DVD-167C Coaxial Cable Assembly

Transcription

1 DVD-167C Coaxial Cable Assembly Below is a copy of the narration for DVD-167C. The contents for this script were developed by a review group of industry experts and were based on the best available knowledge at the time of development. The narration may be helpful for translation and technical reference. Copyright IPC Association Connecting Electronics Industries. All Rights Reserved. Introduction This video will teach you how to terminate the most common types of coaxial cables and connectors used in modern electronics assembly. A coaxial cable consists of two conductors separated by an insulating, or dielectric material. The center conductor and the outer conductor braid, or shield, are configured in such a way that they form: concentric cylinders with a common axis. That s why this cable is called co-axial. Coaxial cable is designed to carry radio frequency or RF signals. The center conductor carries the high frequency signal while the dielectric insulator isolates the center conductor from the metallic shield. This metal shield will protect the internal signal from radiating outward and losing signal strength. The metallic shield will also protect the internal signal from outside interference by external radio waves -- like cell phones and other airborne transmissions. The thickness of the dielectric insulator and the thickness and type of metallic shielding will vary depending on the signal frequencies and electrical requirements. Common uses of coax are for cable television, audio and video applications, computer networks, instrumentation and other types of RF applications such as satellite communications. Coaxial cable can be flexible, formable and semi-rigid. In this video, we ll be focusing on flexible cable because it s more commonly used. We will also explain how formable semi-rigid coax is terminated. Coaxial cable comes in a variety of sizes. Some common sizes include RG58 and RG142 with an outside diameter of almost two tenths of an inch; and RG316 which has a much smaller diameter of less than one tenth of an inch. RG stands for radio guide. Coaxial cable operates similarly to a component in a circuit in that it has capacitance, inductance and resistance in other words impedance. Resistance is measured in ohms and coaxial cable is commonly rated for 50 or 75 ohms. You don t really need to understand all the details of capacitance, inductance and resistance because they are beyond the scope of this video. Now that we have a basic understanding of coaxial cable, let s take a look at the most common connectors used to terminate the cable. These include F type connectors; BNC and TNC 1

2 connectors; SMA and MCX connectors; and N type connectors. All of these types of connectors are available in male and female configurations. The connectors are sized for specific diameters of cable as well as being designed for several methods of attachment. F type connectors are the most recognizable of the coax connectors and are used for cable television, broadcast television and cable modems. Because they are used primarily in home entertainment applications, the assembly of F type connectors to coaxial cable won t be covered in this video. The B in BNC connectors stands for bayonet and the N and C are the initials of the last names of the two inventors of this quick connect/disconnect connector. The bayonet mating is achieved by lining up pins and slots and twisting one quarter turn to lock or unlock. BNC connectors are capable of handling frequencies up to 4 gigahertz, and are used in audio and video applications and in test instruments. The TNC connector is a threaded version of the BNC. TNC connectors are used in wi-fi routers, networks, telecommunications and wireless microphones. Because the threads make for a tighter connection, standard TNCs are capable of handling frequencies up to 11 gigahertz, while high performance versions are rated to 18 gigahertz as opposed to 4 gigahertz for BNC connectors. The SMA, or subminiature version A, connector looks like a smaller version of an F type connector. However, SMAs are high performance connectors capable of handling frequencies up to 18 gigahertz. These connectors are used in test equipment and other instrumentation. MCX is a micro-miniature connector similar to an SMA, but at least 50% smaller. MCX connectors can handle frequencies up to 6 gigahertz, and are used where weight and physical space are limited. Our last common connector is the N type connector. Standard N type connectors are rated to 11 gigahertz, while higher performance versions are rated to 18 gigahertz. They are threaded and rugged. Originally invented by the US Navy, they have an expandable soft insert that makes the connector watertight as it is being assembled. N type connectors are now used in a wide range of products. Now that you have an understanding of coaxial cable and the most common types of connectors used to terminate the cable, let s take a look at what you ll be learning in this video. The focus of this training will be on the manual and semi-automatic preparation of coaxial cable, along with the methods used to attach each type of connector to the cable. Cable lengths will be specified in the assembly documentation. Strip lengths will either be indicated in the assembly documentation or specified by the connector manufacturer. And the method used to terminate the connector to the cable is usually based upon the specific connector called out in the bill of materials. Your company will let you know whether to use the manufacturer s instructions, or to follow procedures in the assembly documentation. The purpose of this video is to demonstrate some typical assembly sequences for common connectors and coaxial cables in order to understand what s involved in making acceptable terminations. Recognizing defect criteria for preparing and joining cable and connector will also be covered. 2

