IMPROVED DESIGN OF FLANGE MOUNT COAXIAL CONNECTOR WITH LOW PASSIVE INTERMODULATION DISTORTION
Abstract—Imperfect electrical connections cause multiple problems in radio frequency (RF) measurements, passive intermodulation (PIM) being the most common one. This paper proposes a new design approach to reduce the PIM distortion caused by the imperfect electrical connection in flange mount coaxial connectors. The proposed approach employs a flexible ring embedded in the outer conductor of the flange mount coaxial connector to improve the reliability of the metal electrical connection. A simulation model is further developed to analyze the effects of the embedded ring on the signal transmission performance. The effectiveness of the proposed approach is verified by experiments, demonstrating that the improved design can not only decrease the PIM level by up to 9 dB, but also show a good reliability and stability in the conditions requiring repeated connection and disconnection, and thus, this method has the potential for the application of all flange mount devices.
Index Terms—Passive intermodulation, flange mount coaxial connector, imperfect electrical connections, contact faults.
Passive intermodulation (PIM) is the distortion generated by weak nonlinear characteristics in passive devices such as connectors, multiplexers, etc. -. When two fundamental frequencies (f1, f2) are considered, the possible frequency components of PIM distortion can be described as fN(PIM) =mf1+nf2, where m and n are the coefficients of the fundamental frequencies, N is the order of PIM products, which can be calculated by N=|m|+|n|. The third-order PIM frequencies (2f1-f2, 2f2-f1) are very close to the fundamental frequencies, and thus are difficult to be eliminated by filters , . Therefore, reducing PIM distortion in microwave circuits and RF measurement systems is challenging and relies on the design of low PIM devices.
Flange mount coaxial connectors, which have been used extensively to connect printed circuit board (PCB) or flange devices, are the main contributions to the PIM distortion , . Three different kinds of sources, the nonlinear oxide (contaminant) films, the ferromagnetic material plating, and the imperfect electrical connections, are considered as the dominant reasons for PIM distortion in coaxial connectors . Sources of both nonlinear oxide (contaminant) films and ferromagnetic material plating have been well concerned - . For example, in , the effects of coating materials and iron content in base brass on PIM performance have been investigated. In , the PIM distortion caused by aluminum plating-oxide films has been analyzed. In , the design guideline and plating standards have been proposed to minimize PIM generation in RF cables and connectors. However, to the best of authors’ knowledge, only a limited number of contributions have focused on the theoretical analysis for the source of imperfect metallic contact , , the manufacturing approaches for reducing the PIM distortion caused by imperfect electrical connections have not been sufficiently estimated. In practical situations, the manufacturing error, slight deformations of the contact surface, or tightened with insufficient torque, could potentially leads to an imperfect electrical connection in the flange mount surface when connected. Therefore, it is of great importance to design the high-reliability flange mount coaxial connector to reduce the PIM distortion and then improve the performance of the devised RF measurement system.
This paper proposes a modified design to improve the connection reliability and lower the PIM distortion generated by the imperfect electrical connection in N-type flange mount connectors, and this work is organized as follows. In Section II, the structures of the flange mount coaxial connectors are analyzed to investigate the physical mechanism and potential reasons for generating PIM distortion due to the imperfect electrical connection. In Section III, the improved design method is proposed, and the dimension and plating materials of the modified ring are further analyzed to minimize the PIM distortion in the flange mount coaxial connector by imperfect metallic contact. In Section IV, transmission characteristics, including the S21-parameter, the contact impedance, and the transmission loss for the improved design method are discussed to investigate the effect of structural changes on the quality of signal transmission. In Section V, experiments are conducted to validate the effectiveness of the proposed design method. Section VI draws a conclusion for this paper.
