Achieving High Density in Mission-Critical Circuits

By David Kiesling

Originally published in Microwave Journal

Technology providers are creating advanced new wireless system designs within restricted space constraints in avionics, 5G, space and many other industries. These applications rely on high density RF interconnections capable of high signal integrity and reliability in ever more miniature housings. There are many challenges to providing practical RF interconnections in such dense housing environments. Fortunately, innovations in RF interconnections have led to reliable, high performance solutions that can fit the tightest spaces available, even at the most difficult interconnection angles.

AVIONICS

Avionics applications have limited space as they accommodate more application needs throughout the airframe. In the past, it may have been common to have 12 antennas on an aircraft, but there are now 50—even hundreds in some cases—antennas serving advanced avionics systems. More antennas in aircraft environments leads to more signal paths and the need for more RF interconnect solutions to accommodate them.

5G

As more users rely on 5G services, more antennas will be needed to provide coverage, both at lower FR1 frequencies (under 6 GHz) and at higher FR2 mmWave frequencies. Antenna densification is required to deliver increased peak data speeds, ultra-low latency, enhanced reliability, enormous network capacity and increased availability for 5G. Many 5G networks employ MIMO antennas, which are shrinking in size as higher frequency bands are used to accommodate larger bandwidth requirements. This translates into more antennas in smaller spaces and more RF interconnections within those smaller spaces.

5G small cells, such as micro, pico and femto cells, are examples of the electronic densification within 5G networks as they are spaced much closer than traditional wireless macrocell towers, often only 100 yards apart. Demand for high density cabling solutions to accommodate the necessary
connections in smaller, more compact installations will continue to grow.

SPACE

Equipment used to support space technology must be lightweight, compact, reliable and capable of withstanding high levels of shock, vibration and radiation, as well as wide
temperature ranges. RF coaxial cable assemblies must be designed to perform reliably in the smallest possible footprint. The high frequency cables required for space applications
must support low loss communications, requiring a dense network of antennas.

HIGH DENSITY INNOVATIONS ABOUND

High density RF interconnection solutions have evolved from individual assemblies with multiple coaxial connectors to a single connection port. There are a wide variety of unique high density options suited to fit the specific needs of an industry/application, including multiport and mini-multiport connectors, bundled cable assemblies, locking miniature blind mate connectors and cable assemblies for densely packed in-the-box applications. Common requirements for these environments include ease of installation, high vibration (cannot come apart) and environmental seals.

MULTIPORT AND MINI-MULTIPORT CONNECTORS

Multiport and mini-multiport connector solutions are ideal for high density avionics environments, where space is at a premium, accessibility for maintenance is limited and performance is mission-critical. These connectors consist of multiple coaxial contacts of the same interface integrated
into a single connector module or shell. There are numerous options for these types of connectors, including those with reduced size and weight that provide excellent electromagnetic shielding and phase stability with low VSWR and insertion loss to 20 GHz for multiport connectors and to 40
GHz for mini-multiport connectors. See Figure 1 and 2 for examples.

BUNDLED CABLE SOLUTIONS

Densification creates numerous challenges related to installation, torquing, ensuring proper weather sealing and more. In addition, an increasing number of technologies such as 5G small cells have limited space for equipment, so minimizing size and weight are also key goals.

With so many components in such a small space, maintenance can be challenging. If an interconnect fails, it can be hard to troubleshoot the exact one. Moreover, installation can be a time-consuming, labor-intensive and logistical nightmare. Hooking up the right cables, ports and torquing can be difficult when working with multiple connections. Proper weather sealing is also necessary; it is imperative to ensure that the seal is secure but not over-torqued.

A bundled cable solution can help create the perfect flexible antenna jumper for applications requiring multiple runs, such as 5G. A spiral configuration of multiple flexible and ultra-flexible jumper cables can be created under a common polyurethane outer jacket to promote easy installation and
improved operation. The individual coaxial cable runs are spun together in a way that easily flexes, essentially creating a bundle, which is then run through a large jacket extruder where a ripcord is placed.

This design enables four or five individual cables to be fed into the back of an industry standard MQ4/MQ5 bundled connector, incorporating multiple RF ports and significantly reducing the number of cables that have to be hooked up. MQ4/MQ5 bundled solutions also save a lot of labor and enable
a more rugged solution. They also make the assembly more weatherproof and UV resistant.

