With the increasing popularity of millimeter-wave frequencies, it has become extremely important to correctly treat the delicate cables used at such high frequencies.
The use of microwave cable assemblies is often arbitrary and unthinking. However, various improper use methods may result in degradation of the performance of such cables and shortened service life. At lower frequencies, such problems can be tolerated due to the large size of cables and connectors used. However, due to the small size of the cable used in the millimeter wave frequency and its generally more vulnerable to damage, its tolerance for cable abuse is also lower.
The cellular communications industry, which has been operating at a frequency of 3 GHz, is about to use 28 to 100 GHz of spectrum. In addition, the U.S. Department of Defense (DoD) has increased spending efforts to defend against EW and radar threats at higher millimeter wave wavelengths. As such, since activities involving millimeter waves will be more extensive than ever, it is time to revisit the maintenance and treatment of such critical devices.
Due to the high degree of integration at the above frequencies, various functions are integrated in the same package or circuit board, so the number of microwave interconnection devices required may be less. However, even for equipment such as small base stations, the connection of various subsystems still requires the use of flexible, semi-flexible, semi-rigid, and other cables. In addition, each device and system that operates at the above high frequencies must be tested before use, so designers who have not previously experienced the millimeter wave measurement challenge will soon face this challenge.
Test equipment treatmentThe test and measurement environment is one of the few areas where it is always important to consider the careful use and maintenance of cables, connectors, and components (although people do not fully comply with them). Since the uncertainty of the measurement is the ill of the measurement system, attention to detail is crucial. In addition, because the VNA test cable is very expensive, cost is another reason that people must be careful about this kind of cable. Since the interconnect devices used at millimeter-wave wavelengths are precisely machined, consumable devices, the same applies to all such interconnect devices.
For example, the PE3TC1220 series 110GHz test cable assembly from Pasternack, USA is a representative cable assembly used for VNA and semiconductor probe testing (Figure 1). A 1.0mm male connector is installed at both ends of this type of cable assembly; Nomex is used to protect the armor; the insertion loss at 110GHz is less than 5.6dB; the typical voltage standing wave ratio (VSWR) is 1.5:1; the minimum bending of one bend The radius is 1 inch (2.54 cm); the connector is made of gold-plated beryllium copper and is fitted with a passivated stainless steel coupling nut; it is available in 6" (15 cm) and 12" (30 cm) lengths.
1. PE3TC1220 Series 110GHz Test Cable Assembly for VNA and Semiconductor Probe Testing
At these frequencies, the importance of careful handling of cables and connectors cannot be overemphasized. The device size is related to the wavelength and the cables (and connectors) used for millimeter wave frequencies are extremely small. This is very popular from a dimensional point of view, but in all other aspects of materials, manufacturing, and performance, it has caused problems. Such a small-sized device requires all users to pay careful attention to the twelve points, and even slight dust, almost undetectable scratches, or other damage will impair the accuracy of measurement.
For example, the full wavelength of 60 GHz is about 5 mm, and the measurement accuracy of the half-wavelength antenna at this frequency is about 2.5 mm, and a millimeter wave connector having a size of 2.92 to 1.0 mm is used. In addition, 0.8mm connectors have been developed for frequencies above 110 GHz. Compared to this type of connector, the connectors used for the HF to UHF frequencies are large—the full wavelength of 100MHz is 3m, and the full wavelength of 1GHz is 30cm.
Since loss is an important consideration for millimeter-wave frequencies, the fewer interconnection devices used between the device under test (DUT) and the measurement instrument and all other components in the transmission chain, the better. In this way, not only can the insertion loss be reduced, but also the number of connection points that are at risk of disconnection can be reduced, the test apparatus can be simplified, and the entry path of dust can be limited.
The mating process is basic but crucialWhen the millimeter wave cable or connector is mated with the corresponding cable or connector, once it is deviated from the correct alignment position, damage to the cable or the connector center conductor and the dielectric layer can easily occur. Even slight deviations in the alignment between the two connectors to be mated may cause adverse effects. The correct approach is to firmly grasp the connector so that it does not rotate, to prevent damage to the surface treatment layer and the coating of the contact, or to transmit torque to the cable assembly. Once the connector is soiled, suspected of damage, missing pins, excessive torsional damage, or damaged threads in the coupling nut, it cannot be used.
Although few engineers will admit it, fixing a coupling nut does not always use a torque wrench, and in some sensitive test measurement environments, sometimes even avoiding the use of a torque wrench. This is because novices and veterans want to exercise a "feel" to determine when the connector has been rotated into place. However, once the connector is over-torqued, the connector will transmit its own error to the corresponding connector and any other connector to which it is mated, thereby creating a "global series error" in the measurement results.
