Intermodulation distortion, including PIM in passive PCB antennas, is of increasing concern with the growing numbers of wireless emitters. As Figure 2 shows, intermodulation results from the mixing of two or more closely located transmitted fundamental-frequency signals, f1 and f2, and their harmonics, 2f1, 2f2, 3f1, 3f2, and so on. The total of the coefficients of two mixing signals, such as 2f1 – f2 (2 and 1), determines the order of the resulting intermodulation product. As wireless services and their numbers of users increase, opportunities for intermodulation increase, with greater numbers of transmitted high-frequency signals. Intermodulation can occur in active circuits, such as frequency mixers or amplifiers, or as PIM in passive circuits, such as filters, couplers, and antennas.
Circuit materials and PIM
From Figure 2, it can be seen that PIM levels decrease with increasing order number, and that the third-order PIM products are usually of the greatest concern. Figure 2 does not show them, but even-order intermodulation products are also produced, and these can be a concern in some broadband applications. When the frequencies of any intermodulation products are within the frequency range of a nearby receiver, and the levels of the intermodulation products exceed the sensitivity of the receiver, they essentially function as noise to the receiver and can cause dropped calls, loss of data, and generally degraded wireless service.