Using the four- or five-conductor solution eliminates the need to create individual weather seals, resulting in tremendous labor savings. Furthermore, it reduces the need to worry about coupling torque, which is critical because all it takes is an error on just one weather seal to create a point of
ingress for water that could create a multitude of problems and even potentially shut the system down. With a bundled solution, the connection between the male and female cluster connectors is sealed to IP-67, as are the connector bodies and the transition from the cluster connector to the bundled cable. Any potential system troubleshooting becomes much easier. Finally, the possibility of hooking up the wrong cable to the wrong port is eliminated. The solution is keyed, so the cables can only be hooked up a certain way—no torque wrenches, know-how or special technique required.

Bundled solutions are optimal for high density challenges as they permit installation in tight spaces; instead of connecting multiple threaded connectors, just one will do the job. They are faster and easier to install and maintain and provide one firm, reliable connection to support consistent high performance. Their design has many use cases, thus becoming particularly popular in applications
where cable installations and rising operating frequencies demand coaxial cables and connectors to deliver high signal integrity and reliability. An example of this type of solution is the TMQ4 and TMQ5 bundled cable assemblies from Times Microwave Systems^® shown in Figure 3.

LOCKING MINIATURE BLIND MATES

A new generation of locking miniature blind mate connectors (TLMB) is specifically designed to overcome performance issues arising from typical SMP connectors’ susceptibility to electromagnetic interference (EMI) and electromagnetic compatibility (EMC) interference, liquid and salt ingress. Their rugged, sealed design is more durable to withstand harsh conditions and operate in severe environments. TLMB connectors retain the small form factor of the SMP for highly dense environments but add improved environmental, shielding and power capabilities, with a frequency
range from DC to 60 GHz.

While SMPs are still a valuable connector option for many designs, they pose problems as applications demand higher and higher frequencies. One of the critical issues is shielding and EMI.
Similarly, the SMP’s design reduces its ability to function without affecting other equipment in the same environment. The connector’s signal leakage issues often result in failed EMC tests. In short,
the SMP’s lack of proper electrical bonding and shielding exposes the conductor’s signal to external influence.
This signal leakage limits how closely the connectors can be placed in a single shell; without proper shielding, the contacts must be kept at a greater distance to prevent signal interference. With the improved shielding of a TLMB, more connectors and cables can be added in a much smaller footprint without interference issues.

Another major failure area in the SMP’s design makes them susceptible to ingress from saltwater, fuel and other contaminants. The lack of an environmental seal due to their mechanical openings makes SMPs prone to corrosion and failure. Another problem arises with using SMPs in high vibration applications, where their easy connect/ disconnect design makes them susceptible to unwanted de-mating in high vibration environments. TLMBs were created for high-reliability, high vibration environments such as military and aerospace. Areas where EMI may be an issue, such as
shipboard or aircraft, need an environmentally sealed and shielded connector.
The standard SMP may also disconnect in high vibration environments such as a carrier landing, weapons launch or any powerful weapons platform, making a locking miniature blind mate connector the ideal choice.

CABLE ASSEMBLIES FOR DENSELY PACKED IN-THE-BOX APPLICATIONS

Additionally, it may be optimal in high density applications to reduce the footprint required behind the connector to help install numerous cables into a very small space. Minimizing space between the cables and connectors is also necessary for the interconnect system to survive the high vibration and other harsh environmental conditions found in applications such as space and avionics.

New cable assemblies can be bent around tight corners and very closely behind the connector to minimize footprint, save space and simplify cable routing in tight spaces while offering low loss and optimized performance. Originally designed for space flight applications, this type of high performance assembly uses a compact, phase stable, highly flexible, micro becoming axial cable that can easily accommodate densely packed in-the-box applications. For example, Times Microwave System’s new InstaBend™ high performance microwave assemblies provide a flexible
preassembled design for interconnects between RF circuit cards, modules and enclosure panels. InstaBend is ideal for in-the-box applications with space constraints, including space flight, thermal vacuum, microwave test and other commercial and military applications. The cable can be bent very closely behind the connector, minimizing footprint, saving space and simplifying cable routing (see Figure 4). This also eliminates the need to protect the back of the connector.

Additionally, InstaBend provides these benefits at a dramatically reduced lead time compared to competing solutions. The high performance microwave assemblies are available in standard configurations or customized to meet an application’s specific needs. This new product’s ability to bend from connector to connector provides maximum flexibility and minimum use of available volume in high density, inside-the-box applications.

SUMMARY

As advanced new mission-critical technologies are introduced, RF interconnect requirements are changing drastically, including the need for novel solutions to accommodate extremely restricted space constraints and rising operating frequencies. New innovations in high density RF interconnects are emerging to deliver high signal integrity and reliability in increasingly dense environments.

