Anritsu HFE0104_Moyher 电路图.pdf

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1、20High Frequency Electronics High Frequency Design RF CONNECTORS RF Connector Selection for Higher Frequencies By Kevin Moyher Times Microwave Systems W hen designing an RF system, an engineer will frequently be very careful in the selection of the coaxial cable, basing any decision on the cables ab

2、ility to meet system requirements such as return loss or VSWR, insertion loss, shielding effectiveness, velocity factor, passive inter- modulation, power handling capability, bend radius, bending moment, diameter and other characteristics. Its a wise decision to spend this “up front” time on cable s

3、election, because choosing the optimal cable for the application will help to insure that system design param- eters are met. Unfortunately, the design engineer will fre- quently pay much less attention to the selec- tion of the RF connector, even though the selection of an appropriate connector and

4、 ensuring proper attachment of that connector to the cable are equally critical to achieving required performance. More often than not, transmission line problems can be traced back to improper design or installation of the cable connectors. The focus of this article is the effect of connector desig

5、n and termination on voltage standing wave ratio (VSWR) and insertion loss (IL). Selecting the Right Cable Construction The primary consideration in selecting a coaxial cable is usually the loss budget for the run. Times Microwave Systems LMR family of cables offers a wide range of sizes and con- st

6、ructions that can satisfy the requirements of a broad range of systems and is generally very cost effective, so we will consider termination issues with respect to this range of cables. The construction of the basic LMR cable consists of a copper or copper-clad aluminum center conductor or copper tu

7、be that is coated with an adhesive over which a closed cell polyethylene foam dielectric is extruded. Bonded adhesively to the outside of the dielectric is an aluminum-mylar-aluminum composite tape that serves as the outer con- ductor of the cable. Covering the tape is a tinned copper, round wire br

8、aid. A heavy-wall black, UV-protected polyethylene jacket is extruded over the braid. This construction is low-loss, flexible and cost effective, suitable for many different applications. There are many variations on this con- struction in the LMR family that may be used with the same standard conne

9、ctors. Each is optimized for specific requirements or applica- tions. A few of these constructions are shown in Figure 1. Selection of the proper RF connector, and proper attachment of connectors, can affect transmission line performance as much or more than choosing the right coaxial cable Figure 1

10、 Different types of coaxial cable construction. From January 2004 High Frequency Electronics Copyright Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 22High Frequency Electronics High Frequency Design RF CONNECTORS How Connector Quality Impacts Performance The preparation of the cable can great

11、ly affect the overall performance of the assembly or jumper. Improper workmanship can readily result in poor performance of the finished cable assembly. The efficiency of a transmission line is partly a func- tion of impedance uniformity. Impedance is a function of the center conductor (“d” in Figur

12、e 2), the outer conductor (“D” in Figure 2) and the dielectric constant () or veloci- ty of propagation (Vg), where = 1/Vg. An ideal RF transmission line has uniform impedance along its length and matches the impedance of the system itself. In practice, however, this will never be the case. But over

13、 the years cable manufacturing processes have improved to the point that the cable is seldom the culprit when impedance non-uniformities are detected. Due to the line size transitions that are taking place and the mechanical techniques that are required to secure the connector to the cable, the conn

14、ector and the junction between the connector and the cable will exhibit impedances that are different than the cable impedance. In a well-designed connector, proper design of the transition between the different line sizes of the cable and the connector interface will minimize the deviation of the i

15、mpedance from the nominal value. However, many con- nector designs have less than optimal design of these transition sections. We will look at a few examples below. Another contributor to impedance non-uniformity is the termination process. There are many opportunities to alter the impedance constan

16、t during the termination pro- cess. Figure 3 shows the relationship between impedance non-uniformity and the VSWR. The VSWR value can be used to express the level of impedance non-uniformity within a cable or cable assembly.A cable assembly having perfect impedance uniformity will have a VSWR of 1.0

17、0:1. Increased levels of impedance non-uniformity will be rep- resented by increasingly higher levels of VSWR. Mismatch loss (MML) is an often overlooked factor in system planning. The formula of Figure 4 shows how the MML value is a component of the overall IL . MML is the additional loss experienc

18、ed by reflected waves as they travel through the cable/connector system and, therefore, is a function of both matched loss and VSWR. Figure 5 is a table showing the MML number for a range of VSWR values in a system with a fairly high matched loss (not unusual at microwave frequencies). It also lists

