Until recently, PIM (Passive Intermodulation Distortion was only a concern for wireless networks, and ultimately, the wireless user. Today, cable manufacturers are involved as certain low PIM cables are necessary for maintaining and testing this annoyance and potential service inhibitor.
And even though PIM is not a new problem and is very well known to the satellite communications and microwave trunking industries, until the use of cellular phones became widespread, Passive Intermodulation Distortion (PIM) was of little concern to the connector designer. That is not the case today. Until five years ago, most connector engineers had never heard of this PIM phenomenon. That is not the case today. Now, due to the frequency plans of today’s modern base stations, the use of higher transmitter power levels and more sensitive receivers, PIM has surfaced as a potential problem for GSM, DCS, PCS and other wireless services. “In order to effectively design low PIM connectors and cable assemblies, an in-depth understanding of PIM is needed. ” [1]
What is PIM?
PIM is the unwanted signal or signals generated by the non-linear mixing of 2 or more frequencies in a
passive (or linear) device such as a connector or cable. PIM occurs when two or more high power RF signals encounter non-linear electrical junctions or materials in an RF path. These non-linear junctions behave like a mixer causing new signals to be generated at mathematical combinations of the original RF inputs. When these PIM signals fall in the Rx band of the cell site the noise floor rises causing increased dropped calls, reduced data transmission rates and decreased system capacity. The impact of PIM on the network performance can be severe, especially for systems such as CDMA, UMTS or LTE. “Filtering can reduce or eliminate the PIM generated by the active devices in the system such as amplifiers, but components such as cable assemblies in the transmission line that cause PIM cannot be filtered. These passive coaxial components can be common to many if not all channels. The only way to reduce PIM in the transmission path is to design low PIM devices.”
What are the concerns?
“If the circuit has non-linear characteristics, then the fundamental frequency components will become distorted in the time domain and generate a decaying series of higher order harmonic frequency components in the frequency domain. If these generated harmonic frequency components fall within the receive band and are of sufficient magnitude, they can effectively block a channel by making the base station receiver think that a carrier is present when one is not. Generally, the components of concern are 3rd, 5th and 7th order where the third order is of greatest signal strength, and therefore, of primary concern.”
How are low PIM cables constructed?
Low PIM coax jumpers are typically fabricated with advanced soldering technology incorporating specially designed RF connectors, providing a durable corrugated coax cable assembly. The low PIM coaxial jumper cable assemblies have an injection molded boot on each connector which provides added strain relief and weather sealing ensuring long term durability.
For example Pasternack Enterprises provides these cables in 1/2″ flexible and 1/2″ superflexible coaxial cables. Pasternack low PIM coax cables can be ordered in standard and custom lengths with 7/16 DIN and Type N connectors to meet your specific requirements. Pasternack Enterprises new low PIM 1/2″ coaxial jumpers are used on many RF communications applications. Examples of these low PIM applications include tower top jumpers, shelter/cabinet jumpers, antenna feed lines and 2 way land mobile radio installations and other wireless or cellular infrastructure needs. “The new 1/2″ corrugated coax jumpers from Pasternack are perfect for cable runs when, low PIM, low VSWR, low insertion loss, and/or RF Isolation are critical.” Pasternack’s new line of low Passive Inter-Modulation coaxial cable jumpers mate perfectly to our low PIM precision adapters. [2]
[1] David Weinstein, Amphenol Corporation, Communication and Network Products; Passive Intermodulation Distortion
in Connectors, Cable and Cable Assemblies, http://www.ieee.li/pdf/essay/passive_imd.pdf
[2] Pasternack, http://www.pasternack.com/t-Low-pim-cables.aspx
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