The torque wrench used for the millimeter wave connector is a precision tool. One example here is the US Patersonack Company's PE5019-16 bending torque wrench for 1.0mm connectors. Because it is preset to 4 inch pounds (0.45 Newton meters), it can prevent the application of excessive mating force (Figure 2). The advantage of the bending type is that when the preset torque is reached, the bending point will be bent to prevent further application of force. The wrench has an accuracy of ±0.15 inch pounds (0.017 Newton meters) and a hexagonal size of 6 millimeters.
2. The PE5019-16 torque wrench for a 1.0mm connector is a bent wrench that eliminates the possibility of excessive twisting of the connector.
Each type of cable assembly has a maximum bending radius. Once the bending radius is exceeded, the measurement results will be inaccurate first, and eventually the cable assembly will be damaged. Although in some cases, a large degree of bending may seem unavoidable, due to an error, the entire millimeter wave cable can be damaged, so even in this case, it should be avoided. In other words, you should avoid bending the cable to the maximum bend radius under any circumstances.
In addition, cable twisting should also be avoided because twisting can lead to damage to the cable and damage to the integrity of the component, and sometimes damage to the connector. When a cable jacket is found to show signs of wrinkles or other stress, the cable has been severely distorted or has exceeded its bending radius rating.
Cleanliness is especially importantIn the case of tight tolerances such as millimeter wave components, all interfaces must be kept clean. Although it may seem extreme, microscopy is a good practice. The cleaning process is very simple: find a lint-free cloth (polyester cloth is recommended); use isopropyl alcohol to wet it; use a damp cloth to gently wipe the part to clean it. In addition, filtered compressed air or nitrogen can also be used. However, it should be noted that pressurised tanks are often supplemented with chlorinated solvents that are used to deepen the infiltration, which may result in unfavorable results.
After the above cleaning process is completed, a magnifier can be used for surface inspection. The inspection process may also be repeated when it is necessary to ensure removal of all metal particles or other substances. Although magnifiers are readily available, their effectiveness in interface inspection can be described as a multiplier. Possible cable defects include bent pins, missing pins, damaged dielectric layers, worn or damaged threads, and other visible damage. Once any such defect is found, the cable must be replaced.
In general, the cables used in the measurement systems in the aerospace sector are the most seriously abused. This is because only some of the surveyors in this field understand the correct treatment of cables, and others do not have this awareness. In this area, examples of ruthless handling of cables abound, such as: dragging a mobile measuring station by dragging cables; placing cables under other objects; pinching, squeezing, stepping on cables; pulling cables on sharp edges Always bend the cable beyond its maximum bend radius.
Although the above situations are difficult to eradicate, anyone responsible for measuring system maintenance should at least try to ensure that all cables used are undamaged. In addition, in the outdoor measurement environment, a "drop loop" should also be created for the cable. This is because water droplets can flow along the cable into the connector, which ultimately causes changes in cable performance such as increased insertion loss.
Under aviation conditions, the environment where the cable assemblies are located is relatively harsh due to the presence of oil and other liquids and gases. Cables should not be directly exposed to such environments, but cable users may only have this awareness. Although the cable in a test and measurement application has a durable design, it does not necessarily have the ability to withstand exposure in all harsh environments.
Microwave cable jackets can use a variety of materials, each of which has its own advantages and disadvantages in harsh environments. For example, polyurethanes are resistant to solvents, UV rays, radiation, and fungi but are not resistant to cleaning chemicals. Fluoropolymers such as fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), and polytetrafluoroethylene (PTFE) are well suited for these environments - they can withstand high temperatures as well as chemicals, acids and invasive solvents Exposures are non-flammable substances.
Moisture can oxidize cable outer conductors or shields, which can increase cable losses over time. Therefore, all types of cables treat moisture as a threat. At RF power, the moisture absorbed by the dielectric layer can be heated up. Water or water vapor can enter the cable through the following: holes in the jacket material (including tiny holes); connection points between the cable and the connector; and mating points between the connector and the corresponding connector.
Waterproof bushings may be provided at the interface between the cable and the connector and the connector and the connector. Even so, water vapor may still enter the cable through tiny wear holes in the cable jacket that can only be observed through a magnifier in well lit conditions. Therefore, it takes a certain period of time to check the condition of the sheath. Otherwise, the damaged cable will be degraded or eventually fail over time. In addition, it goes without saying that in a shipboard or coastal environment, when the plating is damaged, the salt mist that enters the cable can cause corrosion of the metal in the cable, on the connector parts, or in the connector parts.
in conclusionThe operation at millimeter-wave wavelengths is a challenge, which is why the development of the millimeter-wave band is limited to only a few applications with suitable properties, and these uses rarely involve the field of communications.
However, as the cellular communications industry gradually expands to almost all available bandwidth, the above scenario will change over the next decade. This momentum of development, together with the development in the field of defense, means that designers must take stricter precautions than ever before to ensure that their cable assemblies not only achieve the targeted performance but also provide long-term, trouble-free service life.
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