Passive Intermodulation in PCBs for 5G Systems
5G networks are coming. Are your PCBs ready?
Anyone pays attention to the top bar on their phone has noticed the 4G indicator. If you use a major wireless carrier in the U.S., you’ll likely see 5G in the top bar once you buy your next smartphone. 5G systems designers, especially PCB designers, will need to work with higher frequencies, faster data rates, lower power budgets, and plenty of other performance requirements.
Nonlinearity will also rear its ugly head in 5G systems and lead to a certain type of interference between TX and RX bands called passive intermodulation (PIM). This unique interference effect is becoming especially problematic in FirstNet, the new national first responder telecommunications system in the U.S. PIM can cause performance problems in any wireless network if left unaddressed.
What is Passive Intermodulation?
Most designers consider PCB traces and passive components to be purely linear devices, but this is not always the case, especially in high frequency circuits. Sources of PIM are still subject to debate, but the most widely accepted source of PIM in PCBs is nonlinearity in PCB traces and passives. Note that the term “nonlinearity” here refers to a nonlinear relationship between voltage and current, which naturally leads to frequency mixing.
Modern transceivers and radio systems which use carrier aggregation to provide increased bandwidth (thus increasing bitrate) alongside frequency division duplexing (FDD) are susceptible to PIM. When PIM occurs, carrier signals in one or more TX bands generate sidebands in the RX bands, and vice versa. This mixing between two or more signals arises in a nonlinear passives component, e.g., diodes and transistors. Note that resistors, capacitors, and inductors can exhibit nonlinearity, leading to PIM.
PIM in a ~2 GHz system with carrier aggregation in LTE band #2
Note that the 3rd order intermodulation product (orange line) is strongest, with the remaining intermodulation products falling off in intensity. Higher order intermodulation products are also broader than the carrier. There are also even ordered intermodulation products (not shown) at much higher and lower frequencies, but these are not problematic unless a system is using multiplexing over a very broad range of carrier frequencies.
Although the strength of an intermodulation product can be significantly weaker than the carrier signals in a wireless/RF system, it can adversely affect download/upload speeds. With certain carrier frequencies, intermodulation products can fall into the receive band, which disturbs or even blocks discrete channels, as shown in the above figure. While an intensity difference of approximately -140 dB might seem like a huge difference between a carrier and an intermodulation product, this is enough to cause an approximately 20% decrease in download speeds.
Addressing PIM in Passive Components
PCBs in 5G systems and subsystems will need to be designed to operate frequencies that are higher than current 2.6 GHz 4G LTE wireless networks. The goal is to provide 10 Gbps speeds directly to mobile devices using mmWave frequencies (i.e., on the order of 10’s of GHz). PIM will continue to remain a problem for PCBs operating in 5G communication bands.
Much like other noise sources, PIM cannot be entirely eliminated from a wireless system, but it can be minimized using creative design techniques. The proper substrate material with minimal dielectric soakage is one important aspect of 5G PCB design. Dielectric soakage is one nonlinear effect where some charge remains in a capacitive element (discrete or stray) during discharge, essentially forming a hysteresis loop. This is regarded as a source of second order PIM.
Capacitors and inductors used in termination networks for RF components should be chosen judiciously. Capacitors can exhibit self-resonance at certain frequencies; for example, ~10 pF discrete surface-mount capacitors have self-resonance near 2 GHz. They also exhibit dielectric soakage at particular frequencies, leading to hysteresis. Inductors with ferritic cores suffer a similar problem of hysteresis.
Poorly manufactured passives, over-etched copper, rough mating surfaces in connectors, and even residual dust or metal flakes can also cause PIM through the so-called rusty bolt effect. This effect has been known to cause frequency mixing problems in large-scale antennas that worsens over time and has nothing to do with the degradation of electronic components. This should be kept in mind when sourcing components and planning for manufacturing.
As traces are regarded as a primary source of PIM, it is better to opt for wider traces where possible as this reduces current density, thus decreasing the amplitude of sidebands produced via frequency mixing. This may cause some issues with controlled impedance design, thus careful stackup planning is required to ensure accurate impedance calculations.
A 5G base transceiver is one system that suffers from passive intermodulation
With the rollout of 5G mobile networking already in progress by major telecommunications carriers in the U.S., companies producing prototypes for use in 5G networking have a role to play in ensuring PIM does not cripple PCBs in communications systems. A number of companies are developing test equipment for measuring PIM suppression mechanisms and PIM itself. PIM measurement standards are specified in IEC 62037. These measurements are typically performed in shielded enclosures, although they can also be performed in the field when more realistic data is required.
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