Passive intermodulation is an interference source that affects many wireless systems, especially cell base stations. These base stations are characterized by their large transmit powers and sensitive receivers. The transmit and receive bands are frequency separated, but in many standards are not separated enough (notably the GSM standard shown inf Fig. 1). This lack of separation allows third order products generated by high power tones in the transmit tones to show up in the receive band.
This third order mixing can be a direct result of passive intermodulation or PIM. Our research focuses on the physical phenomena underlying this mixing as well as new measurement techniques for studying it. Many potential sources have been identified previously including worn connectors, ferrous materials, thermal heating, and recently surface roughness. Recently we have demonstrated that placing a permanent magnet near a ferromagnetic PIM source, the intermodulation level can be reduced as much as 35 dB. This is shown in the youtube.com video above. The video also gives a brief introduction to PIM and measurement system.
For the measurement system we chose a Summitek SI-400C. This world class PIM measurement system allows us to measure third order distortion with a dynamic range of -160 dBc (40 dBm input, -120 dBm residual IM). This dynamic range is difficult to grasp in units of power so transfering it to a more tactile unit like length is instructive. Suppose our input tone is a sine wave with an amplitude the size of a 100 story sky scraper (380 m). The smallest third order tone we are able to measure is 160 dB below this. This is roughly the size of the nucleus of a carbon atom (33.3 femtometers). This tiny ripple is what is meant by the -160 dBc dynamic range.
Fig. 1. Frequency layout for a GSM communication system. Transmit (blue) signals can mix to create interference which falls in the receive (red) band.
This amazing dynamic range allows us to investigate extremely small effects that might be unnoticeable with standard measurement systems.
J. Henrie, A. Christianson, W. J. Chappell, "Prediction of Passive Intermodulation From Coaxial Connectors in Microwave Networks," IEEE Trans. on Microwave Theory and Tech., vol. 56, no.1, pp. 209-216, Jan. 2008.
A. Christianson; J.J. Henrie; and W.J. Chappell, "Higher Order Intermodulation Product Measurement of Passive Components," IEEE Transactions on Microwave Theory and Techniques, vol. 56, no.7, pp.1729-1736, July 2008.
J. Henrie, A. Christianson, and W. J. Chappell, "Linear-nonlinear interaction's effect on the power dependence of nonlinear distortion products", Appl. Phys. Lett. 94, 114101 (2009), DOI:10.1063/1.3098068.
J. Henrie, A. Christianson, and W. J. Chappell, "Engineered Passive Nonlinearities for Broadband Passive Intermodulation Distortion Mitigation", IEEE Microwave and Wireless Components Letters, Accepted for Publication, 2009.
A. Christianson; W. J. Chappell, "Measurement of Ultra Low Passive Intermodulation with ability to
Separate Current/Voltage Induced Nonlinearities", IEEE MTT-S International Microwave Symposium Digest, 2009, pp. 1301-1304, 8-12 June, 2009.
J. Henrie, A. Christianson, and W. J. Chappell, "Cancellation of Passive Intermodulation Distortion in Microwave Networks," in European Microwave Conference. Amsterdam, The Netherlands: IEEE, 2008.
A. Christianson, J. Henrie , and W. J. Chappell, "Active Monitoring and Control of Tunable Cavity Resonators " Proceedings of Union Radio Scientifique Internationale (URSI), Chicago, USA, no. C07.3, Aug. 2008.
J. Henrie, A. Christianson, and W. J. Chappell, "Passive Intermodulation Distortion in Microwave Networks from Coaxial Connectors," Proceedings of Union Radio Scientifique Internationale (URSI), Chicago, USA, no. E08.7, Aug. 2008.
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