Connector. It is also called connector, plug and socket in China. It generally refers to electrical connector. It refers to the device connecting two active devices to transmit current or signal. It is widely used in aviation, aerospace, national defense and other military systems. Connector is a kind of component that our electronic engineering technicians often contact. Its function is very simple: to build a bridge of communication between blocked or isolated circuits in the circuit, so that the current can flow and the circuit can achieve the intended function. Connectors are indispensable parts in electronic equipment. When you observe the current flow path, you will always find one or more connectors. The form and structure of connectors are ever-changing. With different application objects, frequencies, power and application environments, there are various types of connectors. So, how should we correctly select RF connectors, determine cable components, and calculate insertion loss and VSWR? The following is a brief description of these connector technical issues.
What factors should be considered when selecting RF connectors?
There are many factors that determine the connector series and style, of which the mating cable and the frequency range are the main factors. In engineering practice, the diameter of connector and cable shall be as close as possible to minimize reflection. The greater the difference between cable diameter and connector diameter, the worse the performance. Reflection is usually increased as a function of frequency, and generally smaller connectors have good performance in higher frequency bands. For very high frequencies (above 26GHz), precise air medium connectors are required.
Usually the cable specification determines the impedance of the connector. 50 ohm and 75 ohm are the two most commonly used standard impedances, while many connector families have 50 ohm and 75 ohm impedances. Common cables and their characteristics can be found on our website. Sometimes, 50 ohm connectors can be used on 75 ohm cables at frequencies below 500 MHz (or vice versa) and the performance is acceptable. The reason for this is that 50 ohm connectors are generally cheap, and they are widely used.
In addition to making the cable and connector match in size as much as possible to minimize errors, the interface and insulator materials of the connector are also important considerations. The interface of linear docking and air connection (such as SMA and N-type interface) can provide high frequency and low reflection performance, while the frequency and reflection performance of overlapping dielectric interfaces (such as BNC and SMB) are generally limited. Usually, the chart reflecting the connector performance is the reflection coefficient table. This is a measurement method to describe how much the signal is reflected from the connector. It can be expressed by reflection coefficient, voltage standing wave ratio (VSWR) and return loss
In some special applications, power and voltage requirements are also a factor in determining the use of connectors. High power applications will require large diameter connectors (e.g. 7-16 DIN and HN types). Generally, the transmission power is determined by the transmission power of the cable, usually based on experience. The voltage breakdown level depends on the peak voltage. Power transmission capacity decreases with frequency and altitude.
Voltage Standing Wave Ratio (VSWR) and its determination?
VSWR (Voltage Standing Wave Ratio) is a measurement standard for measuring the amount of signal returned from the connector. It is a vector unit including amplitude and phase components. It is very important to understand this, especially when considering the combined effects of multiple connectors on transmission lines. Impedance mismatch will cause reflection. If the cable used is 50 ohm impedance, the connector must also maintain 50 ohm impedance. The size change from cable to connector transmission line, the insulator medium string in connector and the contact loss of conductor are the main factors leading to mismatch. Generally, there are two methods to determine the VSWR of a connector. The first method is to use the "flat straight line limitation" method in the entire frequency band. For example, for a straight BNC plug connected with a flexible cable, the maximum VSWR specified to 4 GHz is 1.3:1 (usually written as 1.3). The second method is to consider that VSWR is a typical frequency direct function in the actual situation, and the VSWR can be described as: VSWR=1.15 0. 01 * F (GHz) to 12.4 GHz maximum frequency when matched with a straight SMA plug of RG-142 B/U cable. For example, at 2 Ghz, the allowable maximum VSWR would be 1.15 2 *. 01 or 1.17 maximum. At 12.4 Ghz it will be 1.15 12.4 *. 01 or 1.274 maximum. Naturally, these values can be converted into return loss or reflection coefficient.