3 Cable Preparation In this section we ll be examining how to prepare coaxial cable for the assembly operation. This involves cutting the cable to specified lengths and stripping the cable to expose the correct lengths of center conductor, dielectric and braid. The first step is to cut the cable to the specified length. Use approved cable cutters to execute a clean cut, making sure the cut is straight across and not slanted to one side or the other. After cutting the cable to the proper length, we ll strip the cable. Stripping can be done using manual stripping tools or semi-automatic machines. We ll start with manual tools. In this example, the RG6 cable will be stripped using a manual cable stripper. The three blades inside this stripper need to be adjusted for the specified strip dimensions. These dimensions are typically referred to as A, B and C on the assembly documentation. Since strip length is critical for terminating connectors to coax, it s important to be precise when adjusting the blades. Position the stripping tool over the cable. The end of the cable should fit snugly inside the stripper and touch the end of the stripping chamber inside. With the cable in place, close the tool and gently rotate it in a clockwise direction for several turns. Then open the stripping tool and pull off the slugs to reveal the three strips. Then visually inspect the cable. Make sure that the stripping dimensions are accurate. This strip is just about perfect. The cut is smooth and clean with no jagged edges. The wire mesh, or braid is evenly cut with no long strands and lies smoothly. Notice there is no damage to the outer insulation or the dielectric, and the center conductor is not nicked or cut. The IPC/WHMA-A-620 standard contains the acceptance criteria for stripping and terminating coaxial cable for Class 1, 2 and 3 products. Now that you re familiar with manual cable stripping, let s examine a semi-automatic stripping machine. Once these machines are properly set up, they make accurate, repeatable strips much faster than the manual stripping tools. As you can see, the machine makes three separate passes to strip the coax. The first pass strips outer jacket and braid from the dielectric. The second pass strips the outer jacket from the braid. And the third pass strips the dielectric from the center conductor. On this machine, the second and third passes are reversed meaning the dielectric is stripped from the center conductor before the outer jacket is stripped from the braid. On these types of machines, the cut depth and cut length are specified for each of the three strips. When calculating cut depth, the lower the number, the deeper the cut. For cut length, the higher the number, the longer the cut. Therefore, if the result of a strip is unacceptable, you may alter the numbers to achieve a more optimum result. Before moving on to the sections that describe how to properly terminate the most common coaxial connectors, we ll provide some tips on what you need to be aware of to create a successful outcome. For starters, all materials and tools must match for the job. In other words, the diameter and impedance of the cable determines the size and impedance of the connector needed. The diameter of the cable also determines the settings on the stripping tool or semiautomatic machine. The strip length is determined by the size of the connector. And the size and type of connector determines which crimping or compression tool will be used, or whether soldering is required. 3

