4.8: Passive Intermodulation Distortion

Passive intermodulation distortion (PIM) is the term used to describe nonlinear distortion that occurs when passive components are supposedly linear. Just as with nonlinear components such as amplifiers, the generation of spurious tones can swamp small received signals. The typical test of PIM is a two-tone test in which two large equal-amplitude sinusoids are applied to the device under test (DUT) and the level of the intermodulation signals measured. Except for the DUT generating the distortion being passive, distortion appears as described in Section 4.5. PIM is observed in a wide number of situations including coaxial cables [33, 34], microstrip transmission lines [35], attenuators [36], terminations [37, 38], rectangular waveguides [39, 40], antennas, and filters [41, 42].

4.7.1 Sources of PIM

Several causes of PIM have been identified or suggested. These include self-heating and nonlinear junction effects when currents flow from one metal into a dissimilar metal. The concept being that the dissimilar metals form a weak diode-like current-voltage characteristic [39].

The most convenient method for measuring small levels of PIM uses a filter to separate a small distorted intermodulation signal from the two driving tones. The finite filter response requires that the RF tones be at least \(1\text{ MHz}\) apart. However, distortion that is less than \(1\text{ MHz}\) from a signal is of concern in RF systems. Several measurements have shown that the level of distortion increases rapidly as the distortion component gets closer in frequency to the driving signal [37].

Figure \(\PageIndex{1}\) shows the third-order PIM of terminations measured in a two-tone test [43]. The PIM level is plotted against the frequency separation of the two tones. Similar results are seen with other components including transmission lines and antennas [35, 36, 37, 38]. The low-PIM termination, resulting in Curve (a) in Figure \(\PageIndex{1}\), is the lowest PIM termination available

Figure \(\PageIndex{1}\): Measured third-order passive intermodulation distortion (lower IM3) of common laboratory high-power, finned N-type terminations using a two-tone test at \(460\text{ MHz}\): (a) low-PIM cable-based termination; (b) part PE6097, component A; (c) part PE6097, component B; (d) part PE6035, component A; and (e) part PE6035, component A. Measurement taken with \(26\text{ dBm}\) input power for each tone. After [43].

comprising, here, a terminated \(100\text{ m}\) long slightly lossy cable with a loss of \(0.1\text{ dB/m}\). The distributed low-level loss spreads the heat generated out over a considerable distance and so there is a negligible electrothermal PIM effect. Curves (b) and (c) plot the measured PIM for two otherwise identical finned N-type connectors. Curves (d) and (e) plot the measured PIM for another pair of connectors. The reason for the discrepancy is unknown. The rapid rise in PIM below \(200\text{ Hz}\) tone separation is due to electrothermal effects.

The rapid increase in the level of PIM as the tone separation reduces is due to electrothermal effects. When close in frequency, the two tones produce a beating waveform that periodically heats a conductor, changing its resistance through the thermal coefficient of resistance of the conductor. This change in resistance results in a periodic variation of the current-voltage relation ( i.e., Ohm’s law) which results in the generation of intermodulation tones. It has been shown that if dissimilar metals and magnetic materials are avoided, the PIM generated is entirely due to electrothermal effects [37].

With tone spacings exceeding a few hundred kilohertz, or in some cases a few megahertz, the PIM is being generated through another mechanism. Several sources of this PIM have been postulated, and while all could be physical sources of PIM, which is most important is not clear. PIM has been shown to be almost entirely due to current density effects so reducing current density is an effective means to reduce PIM [44]. Plausible mechanisms generating such PIM include metal-insulator-metal and metal-metal contact nonlinearities [45], particularly due to surface topography at contacts. For example, it is known that high force applied across contacts reduces PIM. In cellular base stations where the power of signals is very high, it is known to be important to have very tight connections of cables carrying high-power signals.

Tunneling at a metal-insulator-metal contact could produce PIM. This is supposed to be a particular problem with aluminum-oxide-aluminum contacts, as the oxide thickness may be just right, \(2\text{ nm}\), for tunneling to occur [40, 45]. However, this is expected to be a minor contributor to PIM. Another possible source of PIM is thermionic emission [46]. Thermionic emission is secondary to tunneling and will result in a small increase in tunneling current due to other sources [46].

The metal-to-metal contact of dissimilar metals acts as a weak diode due to the difference in the work functions of the metals. Alignment of Fermi levels requires charge to transfer from the high work function metal to the low work function metal. Since charge transfer has occurred between the metals, a field therefore exists at the interface and so there is a contact potential. It is known that copper, silver, and gold contacts form low PIM contacts and these metals have very similar work functions, supporting the view above. Also at metal-to-metal contacts there is a constriction of current flow due to rough surface topology that exaggerates PIM due to increased current density.

Ferromagnetic materials such as iron, steel, cobalt, and nickel produce high significant PIM [47]. This is also true for ferroelectric and piezoelectric materials.

4.7.2 Summary

There are many possible sources of PIM, and provided that care is used in avoiding dissimilar metals and avoiding ferromagnetic materials, the only confirmed source of RF PIM is the electrothermal self-heating effect [35, 37]. However, it is not always possible to build such components as desirable mechanical, electrical, and packaging requirements necessitate the use of dissimilar elements. Ferromagnetic metals such as nickel, a desirable processing material, produce significant PIM. With dissimilar nonmagnetic materials there is clearly another (unknown) source of PIM at high tone spacings. Even with carefully designed components, PIM exists at large tone spacings, and the main source of PIM in these circumstances has not yet been identified. There is a reasonable confidence that such PIM is a current nonlinearity and not a voltage nonlinearity [44]. So a strategy for reducing PIM is to avoid high current densities, and for electrothermal PIM, to provide rapid dissipation of heat as close to the source of heat as possible. Other sources of PIM-like responses, that is, RF distortion where it is not expected, have been found due to vibration [48, 49] and the transient response of a filter [41, 42].