II. STRUCTURE ANALYSIS OF FLANGE MOUNT CONNECTORS
To aid visualization and completely understand the physical mechanisms of imperfect electrical connection in the flange mount devices, a photograph of a typical N-type flange mount coaxial connector is shown in Fig. 1. As can be observed in Fig. 1(a), the dimension of the flange mount surface is larger than that of the insulator. The flange mount surface is a square of a side-length 25.4 mm with an area of 645.2 mm2 , and the diameter of the insulator is 9.8 mm with an area of 75.4 mm2 . Accordingly, the area of the flange mount surface is approximately nine times larger than that of the insulator so that the imperfect metallic contact may occur in the flange mount surface with ease. This because the large flange mount surface could be slightly deformed as a result of manufacturing error, tightened with unequal torque on 4-mounting holes. Meanwhile, the outer conductor of the coaxial connector is the inner circle of the flange mount surface. The above two reasons lead to the imperfect metallic contact being easily occurred in the outer conductor of the N-type flange mount coaxial connector, and this phenomenon is further depicted in Fig. 1(b). On the left side of the flange mount surface, a small gap could exist between the flange mount surface and the corresponding connection female due to the deformation of the interconnected surfaces, resulting in the imperfect metallic contact of the N-type flange mount coaxial connector.
From the previous studies , , two critical factors are responsible for the PIM distortion generated by the imperfect metallic contact, i.e., the electro-thermal (ET) coupling and the variation of the nonlinear oxide (contaminant) films. For the ET PIM, the additional junction impedance emerges and increases with the imperfect metallic contact level, this causes significant self-heating of the connective junction in high-power microwave circuits. On the contrary, the material property, such as metal resistivity, is changed because of the self-heating. Therefore, the resistivity of the junction is nonlinear, and then the ET PIM is generated. For the oxide (contaminant) films PIM distortion, the self-heating accelerates the process of oxidation and increases the thickness of the nonlinear oxide films. Nevertheless, the additional junction impedance makes much more current through nonlinear oxide (contaminant) films, which further deteriorates the PIM level.
Specifically, as shown in Figs. 2(a) and 4(a), a method that designs a gasket on the outer conductor of the traditional flange mount surface has been proposed in industry to reduce the probability of the imperfect metallic contact. Generally, the width d and the height h of the gasket are designed ranges from 0.80 mm to 2.00 mm and 0.12 mm to 0.20 mm respectively.
Although the gasket method can enhance the connection reliability and reduce PIM distortion in the first few uses, the stability of repeated connections and disconnections in the course of service is unsatisfactory. This is due to the fact that, 1) the gasket is the main acceptor of the connection forces between the flange mount surface and the corresponding connection female. 2) the area of the flange mount surface is much larger than that of the gasket. These factors make it easy to have a plastic deformation in the corresponding part of the gasket under the conditions of over-standard tightening torque or requiring frequent connections and disconnections (the element after plastic deformation does not fully recover its original shape).
In summary, the gasket method can improve the connection reliability and diminish PIM level in the first few uses. However, when in the situations requiring frequent disconnection and reconnection, the gasket method may have more serious PIM distortion than the traditional N-type flange mount coaxial connector.
III. DESIGN FOR THE IMPROVED METHOD
In an effort to cure the deficiencies of the proposed gasket design method, in this section, we will provide a modified ring method that is composed of two parts, namely the stress ring and the conducting ring. As shown in Fig. 2(b), the stress ring has the same size as the gasket in Fig. 2(a), the conducting ring is embedded in the stress ring as the outer conductor for the flange mount coaxial connector to transmit the signal. Nevertheless, as illustrated in Figs. 3(a) and 4(c-e), the conducting ring is divided into many ring petals by slots so that there is a small gap between two neighboring ring petals. Therefore, when a connection force is applied in Figs. 3 (c), and 4(e), the ring petals are bent in response to the contact pressure, and then the stress ring will be the main acceptor for the contact pressure. In other words, the imperfect metallic contact and plastic deformation can be avoided in the signal transmission component because the parts for receiving the contact pressure and transmitting the signals are separated (the conducting ring is employed to transmit the signal and only occurs elastic deformation; the stress ring is used to receive the contact pressure and may generates plastic deformation). In addition, the parameters design on the dimension and plating for the modified ring method is discussed as follows.
A. Dimension Design for the Modified Ring
Similar to the gasket in Fig. 2(a), the stress ring is also the main acceptors for the connection forces, therefore, the stress ring and gasket have the same dimension and material, and both need to be heat-treated to improve the hardness. In practical situations, the hardness of them ranges from 28 HRC to 32 HRC, here, HRC represents Rockwell C scale hardness. As shown in Fig. 2(b), the height and width of the conducting ring are l and w, respectively, which refer to the skin depth and plastic deformation of the ring petals. The reasons are summarized as follows.