4 To put it more succinctly, the variables include the cable, the strip length, the connector and the tools. The fifth variable, which is less tangible, is knowing how to use tools properly. It doesn t matter if you have all the right parts and tools, if you don t have the training to use them you can easily cause damage. The right tool with the right technique is critical when performing coaxial cable assembly. BNC and TNC Connectors Our first demonstration involves assembling a compression style BNC connector to broadcast cable. The cable has been cut and stripped. Notice that this cable has two outer conductor layers a braid and a foil underneath. The first step is to use a scribe to splay out the braid away from the foil and the dielectric. Next, carefully slide the compression BNC connector over the center conductor, over the splayed braid and onto the outer jacket of the cable. It s important to push hard enough to make sure that the braid is fully visible in the inspection window. Here s a closer angle that shows how the braid should fill the inspection window. And here s what it looks like when the braid isn t far enough into the inspection window. At this point, open the compression tool and place the BNC connector in position so that it s aligned with the plunger head. Then lock the connector in place. Now, close the handles of the compression tool so that the plunger head fully compresses the locked in connector. Then open the tool and remove the finished BNC connector cable. As you can see, the connector is fully compressed over the cable meaning there s a reliable mechanical and electrical connection. And here s what it looks like when there s inadequate compression. Let s examine a crimp style BNC connector. The first step is to cut and strip the RG58 cable. We ll be crimping a pin onto the center conductor and a ferrule onto the connector shaft. Begin by sliding the pin onto the center conductor. Next, place the positioned pin into the crimping tool. Then close the jaws of the crimping tool to crimp the pin to the center conductor. As you can see, the crimp is centered on the crimp area of the pin; there is equal compression on all crimp surfaces; and there is no damage to the pin or dielectric. And here are a couple examples of defects. Now, slide the ferrule over the pin and onto the outer jacket of the cable. The next step is to use a scribe to splay out the braid. Slide the connector over the contact pin. Keep pushing until the center pin is fully seated. You ll usually be able to feel it snap into place. Verify that the pin is in the correct position. Then slide the ferrule over the splayed braid and onto the shaft of the BNC connector. The ferrule should rest against the rear of the connector. Then position the ferrule into the die of the crimping tool. Push the handle of the tool to close the jaws and crimp the ferrule to the connector shaft. For the target condition the crimp on the ferrule is located tight against the connector body and there is a bellmouth formed over the outer jacket. And here are defective ferrule crimps. To verify the connection, visually check the connector cable and perform an electrical continuity test. Now we ll turn our attention to a TNC connector. For this operation we ll be using RG142 cable that has been cut to size and stripped. This connector will be terminated using the same sequence we just saw for the BNC connector. 4

5 First, slide the pin onto the center conductor. Next, place the positioned pin into the crimping tool. Then close the jaws of the crimping tool to crimp the pin to the center conductor. As you can see, the crimp is well formed and is flush to the dielectric. Now, slide the ferrule over the pin and onto the outer jacket of the cable. The next step is to use a scribe to splay out the braid. Then slide the connector over the center pin. Make sure that the center pin is fully seated. Then slide the ferrule over the splayed braid and onto the shaft of the TNC connector. The ferrule should rest against the rear of the connector. Now, position the ferrule into the die of the crimping tool. Push the handle of the tool to close the jaws and crimp the ferrule to the connector shaft. For the target condition the crimp on the ferrule is located tight against the connector body and there is a bellmouth formed over the outer jacket. And here are some defective ferrule crimps. The last steps are to visually inspect the crimp and test the termination for electrical continuity. SMA and MCX Connectors Now we ll examine an SMA connector. The SMA connector is less than half the size of a BNC connector. And the RG316 cable we ll be using is less than half the size of RG142 cable. The RG316 cable has been cut and stripped and a pin has been placed on the center conductor. Notice that a much smaller crimping tool is used to crimp the pin to the center conductor. Also notice that the form of the crimp has a different appearance. Sometimes a customer requires soldering the pin to the center conductor. Here s one way of doing this Using pliers to hold the pin, insert solder wire into the opening of the pin. Then cut the wire. Next, transfer the pin from the pliers to a thermal tweezers. Now, move the pre-tinned center conductor to the opening of the pin. When the solder melts, push the center conductor fully into the pin and remove the thermal tweezers. Finally, verify there is a concave solder fillet in the inspection window. Regardless whether the pin has been crimped or soldered to the center conductor, the next step is to slide the ferrule over the pin and braid onto the outer jacket of the cable. Then splay out the braid using a scribe. Now, slide the SMA connector shaft over the pin and dielectric until it stops against the outer jacket. Then slide the ferrule back over the braid and connector end. The last step is to crimp the ferrule to the connector. Perform a visual inspection to make sure that the pin is fully seated into the housing of the connector and that the ferrule crimp is located tight against the connector body. Make sure that the connector and ferrule do not turn or move on the cable. Then verify the electrical continuity of the cable assembly. Now that you understand how to terminate this type of SMA connector, let s take a look at a different assembly process for a right angle MCX connector. First, we ll slide the ferrule onto the stripped cable and use a scribe to splay out the braid. Next, we slide the right angle connector entry shaft over the center conductor and dielectric until it stops against the outer jacket. Now, slide the ferrule over the splayed braid and onto the shaft of the MCX connector. Make sure that 5

