Intermodulation distortion (IMD) is well known to audio circuit designers as a barrier to high fidelity. It is also becoming better known by higher-frequency circuit designers, even in passive circuits such as antennas, cables, and connectors, in the form of passive intermodulation (PIM). Quite simply, PIM causes interference. PIM results from nonlinearities in a passive circuit. Transmitted signals consist of fundamental tones and their harmonics. When two or more signals are physically near each other and transmitted at the same time, nonlinearities in the circuit can cause the different signal tones to mix and generate additional, spurious signals which can block or interfere with the reception of the desired signals at the receiver.
Unwanted levels of PIM can result from several different factors, including the amplitudes and frequencies of the multiple transmitted tones, the configuration of a circuit’s transmission lines, and the current density and power level of the application. Multiple signal tones are usually denoted by their fundamental frequencies, such as f1 and f2, with the frequencies of their PIM spurious signals resulting from the differences between different harmonics of the fundamental tones.
If f1 and f2 represent fundamental tones of increasing frequency, the total number of signals resulting from a high-PIM design and just the second and third harmonics of each fundamental tone, such as 2f1 and 3f1 and 2f2 and 3f2, would generate as many as four additional signals close in frequency to the original two tones: 3f1 – 2f2; 2f1 – f2; f1; f2; 2f2 – f1; and 3f2 – 2f1. Depending upon the design of the transmitter/receiver combination, the PIM-generated spurious signals may fall within the bandwidth of the receiver and interfere with the reception of the two desired tones.
For example, in the cellular frequency range, initial f1 and f2 transmit tones of 890 and 892 MHz, respectively, would result in 2f1 of 1780 MHz, 3f1 of 2670 MHz, 2f2 of 1784 MHz, and 3f2 of 2676 MHz. In addition to the f1 and f2 tones of 890 and 892 MHz, the resulting PIM-generated spurious signals would be 3f1 – 2f2 = 886 MHz; 2f2 – f2 = 888 MHz; 2f2 – f1 = 894 MHz; and 3f2 – 2f1 = 896 MHz. The third-order (2f1 – f2) IM spurious signals have higher amplitude and are more problematic than the fifth-order (3f1 – 2f2) IM spurious signals. Because of this, it is typically the third-order IM which is considered for PIM evaluations rather than fifth- or higher-order IM spurious products.
As might be evident, receiver channels that closely surround the transmit frequencies in a communications channel can fall victim to high PIM levels. The amplitudes of these PIM-generated signals are largely dependent to the power levels of the initial fundamental tones. It should also be noted that this example is just showing two tones and their second and third harmonics; generated PIM can quickly become quite complex with a greater number of fundamental tones and at higher signal amplitude where more harmonics are involved in the mixing process.
Pointing to the Metal
PIM is often blamed on the quality of metal-to-metal contacts at wireless base stations and other communications systems, such as coaxial connector interfaces. Those parts of a system that handle higher signal power levels at higher current densities, such as the connector interfaces at transmitters, can suffer nonlinearities from poorly fitting contacts or from inconsistent metal-to-metal contacts because of dirt or oxidation or other forms of contamination on the metal surfaces that are part of the circuit. When these loose or soiled metal-to-metal surfaces develop a nonlinear relationship between the applied voltage and the current flow through these interfaces, at high current densities, PIM can be generated from multiple signals.
Extensive studies of circuit materials have shown PIM is more due to a circuit, assembly, or system design than a property of a circuit material itself, such as its dielectric constant (Dk) or dissipation factor. Still, a circuit material that is properly formulated can help keep the PIM levels low, and the focus on minimizing PIM is again on the metal surfaces within a design. A circuit laminate with smooth copper surface at the copper-dielectric interface is known to exhibit less PIM than a circuit material with rougher copper surface at its copper-dielectric interface. Due to this material characteristic, designers seeking to create a printed-circuit-board (PCB) antenna with low PIM are encouraged to specify a circuit laminate with minimal copper surface roughness at the copper-substrate interface.
Some of those PIM studies have been on RO4534™ circuit laminates from Rogers Corp. It is a low-PIM, antenna-grade, high-frequency circuit laminate with low dissipation factor of 0.0027 at 10 GHz and Dk of 3.4 at 10 GHz that is tightly held to a tolerance of ±0.08. To explore the circuit material’s role in high-frequency PIM performance, three different types of microstrip circuits were fabricated within inches of each other on the same sheet of RO4534 circuit laminate to compare how the differences in circuit configurations impacted the PIM performance when fabricated on the same material.
The three circuits were a relatively simple microstrip transmission line, an edge-coupled bandpass filter (BPF), and a stepped-impedance lowpass filter (LPF). The current densities of the three circuits differed, as did the PIM performance. The microstrip transmission line, with the lowest current density of 4.5 A/m, also had the lowest PIM, of -157 dBc. The BPF, with high current density of 23 A/m at its edge-coupled sections, also had the worst PIM performance of the three circuits, at -128 dBc. In between, the LPF, with current density falling between the microstrip transmission line and the BPF, at 12 A/m, exhibited PIM performance that was also between the two other circuits, at -143 dBc.
Such drastic differences in PIM performance from circuits fabricated from the same panel of circuit material points to the circuits rather than the material as the reason for the differences in PIM. The differences in current density are evidence of the impact of the circuit structures on the linearity of each circuit and the resulting differences in PIM performance, with the simplest circuit, the microstrip transmission line, yielding the lowest current density and the best PIM performance. Essentially, a circuit that enables linear behavior will also make possible excellent PIM performance, just as less-linear circuit structures will suffer worse PIM performance, even when fabricated from the same circuit material.
Rogers Corp. has performed analysis and research on the PIM performance of antenna-grade circuit materials for some time (17 years), even though PIM is known not to be a circuit material property. The large database of test results during that time has helped provide insight into the circuits fabricated on those materials as much as on the materials themselves, and helps us better understand the relationships of signal power and current density on PCB PIM performance, in support of helping our customers develop low-PIM PCB antennas and other passive circuit designs, such as filters. As a result, antenna-grade materials such as RO4534 laminates deliver consistent, predictable PIM performance across wide frequency ranges, enabling the design of diverse circuit structures with the greatest linearity possible.
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