19、 the reduc- tion in transmission efficiency that is a function of the impedance mismatches in the system. In this example, you will lose an additional 25 percent of your incident sig- nal with a 3.0:1 VSWR. How the Termination Process Can Impact VSWR Many steps in the termination process can impact

20、the VSWR. Figure 6 shows a properly prepped and soldered cable end; in Figure 7 the cable end is properly crimped. The length of the various strip backs from the end of the cable should be in accordance with the manufacturers recommendations. In addition, care must be taken to cut the dielectric and

21、 outer conductor square. It should be cut with a sharp instrument so as not to form an indentation in (or deforming of) the dielectric or to produce a jagged outer conductor. Commercially available cable stripping tools are great for obtaining the proper strip length, as well as assuring a square cu

22、t of the dielectric. Make sure that the tool is sharp. The soldering of the pin is another step in the termi- nation process that can have a great impact on the final performance of the transmission line. Aside from cold sol- der joints and the risk of opens, the pitfalls that are pre- sent during t

23、he pin soldering process are (1) excess solder and flux, (2) improper pin-to-core gap, (3) melting of the dielectric and (4) the actual pushing of the solder cup of the pin into the dielectric material. All of these mistakes will create impedance mismatches and higher overall val- ues of (IL). The j

24、acket strip back and the crimping of the connec- tor can also have ramifications in terms of VSWR. The length of the jacket strip back must be in accordance with Figure 2 Impedance is a function of the conductor diameters and the propagation velocity (Vg). Figure 3 The relationship between impedance

25、 non-uniformity and the VSWR. Figure 4 IL is the sum of all con- tributing lossescable loss, con- nector loss and mismatch loss. RadioFans.CN 收音机爱 好者资料库 January 200423 the manufacturers directions. Removal of too much jack- et will have an impact on torsional pull strength (which we are not addressi

26、ng in this article). Removal of too short a piece of jacket can have serious impact on VSWR. It will almost certainly cause a piece of the jacket to be crimped underneath the ferrule. If the jacket is com- pressed, it has nowhere to go but to compress the foam dielectric and create a section of lowe

27、r impedance by altering the value of (D) and (Vg). Basically, the connector should be crimped so that it is optimally connected to the cable in mechanical terms without impacting (D) or (Vg). Figure 8 shows a connector where the ferrule has been pushed away from the connector due to protruding braid

28、, plus the ferrule has been double-crimped. A termination such as this would likely have a very high VSWR value. Figure 9 shows a comparison of VSWR for an assem- bly using good workmanship vs. one of poor workmanship (i.e., melting of dielectric, improper pin gap, jagged cutting of dielectric and o

29、uter conductor,double crimping, etc. Keep in mind that we are using high- quality cable and connec- tors in this demonstration and that the degradation of performance portrayed by the red curve is strictly due to errors in termination. This demonstration is not an exaggeration. The author has evalua

30、ted many cables and cable assemblies both in the lab and in the field and assures you that these errors are commonplace. Selecting the Right Connector The overall performance and reliability of the trans- mission line can be greatly impacted by impedance mis- matches within the connector. Connectors

31、 that look iden- VSWRReturnReflectionMismatchMatch ( :1)Loss (dB)CoefficientLoss (dB)Efficiency(%) 1.011450.0060.000100.00 1.020400.0100.00099.99 1.036350.0180.00199.97 1.065300.0320.00499.90 1.074290.0350.00599.87 1.08280.0400.00799.84 1.09270.0450.00999.80 1.11260.0500.01199.75 1.12250.0560.01499.

32、68 1.13240.0630.01799.60 1.15230.0710.02299.50 1.17220.0790.02799.37 1.20210.0890.03599.21 1.22200.1000.04499.00 1.25190.1120.05598.74 1.29180.1260.06998.42 1.33170.1410.08898.00 1.38160.1580.11097.49 1.43150.1780.14096.84 1.50140.2000.17696.02 1.58130.2240.22394.99 1.67120.2510.28393.69 1.78110.282

33、0.35992.06 1.92100.3160.45890.00 2.1090.3550.58487.41 2.3280.3980.74984.15 2.6170.4470.96780.05 3.0160.5011.25674.88 3.5750.5621.65168.38 4.4240.6312.20560.19 5.8530.7083.02149.88 Figure 5 MML numbers for a range of VSWR values. Figure 6 Properly sol- dered cable end. Figure 7 Properly crimped conne