6 the center conductor is flush to the end of the slotted terminal. It is a defect condition when the center conductor extends beyond the edge of the slotted terminal greater than one center conductor diameter. It is also a defect for class 3 products when the center conductor is not flush or visible beyond the slotted terminal. At this point, position the ferrule into the crimp jaws and make the crimp. Then remove the MCX cable from the tool. Next, place the soldering iron tip at the edge of the slotted terminal and add solder to the center conductor. Here is an example of a target solder joint. Notice that the conductor extends completely through the slot and is visible on the exit side. And here s what it looks like when there is insufficient solder and excessive solder. After visually inspecting the assembly, test the MCX cable for electrical continuity. N-Type Connectors The last coaxial connector we ll be examining is the N type connector. We ll describe two termination examples one using RG142 cable and one with formable semi-rigid cable. The first step is to gently slide the pin over the center conductor so that it rests against the dielectric insulation. Then place the pin in the crimping tool. Now close the jaws of the crimping tool to crimp the pin to the center conductor. As you can see, the crimp is well formed and the pin is tight against the dielectric. Next, slide the ferrule over the pin and onto the outer jacket of the cable. Then splay out the braid. At this point, we slide the N type connector over the contact pin. Keep pushing until the junction of the contact pin and the dielectric is flush with the front part of the connector. Then slide the ferrule over the splayed braid and shaft of the connector until it is up against the rear of the connector. Next, position the ferrule into the correctly sized die of the crimping tool and push the hand of the tool to crimp the ferrule onto the connector. The last step is to visually inspect the connector and to perform an electrical continuity test. Now, let s take a look at how we terminate the N type connector to conformable semi-rigid cable that s been cut and stripped. The first step is to set the contact pin in the correct sized cavity in this template block. Next, insert solder wire into the opening of the pin and cut the wire flush with the end of the pin. Place the ends of a thermal tweezers on the side of the pin and apply heat until solder melt is observed through the inspection window. Then, insert the pre-tinned center conductor into the pin. Once the center conductor is fully seated in the pin, remove the thermal tweezers. The fillet in the inspection window should be slightly concave and show good wetting. And here s what happens when too much solder is used. The next step is to slide the N type connector over the pin and onto the cable. Make sure that the pin is far enough into the connector. Then place the connector into the correct sized cavity of the template block. 6

7 Now, place a heated soldering iron tip on the junction of the conformable cable and the rear of the connector and feed in some solder wire. Carefully spin the connector so that the solder goes into the area around the connector. This process can take some time until sufficient solder is properly wetted to this junction. Once you re satisfied with the connector to cable solder joint, clean the connection. This solder joint is just about perfect. Here s an example of insufficient solder and here is an example of excessive solder. Finally, it s a defect condition when the solder is less than 360 degrees meaning that there are voids along the connection. The last steps are to visually inspect the connection and test the termination for electrical continuity. Coaxial cable is critical to many areas of modern life from high speed-data transmission, to medical monitoring equipment, to military communication and instrumentation, to cable and satellite television and for computer networks. The quality of every cable connection determines how well these systems operate on a daily basis. When you put this information to work, you can feel proud that you re contributing to the conveniences and necessities we all depend upon. 7