When selecting the right high density RF cables and connectors, it is best to work with a partner whose engineers can identify the application’s unique needs and design an optimized, easier to use solution—creating better electrical, mechanical and environmental performance. Look for a supplier
with a long history of building quality cable and connectors, along with the skill, processes, techniques and materials to bring custom solutions for specific application needs to life

Connector Installation

New Connector Field Installations Improve System Reliability

By Kevin Moyher

Here is a short case history demonstrating a solution for reliable field-installed connectors use in high reliability wireless E911 systems

Across the country, CDMA, TDMA and GSM wireless communication sites are being upgraded to meet the requirements of the FCC’s Enhanced 911, or E911, mandate. Phase II of this ruling requires that network-based wireless location systems locate a wireless phone user to within 100 meters 67 percent of the time, and to within 300 meters 95 percent of the time. The quality and reliability of these systems is paramount; people’s lives may depend on it. Applications for this system are personal security, medical alerts, child tracking and accident response.

An Example of E911 Equipment

True Position is a leading supplier of the complex equipment that is integrated into cellular and PCS base stations to give them user location capability. True Position utilizes UTDOA (Uplink Time Difference of Arrival) technology to locate a subscriber’s cell phone. The LMU (Location Measurement Unit) is installed at each cell site. This equipment passively overlays the existing wireless network, sending critical data back to the operator’s Mobile Switching Centers where LCs (Location Calculators) perform the multipath mitigation algorithms. When a wireless phone user whose carrier employs True Position’s equipment dials 911 and activates the E911 network, the equipment in the area surrounding the caller is activated and begins to perform the complex calculations necessary to pinpoint the caller.

The reliability of this equipment is critical, and it has been optimized and ruggedized to perform with high reliability. However, there are only so many safe guards and safety measures that can be incorporated into the components themselves. The ultimate reliability of this system depends on the quality of the interconnecting cable runs. As shown in the photos of Figure 1, the E911 hardware has to be connected to each of the transmit and receive antenna runs, requiring multiple short interconnect cables. These cables are built from small core low-loss, flexible 50 ohm coaxial cable. True Position minimized some of the installation variables by using the QMA interface for these interconnecting runs. This interface can best be described as a high performance, quick connect SMA. The adoption of this interface eliminates the need for threading of small coupling nuts and the concern for achieving the proper mating torque.

Times Microwave has contributed to high performance and reliability of these systems, starting with the basic requirements of high quality cable and connectors with very good return loss. However, since the layout of every base station is different and the lengths of the interconnect cables vary accordingly, the cables must be cut and terminated in the field. This reality prompted the design of a series of EZ (spring finger center con- tact) connectors that interface with Times’ LMR^®-240 cable (Figure 2). Though QMAs are the dominant interface in the system, SMA and Type N connectors are also widely used. These EZ connectors eliminate the need for soldering in the field and solve the issues of pin height and pin to core gap. These three variables are often the largest contributors to inconsistency in the performance of field terminated interconnect cables.

The right angle EZs employ a unique design. Many spring finger right angle connectors have a 90° swept center pin with a mitered outer conductor. This design offers ease of termination at the expense of return loss. Times has taken the typical soldered right angle design and improved upon it. The straight brass center pin has been replaced by a straight beryllium copper pin that is bifurcated at the back end with a lead-in for the cable center conductor. This configuration has the ability to fine tune the impedance across the right angle. Where typical EZ right angles have a return loss weakness over a properly designed right angle solder connector, this new design actually has an advantage: excess solder build-up is no longer an issue. Going a step further with this design, a stop is placed inside the connector so that the pin can not be over extended beyond the center pin.

Optimization of the field terminated cables goes beyond connector design and includes the development of two easy-to-use termination tools. The first of these is a “one-step” cable stripping tool (ST-240EZ). Many small coaxial cable prepping tools are generic tools which are completely adjustable. These tools are capable of being adjusted to work with different cables but can be a real minefield in terms of potential termination problems (i.e., nicked outer braids, nicked center conductor, crushed core, improper strip lengths, etc.). The ST- 240 is a completely customized tool. The cable slides into a cable slot until it hits a stop. The blade package, containing two hardened alloy blades, is then released onto the cable, the index finger is placed through the loop at the end of the tools handle and the tool is spun clockwise around the cable for three to four full revolutions. The tool is then grasped as close to the cable as possible and pulled away from the cable, exposing the center conductor and tinned copper round wire braid.