34、ctor. Figure 8 Improperly attached connector. Figure 9 VSWR for proper vs. improper assembly. 500 MHz8 GHz Frequency VSWR Insertion Loss (dB) 0 0.5 1 1.5 Curve I (VSWR) Curve II (IL) RadioFans.CN 收音机爱 好者资料库 24High Frequency Electronics High Frequency Design RF CONNECTORS tical may actually perform v

35、ery differently. The best way to demonstrate this is by reviewing the data of an exper- iment run to test our suppositions. Figure 10 shows elec- trical performance of three 50 ohm flexible coaxial cable assemblies. The cable used in the demonstration is TMS LMR-400.The assemblies are the same lengt

36、h and termi- nated with the same interfaces, however each connector is from a different manufacturer, built and marketed for LMR cable. Since each of the assemblies was built from the same cable lot and terminated by the same person under the same conditions, the steadily increasing VSWR of the asse

37、mblies from Manufacturer 1 and Manufacturer 2 can be attributed to reflections because of impedance and mismatches within the connectors. These three curves demonstrate how the size and material transitions within a particular connector will affect performance.The VSWR curves on the right show how t

38、he ratio of reflected signal increases with an increase in frequency. The lower curve displays the associated insertion loss for the assem- blies. Notice that at some frequency, the mismatch loss attributed to the higher VSWR creates a noticeable diver- gence in the insertion loss curves. Its concei

39、vable that a short jumper assembly can actually have more than twice the theoretical insertion loss due to the use of inferior connectors or connectors that are not properly compen- sated for higher frequencies. Causes of VSWR Variation in Different Connectors In the next section of the experiment,

40、we ran Time Domain Reflectometry (TDR) plots on the same connec- tors used in the preceding section. We then sliced them open to better understand the TDR plots and the reason for the high levels of VSWR. (Though its not the focus of this article, it is almost impossible to ignore the variation in m

41、echanical robustness between the three connectors. While the general wall thickness and ability to withstand coupling nut torquing of the connector from Manufacturer 2 appear to be marginal, the connector from Manufacturer 1 looks downright fragile. There are also many other mechanical parameters th

42、at may or may not impact electrical performance. Dimensional toler- ances, plating and plating thicknesses, pull strength and pin captivation are just a few.) Figure 11 demonstrates how easily the performance of the transmission line can deviate from the optimal. The vertical scale is 2 ohms per div

43、ision. The connectors were swept across 400 ps on an HP8510 at 18 GHz.What we see is frequency-dependent and not an exact repre- sentation of the impedance, but at 18 GHz we get a very good idea of what is going on. Some of the things that are noticeable are: (1) how the different manufacturers com-

44、 pensated the diameter of the pin where it is encapsulat- ed by dielectric. The pin diameter of the Times connector Figure 10 Electrical performance for three 50 ohm flexible coaxial cable assemblies. Figure 11 TDR plots showing impedance deviations through the connectors. VSWR MFG. #1 MFG. #2 TMS 1

45、8 Frequency (GHz) Insertion Loss (dB) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 2.2 2.0 1.8 1.6 1.4 1.2 1.0 30” LMR-400 with Mfg. #2 connectors 30” LMR-400 with Mfg. #1 connectors 30” LMR-400 / Mfg. #1 conn. 30” LMR-400 / TMS conn. 30” LMR-400 / Mfg. #2 conn. 30” LMR-400 with TMS connectors Curve III (VSWR vs. Fr

46、equency) Curve IV (IL vs. Frequency) 50 50 50 increases slightly in the dielectric whereas the pin in the connector in Figure 12 doesnt use any compensa- tion. There is a large diameter com- pensation in the back of the pin in Figure 13 (over-compensation) and its impact can be seen in the TDR trace

47、 as a very large area of lower impedance.This is the primary cause of this connectors high VSWR. It is also possible to detect the impact of the various means of captivating the pins (i.e., slight protrusion or inden- tation). The impedance mismatch caused by these captivation points is minimal, but

48、 if the connector is prop- erly designed, it is possible to actual- ly have the impact work in favor of the connector in terms of VSWR. How to Minimize Variables in the Field Using a spring finger or “EZ” con- nector in the field is a good means of minimizing the variables that will affect performan

49、ce, because each employs a gold-plated, beryllium cop- per center pin. Using these connec- tors in concert with a sharp strip tool, a simple de-burr tool and the proper crimp die can make the termination process almost foolproof. When possible, such as in the case of cable assemblies that are of stan- dard length or configuration, it is wise to purchase pre-terminated assemblies that have been factory- tested for VSWR and IL across the frequency band of the application. Conclusion The connector and and its termi- nation process can have a large impact on the overall perf