design, focusing on three key aspects:

1.  Enhancing the anti-blocking capability of low-noise amplifiers: Select amplifiers with high blocking levels (Block Level). Higher blocking levels improve the amplifier's resistance to strong interference signals and reduce the likelihood of entering the nonlinear region. Simultaneously, optimize the amplifier's circuit design to expand its input dynamic range.
2. Optimize the front-end filter design by installing high-performance pre-filters (e.g., high-suppression-band-stop filters) at the receiver's front end to specifically suppress strong interference signals from adjacent and out-of-band frequencies, thereby reducing the power of these interference signals invading the low-noise amplifier.

3.  To improve system isolation: In a multi-system coexistence and strong interference environment, measures such as increasing antenna spacing, adopting directional antennas, and optimizing equipment layout can enhance the isolation between the receiver and strong interference sources, thereby reducing the incident power of strong interference signals and preventing blocking interference.

No.5 Summary of RF System Interference Prevention and Control

In RF system design, the generation of four types of interference—spurious, intermodulation, harmonic, and crosstalk—is closely related to the nonlinear characteristics of components, system layout, frequency band allocation, and isolation design. The core principles for controlling these interferences can be summarized as "source suppression, propagation blocking, and terminal tolerance":

1.  Source suppression: Optimize device selection (choose devices with low nonlinearity, low spurious, low harmonics, and high anti-blocking), set the operating state of devices reasonably to reduce interference signal generation;

2.  Propagation blocking: By filtering, shielding, and isolation designs, the transmission path of interference signals is blocked to prevent them from intruding into the useful signal chain.

3.  Terminal tolerance: It improves the receiver's anti-interference capability (e.g., by enhancing reception sensitivity and expanding the input dynamic range), thereby strengthening the system's resilience to interference signals and ensuring normal operation even when interference exists.

In the actual RF system design, it is necessary to analyze the potential risks of various types of interference based on the system's operating frequency band, power level, and networking method, and formulate personalized interference prevention and control plans. Meanwhile, through simulation testing and field debugging, the interference suppression effect should be verified to ensure the system operates stably and reliably in complex RF environments.