This tool assures that the cable is stripped to the proper dimensions every time. It preps the core square and clean without crushing it or rip- ping the outer conductor and it preps the center conductor clean without nicking. The most important function which the tool performs is to expose the braid without nicking it. This is a very important requirement in the termination process that often gets overlooked. The braids on these small core flexible cables are of a very small diameter and a slight miscalculation with the pressure applied to a knife or a slight mis-adjustment of a variable stripping tool could effectively wipe out half or more of the braid, resulting in poor connector retention. The introduction of these EZ connectors for LMR-240, and the simple tools to assist with their termination, has created a nearly foolproof choice for the field assembly of short low-loss  interconnect cables.

Interpretation of Electrical Test Data with Regards to Microwave Cable Assemblies

Prepared By: Dave Slack

Originally published in RF Globalnet

June 3, 2002

A cable assembly provides two essential functions in a microwave or RF system.

These devices serve to mechanically connect an RF source to its load and to serve as a propagation
medium and waveguide for the RF signal.

A cable assembly should not be considered as hookup wire. It is a passive, TEM mode, microwave
device. As such, they are system components whose performance is just as important as directional
couplers, combiners or antennas or a host of other passive microwave components.

Cable assemblies are an integral part of the microwave system and their performance is critical to
overall system performance.

To ensure that the cable assembly being considered for use in a given system will perform well
several measurements of electrical performance are commonly made.

The primary measurement parameters used when evaluating microwave cable assemblies are Voltage Standing Wave Ratio (VSWR) and Insertion Loss. These are commonly referred to in quantitative terms where specifications such as VSWR of 1.4:1 maximum and insertion loss less than 1.5 dB.

While these numerical quantities are of great importance, especially when making a pass-fail judgment, they do not tell the whole story.

When attempting to understand why a cable assembly is not meeting its quantitative requirements it
is enormously valuable to understand the qualitative features of the insertion loss and VSWR
characteristics.

A quick glance at an insertion loss and VSWR plot can yield an abundance of understanding with regards to a cable assembly’s fitness for use.

When a load is connected directly to a source most of the power delivered by the source will be transferred into the load.

In situations where the source and load cannot be co-located the load may then be separated from
the source with a cable assembly “inserted” in between them. The amount that the power delivered to the source is reduced is called the “insertion loss”. This is the loss of power due to inserting the cable between the source and the load.

There are three primary factors contributing to overall insertion loss. These are the attenuation
of the cable, attenuation of the connectors and the mismatch loss due to imperfect impedance
matching.

Attenuation of cable and connectors are functions of their respective material and geometric
properties.

Some of the factors affecting attenuation are:

• Conductivity
• Surface finish
• Dissipation factor
• Propagation Velocity
• Line Size
• Impedance
• Frequency

Figure 1 depicts a typical insertion loss vs. frequency plot of a coaxial cable assembly.

Note that the fine grain ripple on the graph becomes more pronounced as frequency
increases. This is due to the mismatch loss that is a result of less than perfect impedance
matching and will be discussed later.

Total insertion loss is the sum of connector loss, cable loss and mismatch loss.

VSWR

The theorem of maximum power transfer states that the most efficient transfer of energy will occur
when load impedance matches that of the source delivering the power.

A 50 ohms source, connected to a 50-ohm load through a 50-ohm transmission line will be the most efficient system from a transfer of energy standpoint.

In a practical system all of these impedances are not exactly matched. At the point where two unequal impedances meet a discontinuity is created. This discontinuity causes some of the power to
be reflected back from the discontinuity with the balance being transferred forward.

This discontinuity can be quantified in terms of reflection coefficient. Once the reflection coefficient of a discontinuity is quantified the Voltage Standing Wave Ration (VSWR) can be calculated. This is a measure of the reflected voltage and ultimately, of the reflected (and transmitted) power.

Figure 2 illustrates a very simple case of a perfect system containing one, very simple discontinuity. A realistic system is composed of a virtually infinite number of cascaded discontinuities.

Reflection coefficients are complex quantities containing both magnitudes and phase angles. The
composite system makes up an array whose vector sum yields the composite VSWR vs. frequency
response of the system. Figure 3 illustrates impedance vs. time of a typical cable assembly.

Figure 4 illustrates the VSWR vs. Frequency plot of the cable assembly whose impedance characteristics is illustrated in figure 3.

Return Loss is another way of understanding the relative impedance match (or effectiveness of power transfer) of a cable assembly. Return loss is the ratio of power reflected to power delivered and can be calculated as follows:

Return Loss (dB) = 20 log l Γ l

The return loss vs. frequency graph of the above assembly is shown in Figure 5.

