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 Introduction to SAS for high-speed line

SAS(Serial Attached SCSI) is a new generation of SCSI technology. It is the same as the popular Serial ATA(SATA) hard disks. It uses Serial technology to achieve higher transmission speed and improve internal space by shortening the connection line.  For bare wire, currently mainly from the electrical performance to distinguish, divided into 6G and 12G, SAS4.0 24G, but the mainstream production process is basically the same, today we come to share, Mini SAS bare wire introduction and production process control parameters.  For SAS high frequency line, impedance, attenuation, loop loss, crosswish and other transmission indicators are the most important, and SAS high frequency line working frequency is generally 2.5GHz or more under the high frequency, let’s take a look at how to produce a qualified high speed line SAS. 


SAS cable structure definition

Low loss at high frequency communication cable is usually made of foaming polyethylene or foamed polypropylene as insulation materials, two insulated conductor with a ground wire (the market also has a manufacturer USES two double way) into the charter flights, outside the insulated conductor and ground wire winding and aluminum foil and lamination polyester belt, insulation process design and process control, the structure and electrical performance requirements of high-speed transmission and transfer theory. 

Requirements for conductors

For SAS, which is also a high frequency transmission line, the structural uniformity of each part is the key factor to determine the transmission frequency of the cable.  Therefore, as the conductor of high frequency transmission line, the surface is round and smooth, and the internal lattice arrangement structure is uniform and stable, to ensure the uniformity of electrical performance in the length direction;  The conductor should also have a relatively low DC resistance;  At the same time should be avoided due to wiring, equipment, or other device inner conductor bending periodic or aperiodic bending, deformation and damage, etc., in the high frequency transmission lines, conductor resistance is caused by cable attenuation (high frequency parameters base paper 01 – attenuation) of the main factors, there are two ways to reduce conductor resistance: increases the conductor diameter, choose the conductor material with low resistivity.  When the conductor diameter is increased, in order to meet the requirements of characteristic impedance, the outer diameter of insulation and finished product should be increased accordingly, resulting in increased cost and inconvenient processing.  Commonly used low resistivity of conductive materials for silver, in theory, USES the silver conductor, finished product diameter will reduce, will have a great performance, but because the price of silver is far higher than the price of copper, the cost too high, can’t production, in order to be able to take into account the price and low resistivity, we used the skin effect, to design the cable conductor,  At present, SAS 6G uses tinned copper conductor to meet the electrical performance, while SAS 12G and 24G begin to use silver-plated conductor. 


When there is alternating current or alternating electromagnetic field in the conductor, the phenomenon of uneven current distribution will occur in the conductor.  As the distance from the surface of the conductor increases, the current density in the conductor decreases exponentially, that is, the current in the conductor concentrates on the surface of the conductor.  From the view of the cross section perpendicular to the direction of the current, the current intensity in the center part of the conductor is basically zero, that is, there is almost no current flow, only in the part of the conductor edge will have sub-flow.  In simple terms, the current is concentrated in the “skin” part of the conductor, so it’s called the skin effect and the effect is basically caused by the changing electromagnetic field creating a vortex electric field inside the conductor, which cancels out the original current.  Skin effect makes the resistance of the conductor increases with the frequency of the alternating current increase, and results in the decrease of current efficiency of wire transmission, use metal resources, but in the design of high frequency communication cable, but can take advantage of this principle, with the method of plating silver on the surface to meet the same performance requirements under the premise of reduce the metal consumption, thus reduce the cost. 

Insulation requirements

The insulation medium must be uniform, which is the same as that of the conductor. To obtain a lower dielectric constant S and tangent of dielectric loss Angle, SAS cables are usually insulated by PP or FEP, and some SAS cables are also insulated by foam.  When the foaming degree is greater than 45%, chemical foaming is difficult to achieve, and the foaming degree is not stable, so the cable over 12G must adopt physical foaming

The main function of the physical foamed endodermis is to increase the adhesion between conductor and insulation.  A certain adhesion must be guaranteed between the insulating layer and the conductor; otherwise, an air gap will be formed between the insulating layer and the conductor, resulting in changes in the dielectric constant £and the tangent value of the dielectric loss Angle.  

Polyethylene insulation material is extruded to the nose through the screw, and suddenly exposed to atmospheric pressure at the exit of the nose, forming holes and connecting bubbles.  As a result, gas is released in the gap between the conductor and the die opening, forming a long bubble hole along the surface of the conductor.  To solve the above two problems, it is necessary to extrude the foam layer at the same time…  The thin skin is squeezed into the inner layer to prevent gas from being released along the surface of the conductor, and the inner layer can seal the bubbles to ensure the uniform stability of the transmission medium, so as to reduce the attenuation and delay of the cable, and ensure a stable characteristic impedance in the whole transmission line.  For the selection of endodermis, it must meet the requirements of thin-wall extrusion under the conditions of high-speed production, that is to say, the material must have excellent tensile properties. LLDPE is the best choice to meet this requirement. 

2 (1)

Equipment requirements

Insulated core wire is the basis of cable production, and the quality of core wire has a very important influence on the subsequent process.  In the process of adopting core wire, the production equipment is required to have online monitoring and control function to ensure the uniformity and stability of core wire, and control process parameters, including the diameter of core wire, capacitance in water, concentricity, etc.  

2 (2)

Before differential wiring, it is necessary to heat the self-adhesive polyester belt to melt and bond the hot melt adhesive on the self-adhesive polyester belt.  The hot melt part adopts the controllable temperature electromagnetic heating preheater, which can adjust the heating temperature appropriately according to the actual needs.  There are vertical and horizontal installation methods of the general preheater. The vertical preheater can save space, but the winding wire needs to pass through multiple regulating wheels with large angles to enter the preheater, which is easy to change the relative position of the insulating core wire and the wrapping belt, resulting in the decline of the electrical performance of the high-frequency transmission line.  In contrast, the horizontal preheater is in the same line with the wrapping line pair, before entering the preheater, the line pair only passes through a few regulating wheels with the role of national alignment, the wrapping line knitting does not change the Angle when passing through the regulating wheel, ensuring the stability of the phase knitting position of the insulating core wire and the wrapping belt.  The only disadvantage of a horizontal preheater is that it takes up more space and the production line is longer than a winding machine with a vertical preheater. 

Post time: Aug-16-2022