1). Almost all the conducting current is concentrated in the inner side of the conducting ring on account of the skin effect. Hence, for the integrity of the transmission signals, the width w must be larger than that of the skin depth. For example, as the frequency of the transmitted signal is 30 MHz, the skin depth of brass can be calculated as 24.3 μm (resistivity: ρ=6.98×10-8 Ω/m, relative permeability: μr=0.99994). Therefore, the minimum width w of the Conducting ring should be greater than 24.3 μm.
2). With the same connection force, increasing the height l or decreasing the width w can increase the electric contact reliability. However, as the electric contact level or the deformation of the conducting ring exceeds their limits, the plastic deformation will occur. Therefore, the dimensions of the conducting ring are necessitated to enable the N-type flange mount coaxial connector tightly connected and avoid plastic deformation. In other word, the stress ring should withstand the contact pressure absolutely before the conducting ring is deformed with plastic bending.
Taking the designing principles above into account and considering the tightening torque for all four mounting screws is 3.0 N·m, the width w and the height l of the conducting ring are designed ranges from 0.15 mm to 0.30 mm and 0.25 mm to 0.40 mm, respectively.
B. Plating Design for the Modified Ring
Electroplating can significantly increase the electrical conductivity, corrosion protection, and wear resistance of coaxial connectors. As aforementioned, the skin effect makes almost all transmitted currents concentrated on the surface of the conducting ring, and the stress ring is the main acceptors for the connection forces. Therefore, it is important to plate the conducting and stress rings to enhance the conductivity and wear resistance, respectively.
From the aspect of the practicality and economy, the plating thickness of the conducting and stress rings are designed both from 20 uin to 50 uin (uin: micro inch, 0.50 μm to 1.25 μm). However, the stress ring is plated with nickel to improve the performance of corrosion protection and wear resistance. Two metals, gold and tri-metal alloys (55% Copper, 30% Tin, 15% Zinc), are selected as the plating materials for the conducting ring. Specifically, the plating metal of tri-metal alloys is used in a common sensitivity environment of the PIM distortion. Hence, for the applications, where high PIM sensitivity and electrical conductivity are required, the noble metal (silver, gold, and others) is the optimal choice for plating the conducting ring.
IV. SIMULATIONS FOR THE MODIFIED RING
In this section, the transmission characteristics, including the S21-parameters, the contact impedance, and the transmission loss will be analyzed by simulations to investigate the modified ring structure impacts on the transmission performances for the flange mount coaxial connector. Figs. 3 & 4 show the structures of the simulation model. A voltage source is applied at the left terminal of the simulation model (Fig. 4(f)) to generate a Gaussian pulse excitation signal. At the other end of this model, a 50 Ω load is employed to match the characteristic impedance. Three different kinds of flange mount surfaces are considered, namely the traditional design method (Fig. 1(a)), the gasket design method (Fig. 4(b), and the modified ring design method (Fig. 4(c)). In addition, the side-length of flange mount surface is 25.4 mm, the diameters of the inner conductor and insulator are 3.0 mm and 9.8 mm respectively. The width d and the height h of both the gasket and the stress ring are 1.20 mm and 0.20 mm respectively. The width w and the height l of the Conducting ring are 0.15 mm and 0.30 mm.
In this paper, we will take the mobile communication system of GSM1900 as an example to analyze the PIM distortion in the N-type flange mount coaxial connector. The transmitter and receiver frequency bands for the GSM1900 range from 1930 MHz to 1990 MHz and 1850 MHz to 1910 MHz respectively. Two carriers, f1=1935 MHz and f2=1985 MHz, are selected as the excitation signal, and thus the third order PIM distortion product can be calculated as f3(PIM)=1885 MHz.
Figs. 5-6 show the simulated results of the S21-parameter, the transmission loss, and the contact impedance in the flange mount surface for the above three cases. it is important to notice that, the transmission characteristics of the modified ring method are not only in good agreement with that of the gasket method but also with that of traditional method. Not only that, we have further measured the S21 and S11 parameters for the three different types of flange mount connectors mentioned above, and the measurement results are shown in Fig. 7, which is confirm that all connectors used in measurements have the similar transmission and reflection characteristics.