A VSWR plot, to the uninitiated, can be a very confusing plot. One is tempted to seek the maximum
value and make a “pass / fail” decision based on this value. In reality there is a wealth of information contained within this presentation.

A cable assembly, in its simplest sense, is a relatively uniform impedance cable with two
connectors installed on both ends. Each of these connectors typically has impedances that are
slightly different than the cable and probably slightly different than either the source or load.
These connectors are separated be the signal propagation time, or electrical length of the
assembly.

A standing wave pattern created by these two connectors will be a function of the magnitude and
phase angle of the two connector reflection and the electrical length separating them.

As frequency is incremented from lowest to highest operating frequencies the phase angle of the two
reflection coefficients, in relation to each other, also varies. If the swept frequency band is
wide enough the two interacting reflection coefficients will be in phase at some frequencies and out of phase at some frequencies.

When the relative phase angle is zero degrees the two magnitudes will algebraically add and VSWR
will be at a maximum. When the relative phase angle of the two are 180 degrees out of phase the
two, equal magnitude, reflection coefficients will subtract and VSWR will be minimum. If the magnitude of the two reflection coefficients are equal they will cancel each other completely at frequencies where they are 180 degrees out of phase.

When cable assemblies are longer the propagation delay is longer. This causes the frequency
differential between the peaks and nulls to become smaller. For a short assembly the VSWR vs.
Frequency plot has a long period to it and represents a rectified sine pattern. As the assembly
becomes longer the VSWR pattern takes on a fine grain “ripple” effect.

As the cable assembly deviates from this simple model towards a more practical model the VSWR
pattern becomes more complex. The individual parts of a connector will interact with each other.
The time delay between these parts is small. This causes a fine grain VSWR to be superimposed on a long flowing pattern.

Termination Problems

When the two primary reflections (usually the connectors) are equal, or close, in reflection
coefficient magnitude the VSWR maximums will be low and the VSWR minimums will be very close to unity.

This canceling effect is usually not complete because of the attenuation of the cable. This is why
a complete canceling may occur at lower frequencies and become less complete as frequency
increases.

When the VSWR nulls significantly deviate from 1.0:1 VSWR it is indicative of a problem with one of
the connectors. It is letting us know that the reflection coefficient magnitudes of the two connectors are no longer close to being equal.

This can be caused by high resistance contact points within the connector, poor solder quality, or a high reactive impedance mismatch caused by an incorrect dielectric.

Figure 9 illustrates two separate assemblies. One having a high VSWR and the other having a normal VSWR.

The high VSWR assembly is slightly longer in length (and higher in loss) such that their respective
curves do not overlay each other. From the quality of the curves the effect of the high VSWR on
insertion loss is readily apparent.

The fine grain ripple due to mismatch loss that is superimposed on the attenuation curve is much greater on the high VSWR assembly.

“Suckouts”

For purposes of theoretical analysis transmission lines are commonly modeled using lumped constant parameters. The distributed capacitances and inductances are lumped into discrete components from which transfer functions and other mathematical tools may be derived.

The lumped constant model of a coaxial transmission line is that of a multi pole, low pass
filter as shown in Figure 10.

For a coaxial system to work properly all of the mechanical mating parts must have complete contact, throughout 360 degrees of circumference of the inner and outer conductors. When
parts are not correctly assembled, or not mated with sufficient contact pressure, this 360-degree contact is not maintained and a resonance can result.

The resultant lumped constant model is illustrated in Figure 12. A parallel LC tank, at resonance,
presents very high impedance. This causes the transmission line model to have a band stop characteristic that is not desirable in most applications.

The center frequency and bandwidth of the band stop characteristic is a function of the degree of separation severity of the mismatched parts.

The effect on insertion loss and VSWR are illustrated in Figures 13 and 14.

The effect can be further noted when viewing impedance vs. time (distance) plot of an assembly having mismated or loose parts. Fig 15. It is evident upon inspection of the plot that the input connector displays significant “ringing” that is typical of a resonant condition.

Impedance Uniformity (structural return loss)

Until now we have been considering the cable part of a cable assembly to be of uniform impedance. We have considered the connectors, and their interactions, to be the cause of the VSWR effects. In most cases the connectors are the primary contributor to VSWR and the cable is the primary contributor to insertion loss.

There are cases where, for a variety of reasons, the cable impedance uniformity is less than perfect. This is illustrated in Figure 16.

