This presentation, Solutions for Low PIM Applications, was originally delivered in August 2021 as part of Times Microwave Systems’® Times Talks webinar series. Following is a session summary of the talk given by Carrie Obedzinski, distributions sales manager for Times Microwave and Kevin Moyher, product manager for Times Microwave.
As the telecom industry moves to 5G, the need for small cell and DAS systems multiplies. Antenna densification required for 5G is creating a need for smaller and smaller, flexible, low PIM cables. As more 5G networks come online, demand for cabling solutions that can accommodate all the necessary connections in smaller, more compact installations, while minimizing PIM, will continue to grow. The webinar details how cabling solutions are designed at Times Microwave to meet these advanced requirements and enable the next frontier in telecommunications.
Watch the video or read the session notes below.
What is Low PIM? Why Does it Matter?
PIM is short for Passive Intermodulation, which is a type of distortion that may occur in passive, non-linear components such as RF cables and connectors. Essentially, when two or more frequencies exist on the same cable, there is a chance that a third frequency will form. Cables and connectors play a large role in PIM, which can occur because of something resistive in the interconnect, the junctions between different types of passive components such as the connector and cable, ferrous materials, inadequate tolerance, poor torquing, etc.
While PIM is an issue for almost every wireless system, it is more noticeable in cellular applications such as 5G because the frequency bands used are very close to each other. PIM can create interference that limits receive sensitivity, lowering the reliability, data rate, and capacity of the cellular system. This can also result in dropped calls.
PIM is also a great criteria for measuring the quality of an interconnect, especially mechanical integrity and VSWR (Voltage Standing Wave Ratio), which is a measure of return/insertion loss. In an ideal system, 100% of the energy is transmitted. However, if there is a cold solder joint or air pocket in the solder, loose connections or a related issue, the VSWR return or insertion loss may not be detected—but it will be picked up with passive intermodulation. This is another reason that carriers and integrators look for passive intermodulation.
What Can be Done to Minimize PIM?
First, ensure that the right materials and platings are used. Next, eliminate any nonlinear contacts within the RF interconnect, and any poor electrical contacts. This can be caused by loose parts, parts with rough surfaces, oxidation, residual flux, etc. If conductive material is used, particulate on the face of the dielectric or within the interface itself is going to cause a problem and may actually move directly on the connectors when installed.
What Type of Testing is Performed to Ensure Solid Performance?
In the telecom industry, it is pretty much standard to place two 20-watt signals on the RF interconnect. This is done to look at the third order harmonic, typically the harmonic of the largest magnitude. Most testing requirements are looking for 153 dBc-155 dBc or better. At Times Microwave, we look for 160 dBc or better.
There are two types of tests. The first is a static test, basically a bench test. If the right materials, and platings are used and the connectors are properly tightened, this is a fairly low bar to meet. The second test, a dynamic test, is much more difficult.
For example, IEC has a standard for placing cables into the connector interface. The cable is moved off center, creating tremendous stress on the electrical connection within that connector. If there are air pockets or loose connectors, it will also require tapping on the connector to break conductive particles that may be within an interface free. At Times Microwave, we perform 100% static and dynamic testing on all our RF interconnects. Next, we serialize the interconnects, and keep those tests curves on our website for access at a later time, and we also put that data right on the cables.
Times Microwave Standard Low PIM Portfolio
The Times Microwave standard low PIM cable portfolio includes the SPO™ low-loss, low PIM corrugated copper cable which is a workhorse in terms of low PIM interconnects. We also have a similar product in a fire-retardant version, SPF™. It is a UL listed, type CMR (riser). The durable fire retardant, low smoke polyolefin outer jacket is also suitable for outdoor use. Finally, there is the SPP™ for plenum requirements within a building. This is a UL listed, type CMP (plenum) that meets the standard tunnel test. All three products are available in 250, 375, and 500 sizes in any required connector configuration and length. They are also all 100% tested for static and dynamic PIM, VSWR and insertion loss with a serial marker band that includes test data.
5G and Small Cells Drive New Requirements
5G is driving densification of the network, and small cells are the solution to create this densification. The majority of 5G small cell applications are outdoors—installed around lamp poles, roof tops, telephone poles, etc. One thing that’s pretty common across all of these applications is that they require a lot of RF cable feeds, RF jumpers, jumper cables and feeder cables—in tight spaces. This creates challenging requirements, as the corrugated cables used in many low PIM applications are not the proper cables to make these tight bends.
Times Microwave TFT™ Assemblies
Times Microwave unique TFT™ or TFT™-5G flexible, low PIM, plenum rated jumper cable assemblies use a silver-plated copper flat braid outer conductor construction to create an ultra-flexible cable with a durable FEP outer jacket is suitable for both indoor and outdoor use. The TFT delivers the same VSWR and PIM performance as the helically corrugated SPO, SPS, and SPP in a much more flexible and rugged cable.
The quarter inch UL listed, type CMP (plenum) rated cable is available in 401 (similar to SPO-250 and SPP-250 and 402 versions). The 402 is a smaller diameter cable designed for tight places and smaller runs. TFT assemblies are also available in any required connector configuration and length, as well as 100% tested for static and dynamic PIM, VSWR and insertion loss with a serial marker band that includes test data.
Bundled, Multiport Cable Assemblies
The increasing demand for high coverage MIMO antennas used in 5G applications has led to substantial growth in the number of RF ports. Furthermore, 5G antennas are shrinking in size as higher frequency bands are used to accommodate larger bandwidth requirements, which translates into more antennas in a smaller space. This densification creates numerous challenges related to installation, torquing, ensuring proper weather sealing and more. Small cells are one application that is extremely well suited for a bundled cable solution.
Installation can be a time-consuming, labor-intensive, and logistical nightmare, creating the potential for cables to be the weakest link in the system. There are numerous variables to consider: is it the right cable or the right port? Is that connector properly terminated to that cable? Is the coupling properly torqued down? Is the whole thing properly weather sealed? Are those cables properly captivated? Are they hooked up to the right connector and port? Are they flapping around in the wind? Are they protected from the sun, or if not, do they have the proper UV resistance?
All of these concerns can be addressed by using a bundled cable assembly such as the new TMQ4Ô and TMQ5Ô bundled cable assemblies for 5G. This solution combines industry-standard four and five conductor MQ4/MQ5 connectors with Times Microwave’s high-end coaxial cables to greatly reduce the number of individual connections that must be hooked up while creating a more rugged solution. It checks off all the boxes in terms of antenna port densification, saving a lot of labor with quick and easy fool-proof installation. The entire TMQ4/TMQ5 bundle is sealed to IP-67 specification and features excellent UV resistance, adding to the assembly’s durability for long-term performance.
The most common bundled cable constructions are built with inner cables that are ¼” and smaller. This concept can be used on both non-low PIM and low PIM interconnects. There are a number of other constructions to address low PIM bundled harnesses, including corrugated copper outer sheaths as well as ultra-flexible flat braid constructions.
TMQ4 and TMQ5 also use a spring outer contact so that PIM performance is not tied to the how well the tip of the outer contact is making to its mate. These cluster connectors are keyed with a color code dot on the outer coupling nut to make engagement quick and easy.
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