Therefore, the results of both simulation (Figs. 5-6) and measurement (Fig. 7) indicate that the effect of the structural changes (traditional design → gasket design → modified ring) on the critical transmission characteristics can be ignored. In other word, it proves that the modified ring design method can enhance the connection reliability and stability, but does not degrade the performance of signal transmission.
For validations, the test setup is composed of a PIM tester, a device under test (DUT), and a terminal load (50 Ω). CCI PiMPro1921 is used as the PIM tester to generate excitation signals and display the reflective PIM values, and the output accuracy of the used PIM tester is 0.3 dB. The 50 Ω terminal load with the feature of low reflection PIM is employed to dissipate the transmitted power.
The DUT is essentially a micro-strip line and connects the PIM tester and the terminal load with two N-type flange mount coaxial connectors. As shown in Fig. 8, the connector on the right side of the micro-strip is a traditional N-type flange mount coaxial connector, however, the connector on the left side of the micro-strip is the employed test sample. Four types of samples, as detailed in Table I, are considered to evaluate the effects of the modified ring design on the PIM levels. It is important to notice that all test samples in measurements are brand new (so that, the effect of the oxide (contaminant) films on PIM distortion can be ignored), and the standard tightening torques for all four mounting screws are 3.0 N·m.
In experiments, the measurement conditions are set as shown in Table II, where, three connection cases of the standard, under-standard, and over-standard are considered in this work. The measurement on the third-order PIM results for the standard connection case is shown in Fig. 9. It is observed that, as the standard tightening torque is applied, the values of PIM distortion can be decreased up to 9 dB by designing the gasket or modified ring on the outer conductor of the traditional flange mount surface (comparing samples B, C with A), and the noble metal (gold) plating can further reduce the PIM distortion levels (modified ring D).
As shown in Table II, for the under-standard connection case, a slight imperfect contact is generated in the flange mount surface because the tightening torque applied to one mounting screw is insufficient (2.0 N·m). Fig. 10 shows the measured third-order PIM values for the under-standard connection. Comparing the measured results of the under-standard case with the standard cases, it is important to notice that the imperfect metal contact can deteriorate the PIM level (the PIM level of traditional A′ is higher than that in traditional A), but we can improve the connection reliability and diminish PIM level by adding a gasket or modified ring on the outer conductor of the traditional flange mount surface (the PIM distortions of samples B, B′, C, and C′ are almost on the same level).
For the over-standard connection case, the employed samples have been repeated connection and disconnection 20 times with the tightening torque 5 N·m, which means the plastic deformation has generated in the connection surface before the PIM test. Fig. 11 shows the third-order PIM results for over-standard connection case. It is observed that, the frequent connecting and disconnecting or over-standard tightening torque can significantly deteriorate the PIM level for the gasket method (Comparing the measured results of gasket B″ with gasket B). However, the PIM level of the modified ring method is almost the same as its original value (the PIM levels of modified rings C and C″are almost in the same range), this is due to the fact that the components for receiving the contact pressure and transmitting the signals are separated in the modified ring method, and thus the frequent disconnection and reconnection or over-standard tightening torque impacts only on the component of receiving the contact pressure, for the component to transmit the signals, the effect can be ignored.
Specifically, from Figs. 9-11, the differences between each two traces are larger than the measurement accuracy of the used PIM tester, and we can further find that the third-order intermodulation does not meet 3 dB/dB relationship with the input signal power, which can be explained by the fact that, a system is usually made up of linear and nonlinear components, the interaction between the linear and nonlinear elements of a system can dramatically transform the overall nonlinear behavior of the system from that of the nonlinear component in isolation , .
In this paper, an improved design for minimizing the PIM distortion in the flange mount connectors has been proposed. Both the detailed design and verification have been provided. Specifically, to meet the requirement of repeated connections, this design has added two components for receiving the contact pressure and transmitting the signals respectively.
From the investigation in this paper, the proposed design method can not only significantly decrease the PIM distortion level caused by imperfect metallic contact, but also increase the reliability and stability for the flange mount coaxial connector in the conditions of over-standard tightening torque or requiring frequent disconnection and reconnection. Furthermore, this work has the potential to extend for all flange mount devices in engineering.
The authors would like to thank Dr. Yihong Qi, for his helps in technical supports and in the manufacturing the test samples.