These sorts of “structural” discontinuities yield a non-uniform, non-repetitive VSWR pattern. This type of pattern is typically referred to a high structural return loss as illustrated below.

Cable Spikes

When impedance discontinuities are very repetitive and consistently spaced there will be a specific frequency where all of these repetitive discontinuities will add. At this frequency a very magnitude, narrow band VSWR characteristic can be created.

This phenomena is commonly known as a “cable spike”. The larger the discontinuities, the more
consistent the spacing, and the longer the amount of cable affected the larger the cable spike can become.

This can be an especially frustrating problem in the manufacture of cable for use at microwave
frequencies. These cables are typically quite broad band so that any tendency to have a cable spike
will most likely occur somewhere within the operating band of the cable.

Most of the operations employed in the manufacture of cable involve rotating machinery. The repetitive nature of rotating machinery is often the cause of cable spikes. Through the use of quality machinery, diligent maintenance and active quality control measures the spike problem can
be effectively eliminated.

Figure 20 illustrates the impedance plot of the cable spike assembly. Figure 21 is a close up showing the fine impedance discontinuities and their repetitive nature. A period of 90 pico seconds causes the 11.1 GHz spike noted in Figure 18.

Modeing

A coaxial cable assembly is designed to operate in a Transverse Electro – Magnetic (TEM), or coaxial mode. In a TEM wave both the electric and magnetic fields are at right angles (transverse
to) to the direction of travel.

The coaxial structure will not support various “wave guide” modes of operation as long as the half
wavelength of the signal being propagated is less than the mean circumference of the coaxial
structure.

These higher order modes allow circulating fields to exist that are in a direction not transverse
to the direction of propagation. These circulating currents absorb energy that is no longer
available to be transferred to the intended load. These currents tend to be unpredictable and
unstable and are generally not desirable.

The maximum operating frequency, that is free of TE or TM modes, is referred to as the frequency of cutoff (fc). This is illustrated below.

A VSWR and insertion loss plot of a cable assembly operating beyond its cutoff frequency is shown in Figure 19 and 20.

Figures 24 and 25 are a 3 GHz section of Figures 22 and 23. These serve to illustrate that, unlike a cable spike, the effects of operation beyond cutoff are not always evident in the VSWR plot.

Dents

One of the more common causes of failure of a cable assembly is for the cable to become crushed or dented. Cable impedance is a function of geometry and propagation velocity of the dielectric core.
This can be calculated as follows:

where Zₒ is the impedance, Vg is the velocity of propagation, D is the outer conductor diameter and d is the center conductor diameter.

When the cable is crushed or dented the outer diameter (D) is reduced and the Vg is also reduced.
This lowers the impedance and causes an impedance discontinuity.

The following series of figures illustrates the effects on impedance and VSWR when a good assembly has dents imparted into it.

Contamination Ingress

Another common failure mode is that of contamination ingress into the dielectric of the cable structure. This can be caused by compromise of the vapor seal of the cable assembly or by the use
of solvents or other aromatic materials during the manufacture of the cable assemblies.

This failure manifests itself in the form of lower impedance where, the core is contaminated, as
well as an increase in insertion loss by more than the increased mismatch loss would normally indicate.

If the contamination ingress is due to solvents used in the installation of the connector the impedance discontinuity will be evident at the beginning of the cable assembly as shown in Figure
34.

The contamination may also enter the cable through a puncture or tear in the vapor barrier of the
cable. In this case the impedance discontinuity would appear in proximity to the leak.

Conclusion

With a discerning eye towards the qualitative presentation of cable assembly test data, the
discriminating consumer of these products can easily, and quickly, identify the most likely cause
of cable assembly problems.

Once the most likely cause is identified the real work of implementing root cause problem solving techniques and practices can begin.

Broadband Wireless

Don’t Overlook the RF Connector

Technical Primer: Using the Proper Connectors Can Be Critical in High-Frequency Applications

By Kevin Moyher

Connectors on an RF assembly are often taken for granted. In many cases the designer is satisfied if he has found a connector of the proper interface that will physically fit onto the cable that he intends to use. Impedance uniformity across a cable assembly is paramount in the efficient transmission of RF energy. The cable assembly is basically only as good as its weakest link. The time and money spent on high-quality, low-loss cable can be wasted if there are large impedance mismatches within the connectors, at the connector-cable interface and at the connector-device interface.

The Connector and Wave Reflection

Ideally, the RF connector will have a uniform impedance across its entire electrical path and a VSWR (voltage standing wave ratio) rating of 1.00:1. The VSWR value of a connector is the expression of the percentage of the input signal that is reflected back toward the source due to mismatches within the connector. This VSWR value can also be used to express the percentage of reflections across an entire assembly.