Insertion loss and its determination?
insertion loss ρ， Defined as: ρ= 10 * log (Po/Pi), in dB
Po ---- Power output
Pi ---- Power input
There are three main reasons for insertion loss: reflection loss, dielectric loss and conductor loss. Reflection loss refers to the loss of connectors due to standing wave. Dielectric loss refers to the loss of energy propagation in dielectric materials (Teflon, rexolite, delrin, etc.). Conductor loss refers to the loss caused by the conduction of energy on the connector conductor surface, which is related to the selection of materials and the use of electroplating. Typically, connector insertion loss ranges from a few hundredths dB to a few tenths dB. Like VSWR, it can be specified as "Flat Line Limit" or as a function of frequency. As with the VSWR example, for straight BNC plugs with flexible cables, the BNC can be specified as 0.2 dB maximum under maximum 3 Ghz test conditions.
How to determine the performance of the cable assembly?
Cable assemblies have two characteristics of concern: VSWR (or return loss) and insertion loss. Except for the shortest cable assembly (less than 6 inches) using extremely low loss cables, all insertion losses are mainly due to the attenuation of the cable itself, which can be generally determined from the manufacturer's information. On the other hand, VSWR is mainly due to the connector. Remember that VSWR is a vector quantity. When the frequency is scanned, the VSWR of each connector will jump up and down with the fluctuation of the phase shift. Where these maximum and minimum values occur depends on the length of the cable and its dielectric constant. Generally speaking, the calculated maximum standing wave is determined by the reflection coefficient of each terminal connector. The worst case is the addition of two reflection coefficients. Although very small, the return loss of the cable is also a part of VSWR. If the cable loss is ignored, the VSWR will be reduced. For this example, we will ignore the attenuation of the cable as a factor. For example, let's say that the VSWR of one connector at a certain frequency is 1.2, while the VSWR of another connector is 1.25, and the VSWR of cable is 1.05. Convert the VSWR into reflection coefficients of 0.091, 0.111, and 0.024 respectively, and the maximum reflection coefficient is 0.226. Converted back to VSWR is 1.584. A quick way to get results is to multiply the VSWR values of the three. In this case, it will be 1.2 * 1.25 * 1.05=1.575. This is very close to the previous calculation results For return loss, VSWR can be converted to dB. If the return loss of each connector is different or if the cable return loss is not negligible, then each return loss will have to be converted into the reflection coefficient is increased and then converted back to the return loss. It is very important to recognize that the VSWR of connectors and cables are superimposed in vectors, and the VSWR of cable assemblies is higher than that of each individual component.
What is the difference between average power and maximum power?
The power of the connector can determine the long-term (short-term) reliability of the system. The use of connectors that do not give full play to their power will cause serious problems and lead to system failure. Average power and maximum power must be considered. The average power is the maximum safe center conductor temperature that will occur when the connector/cable is in a steady state and the load connected to it is terminated The safe central temperature will not melt the medium. When considering the average power, the following points should be noted: the average power is inversely proportional to the frequency, so the average power must be reduced. Average power is equal to rated power @ 1 MHz/? The rated power of the connector (megahertz for frequency) is higher than that of the cable connected to it. The connector has a metal shell but the cable is wrapped in plastic leather. The connector can be attached with a waterproof wall to help dissipate heat. Due to the presence of air in the connector, there is usually a low attenuation per unit length The voltage level of the connector restricts its maximum power and is determined by the formula PK=V2/Z, where V represents the maximum voltage level Z represents a specific impedance. The following points should be noted when considering the maximum power:
A. The working cycle of the maximum power is usually very short, but you should calculate the average power of the highest pulse to determine that it is in line with the specification.
B. Maximum power is not a function of frequency
The maximum power of C is the inverse function of VSWR and modulation circuit, which must be reduced.
D The maximum power and average power are functions of height and must be reduced E The maximum rated power is generally less than the combination of connectors or cable components
With the above professional definitions and parameters of connectors, we believe that they will be better used in our industry in the future, so that they can bring greater benefits to mankind!