The intermodulation products are spurious frequency components generated when two or more signals pass through a non-linear device.
The intermodulation products can be of different orders, such as second-order or third-order, depending on the non-linearity, number, and frequency of signals present in the system.
In radio communication, the presence of intermodulation products and intercept points can make the system unreliable.
Noise in a signal causes communication errors.
Noise degrades the performance of a system and is one of the major problems plaguing engineering teams. The mixing of signals with noise leads to poor communication lines. The signal-to-noise ratio (SNR) expressed in dB is an index that gives information about how noisy a system is. Channel noise should be less than the signal for high SNR values and good communication. The SNR of 0 dB represents a noisy system, where the desired signal is distorted completely, and retrieving the same at the receiving end is impossible. Signal noises can vary and include shot noise, partition noise, flicker noise, and intermodulation noise. The products of intermodulation are the root cause of intermodulation noise. In this article, we dive into a discussion on the details of intermodulation products and intermodulation noise.
Generating Intermodulation Noise
Any non-linear device present in a communication system generates intermodulation products. A useful signal gets adulterated with the intermodulation products and causes intermodulation noise. The intermodulation products are spurious frequency components generated when two or more signals pass through a non-linear device. The frequency of the intermodulation products can be within, or outside, the frequency band of the device. The intermodulation products can be of different orders—such as second-order or third-order—depending on the non-linearity, number, and frequency of signals present in the system. One of the reasons for interference in the multi-channel system is the intermodulation products which contain both sum and difference products of frequencies.
Consider two signals of angular frequency, ⍵1 and ⍵2, in an amplification system. There are sum and difference products of frequencies corresponding to each order of intermodulation products. The second-order intermodulation products correspond to (⍵1 ∓ ⍵2) terms. The second-order intermodulation products have frequencies ranging from (⍵1 - ⍵2) to (⍵1 + ⍵2). The third-order intermodulation products are at the frequencies (2⍵1 ∓ ⍵2) and (⍵1 ∓ 2⍵2).
Figure 1, below, illustrates intermodulation products for two signals of frequencies ⍵1 and ⍵2.
Figure 1. Intermodulation products can have either additive or subtractive frequency components.
Figure 2. Intermodulation products intersect the signal output at intercept points.
Intermodulation products are noise sources, and they lower the SNR. The amplitude of the intermodulation product plays a key role in degrading the desired signal. The intermodulation products are also intermittent. As long as the component frequencies required for generating any particular order of intermodulation products are present simultaneously in a system, it generates the products and induces intermodulation noise in the system.
Figure 2 plots the power output, second-order, and third-order intermodulation products against the power input. The slope of the second-order and third-order intermodulation product is two and three times the slope of signal output, respectively. Since the intermodulation products have slopes different from the signal output, they intersect with the output at certain points called intercept points. The intercept points are an important factor in the design of radio receivers. In radio communication, the presence of intermodulation products and intercept points make the system unreliable.
The second-order intermodulation product increases 2 dB for every 1 dB increase in signal. Similarly, the increase in third-order intermodulation products is 3dB for a 1dB increase in signal. In the presence of large signals, the third-order intermodulation products mask out the small signals, and the SNR is reduced. Generally, the intercept points may not be reached in practical communication systems, as the signal compresses after a certain value. However, the intercept points, especially third-order intercept points, are considered a figure of merit in high-speed, high-frequency systems.
The non-linear behavior of a transmitter is due to the amplifier being made up of semiconductor components and passive devices such as circulators, connectors, and combiners. The following list includes the types of intermodulation products in a transmitter:
Single-channel intermodulation product: the intermodulation products are caused by the non-linearity in transmitter amplifier circuits and passive devices.
Multi-channel intermodulation product: the signals in the different channels interfere together to produce multi-channel intermodulation products.
Inter-transmitter intermodulation product: one or more transmitters, or transmitters and non-linear components in a communication system intermodulate to produce various frequency components called inter-transmitter intermodulation products.
Intermodulation product due to active antennas: active component transistors are used in active antennas instead of passive components such as capacitors, inductors, metal rods in passive components. Active antennas intermodulate with each other to produce intermodulation products.
Intermodulation product due to passive circuits: the loose connections and aging of passive elements like waveguides, cables, and connectors generate non-linearity in the system. The intermodulation products due to the passive elements degrade the performance of transmitters and receivers.
Prevent Intermodulation Noise
In wireless communication, there are several best practices to prevent and limit intermodulation products:
Reduce the number of non-linear devices such as diodes, amplifiers in transmitters, and receivers. This can reduce intermodulation products due to antennas and inter-transmitter intermodulation products.
Operate the amplifiers in the linear range. The clipping of the amplitude can be avoided by operating the amplifiers in an active region. The audio RF systems can be shielded from intermodulation products by following this practice.
Avoid poorly designed and low-quality components in the circuit, as the design and quality of circuit components can affect intermodulation noise.
Transmitter shielding-Transmitters should be shielded to avoid the interference between neighboring transmitters or non-linear components.
Antenna installation-Antennas should be placed in a way that there are no obstacles in or around them. Proper antenna installation also takes into account the detuning of the antenna operation by nearby objects.
Transmitter and receiver designs in wireless communication must follow recommendations for limiting intermodulation products and intermodulation, as intermodulation noise is strong enough to interrupt wireless communication service. When designing amplifiers or antennas for transmitters and receivers, remember to consider intermodulation products and their effects on the signal.