A uniform impedance across the connector, the cable and the connector-cable interface will allow the input signal of an RF transmission line to be efficiently transmitted to the output. In this case, reflections created by impedance mismatches will be nonexistent, and the losses across the assembly length will be strictly a function of the resistance of the conductors, the electrical properties of the dielectric and the shielding of the cable.

Connectors with greater impedance mismatches will have higher VSWR values associated with them. These VSWR values can be directly correlated with a value called mismatch loss (for example, a VSWR value of 5.85 has a mismatch loss of 3.021 dB).

The overall insertion loss for an assembly can be determined by calculating the theoretical
attenuation of the assembly and then adding all of the mismatch losses that would be associated with the assembly (i.e. cable, forward connector and aft connector). This calculated value represents a worst-case scenario. It would become reality if the peaks of the incident wave and all of the reflected waves were in phase with each other. This scenario is possible, but unlikely. However,
it is almost certain that the overall insertion loss of the assembly will increase as the reflections caused by mismatches along its length increase.

A perfect connector with a 1.00:1 VSWR is not possible, or I should say, it is not economically
viable. There are line-size transitions that are taking place within the connector. There are many variables involved in optimizing these line-size transitions. An abrupt transition may work well at lower frequencies, but this transition must be compensated when working at higher frequencies.
This is not an exact science. However, experienced connector designers can use time domain reflectometry to map the impedance mismatches across the connector. They can then improve the uniformity of the impedance across the connector. The art in doing this is not simply in finding a way to properly compensate the connector but in doing it in such a way that it can be produced economically.

VSWR Performance

VSWR performances of three 30-inch LMR^® assemblies are shown in Figures 1 and 2. The three graphs in Figure 1 display the performance of LMR-400 assemblies terminated with type N male connectors. The bottom graph of Figure 1 shows the performance of an assembly that was constructed with Times Microwave Systems’^® connectors, while the top and middle graphs display the performance of assemblies constructed from connectors supplied by two other leading connector suppliers.

The first thing that comes to mind when looking at these traces in Figure 1 is that the nodes are similarly shaped and very cyclical, indicating that the cable itself has a very uniform impedance. In the event that the cable varied in impedance, the trace would look much more ragged. The VSWR trace of a cable with very poor impedance uniformity would look almost like random noise.

Based on this information and the fact that each of these 30-inch assemblies was made from the same lot of cable, we know that the steadily increasing VSWR with frequency in the middle graph of Figure 1 and the high VSWR in the midband shown in the top graph can be attributed to reflections
due to impedance mismatches within the connectors.

These three curves in Figure 1 demonstrate how the size and material transitions within a particular connector design may be compensated to perform well up to a certain frequency. It is also possible to design a connector that may perform very well in a particular band but reflect a larger percentage of the input signal at frequencies both above and below the designated band.

Figure 2 compares three 30-inch LMR- 600 assemblies. The assembly represented in the bottom graph of Figure 2 has been built with Times Microwave Systems’ EZ-600-
NMH connectors. The assemblies represented in the top and middle graphs of Figure 2 have been built with connectors from two other leading suppliers. The top and middle graphs indicate good performance in the low band, excellent performance at midband and very quick roll off in the high band. The assembly represented in the bottom graph of Figure 2 indicates good performance across
the entire band, with excellent performance at the high ISM (Industrial Scientific Medical) band.

Connector Design for High Frequencies

It gets increasingly difficult with connectors, as with cable, to design, build and maintain the tight process controls that are necessary to achieve a high level of performance over a broad frequency band. It’s especially difficult as the frequencies climb well beyond the 1 GHz level, such as in the
5.8-GHz ISM band. Although it is possible to optimize connectors to perform well in certain bands, it is rarely viable from an economic standpoint.

Most connectors are rated for broadband performance to a specific maximum frequency. Unfortunately, most manufacturers of commercial RF connectors have not been able to keep up with the improvements that would be necessary to obtain reasonably good performance at the 3.7- or 5.8-GHz level, never mind optimal performance.

As little as three years ago, most applications were operating below 1 GHz; as recently as two years ago, most applications for commercial cable and connectors were operating below 2 GHz. It is these cellular and PCS (personal communications service) bands for which most connectors were designed. Someone working in these bands could basically pull any connector off a shelf and it would be a pretty safe bet that the performance would at least be respectable. Most connector manufacturers were slow to understand that a connector that was sufficient in a PCS application at 2 GHz may not meet the requirements for a 2.4 GHz application in the ISM band.
The proliferation of data applications in the ISM bands has placed certain demands on connector performance. These bands are unlicensed, and therefore the Federal Communications Commission has placed power limitations on transmissions in these bands. The need to be assured that there is a
sufficient transmit signal — but at the same time, making sure that the transmit power never exceeds the maximum allowable power — has, in many cases, created a demand for low-loss cable assemblies that are consistent and predictable in performance.

The nature of data transmission itself has also pushed systems to reduce noise levels to their absolute minimum. The best way to do this is to minimize the power levels of the system. Hence, there’s a need for low-loss cable, and, just as importantly, low-loss assemblies. As an added challenge, some of these systems are now operating in the 5.8- GHz band.

A desirable feature to have in any connector, and especially in one that must be field installed, is a captivated pin. The pin may be of a spring-finger design and permanently pressed into the dielectric or it might have a shoulder on it that will make positive contact with the bottom of a counter bore in
the connector dielectric. It’s not a stretch to imagine that the pin height of a connector with a noncaptivated pin, when installed in the field, may be 10 to 20 mils off. This error will create shifts in impedance throughout the connector, especially if there are many diameter transitions in the pin.

The Cable Design Factor

There are an enormous number of 50- ohm coaxial cable designs available. The designs encompass standard RG designations, as well as specific requirements that have arisen over the years. These designs have been devised to optimize certain parameters of the cable, such as attenuation, impedance uniformity, velocity, time delay, diameter, bend radius, flexibility, temperature range of operation and weight.

These parameters are optimized by varying center conductor, dielectric, outer conductor, and jacket materials and sizes. Optimization is also accomplished by varying the processes by which the cable is produced. Although most of these cables are 50-ohm cables, many of them present their own unique physical considerations when they are terminated with a connector.

Times Microwave Systems makes a variety of 50-ohm coaxial cables, more than any other manufacturer. The product line includes RG cables and the LMR line of low-loss flexible cable that is widely used in telecommunications and has become the accepted standard for wireless data.

The Connector-Cable Interface

There are an enormous number of connector interfaces that are in use today.
However, in wireless communications more often than not, we find ourselves working
with SMAs, TNCs, Ns or 7/16 DINs. The need for economic solutions in the development
of wireless infrastructure requires us to use the smallest diameter cable that will keep us within our loss budget and meet any other requirements that the system will place on the cable.

Factoring in the need to work with the smallest cable available, it can be quite a task to find the ideal connector for a particular cable. Considerations include the variety of cable designs, the number of connector interfaces that are dealt with, and issues such as gender, straight or right angle, clamp or
crimp, connector material and plating. Additionally, most cable manufacturers do not design or build connectors, and most connector manufacturers do not design or build cable. Hence, a certain level of frustration can be expected.
For purposes of discussion, we will assume that we are working with cable that has a uniform, 50-ohm impedance. Also, we assume that our connectors have a uniform impedance across their length (including the compensation necessary to carry out the line-size transition from the interface to the
core diameter that is being considered). At this point, the most important consideration is the mechanical fit between the transfer body of the connector (the effective outer conductor) and the outer conductor or shield of the cable itself. Ideally, this interface will be smooth and mechanically intimate around the entire 360-degree circumference of the connector.

Some connectors are designed to snugly fit over the outer shield of a cable. Other connectors are designed to fit snugly over the dielectric and have the shield or outer conductor crimped or clamped to the outer surface of the transfer body of the connector. In any case, the inner diameter of the
outer conductor must be maintained as best as possible when making the cable-connector
transition, and this transition must be mechanically sound. It doesn’t do any good to have an assembly that will perform reasonably well on a workbench but deteriorates quickly when exposed to real-life situations such as wind, vibration and temperature shifts.

Conclusion

As the frequencies of new applications continue to rise and performance requirements
become increasingly demanding, the connector must be looked at closely. Factors include the mechanical variables that will affect overall electrical performance, performance
under extreme conditions and the long-term ability to withstand the environment. Also, each connector has its own impedance characteristics similar to a fingerprint. Various connector designs will be associated with different VSWR values at a given frequency, and, in some cases, may have a significant impact on the overall insertion loss of a microwave assembly.

About the Author:
Kevin Moyher is a sales engineer at Times Microwave Systems. Times Microwave Systems is a division of Smith Industries PLC and has been involved in the design and manufacture of high performance coaxial cables for more than 50 years.