Essential Design Elements of Isolators and Circulators for Unique Military Requirements

Isolators and circulators have long been used for both military and commercial applications whenever RF signal needs to be directed down a circuit path and blocked from another circuit path. To use a street analogy, an isolator behaves as a one-way street while a circulator behaves as a traffic circle. For example, in commercial base stations where the transmit and receive chains share a common antenna, it is desirable to separate the receive chain path from the transmit path and vice versa. A three-port circulator would be used with one port being the antenna and the other ports being the transmit and receive chains. With the explosion of commercial wireless telephony and the Internet of Things (IoT), there has been a greatly increased demand for circulators in base stations. This demand had resulted in significant technological advances in both devices and the materials used within them. Designers of systems for military applications may take advantage of these advances for use in applications such as radar systems. With its heritage as highly regarded supplier of advanced circulator devices for commercial applications, Skyworks combines patented ceramic innovations and cutting-edge circulator design to offer a complete solution for all military circulator and isolator needs.

Basic Circulator Concepts

Isolators and circulators belong to a class of devices known as “non-reciprocal” which are useful in allowing the flow of an RF signal from one circular direction while absorbing or attenuating the flow in the opposite rotational direction. Thus, these devices act as a rotary traffic circle for RF energy. In Figure 1 below, the RF signal can readily travel from port 1 to port 2, port 2 to port 3 or port 3 to port 1. However, the signal is strongly attenuated travelling in the reverse (circular) direction. Circulators can be designed to allow RF signal to travel in either the clockwise or counterclockwise direction.

Figure 1:  Schematic of circulator operations. Source: App Note 202840B.

By Combining military device heritage dating back over fifty years with wide-ranging material expertise, Skyworks is in a unique position to provide a range of circulators and isolators specifically tailored for aerospace and defense applications. There are 3 main areas that differentiate Defense applications related to circulator designs:

1. High Operating Frequency Ranges
2. Reliability and Ruggedness Improvements
3. High Power Operation Requirements

Skyworks has the materials science and device design expertise to provide differentiated products to address each of these key considerations.

1. Extended Range of Frequencies for Military Applications:

Devices for L-band, S-band and the lower portion of C-band are designed to operate in the above resonance region of the ferrite material absorption curve. This means that they are immune to power non-linearities and offer consistent insertion loss performance over the full operating window, it also allows for miniaturization techniques to produce a smaller device than would be possible using below resonance operation.

Figure 2:  Schematic of frequency vs. loss indicating regions for above and below resonance operation in a circulator.

However, at higher frequencies (C-band and X-band) only below resonance circulators may be used. At these frequencies, the magnetic field from the permanent magnet is insufficient to saturate the ferrite for above resonance use. Below-resonance designs require a specific saturation magnetization for each frequency range. Therefore, the correct choice of materials from Skyworks’ RF Ceramics broad range of garnet and ferrite materials is important for designing below resonance devices over a broad frequency range. 

Below resonance operation may also be used at lower frequencies (< 4 GHz) where a broad frequency band (>25% BW) is required. 

When operating below resonance it is important to avoid low field loss and therefore the minimum frequency of use depends on the saturation magnetization of the ferrite according to the relation:

where γ is the gyromagnetic ratio, 4πM_s is the saturation magnetization of the ferrite (typically expressed in units of gauss) and demagnetizing factors are related to the geometry of the ferrite. The above relation sets the saturation magnetization value of the ferrite to be used at a specific frequency. Fortunately, Skyworks manufactures ceramic ferrites with a range of saturation magnetizations from 175 -5000 gauss. There are compositions available over this range of saturation magnetizations which allow for temperature compensated solutions and high-power operation as well.

In addition, the size of the circulator is defined by the ratio of:

For below resonance devices, the only method for reducing the physical size of the device is by using a high dielectric constant ferrite material. Skyworks has patented high dielectric constant ferrite materials showing dielectric constant between 25 and 31 depending on the saturation magnetization of the material. Currently, our TTHiE series of materials are available at saturation magnetizations from 400- 1950 gauss allowing for miniaturized below resonance devices over a broad range of frequencies. This materials knowledge, coupled with circulator design expertise, allows Skyworks to craft custom solutions for any desired frequency range and power level in the smallest diameter device on the market. These solutions may be used for military as well as commercial markets.

2. High Reliability and Ruggedness:

Along with materials and processes discussed above for the commercial market, these same materials and devices can also be tailored to the special demands of the military market as well. For example, solutions can be tailored for high reliability, ruggedized application through cost-effective options depending on the environment of use. Figure 3 shows a typical stripline (also called triplate) circulator which features two magnets, two ferrites and a center conductor to provide a return path for the magnetic flux lines. This design is very common for above resonance circulators for commercial wireless applications. 

Figure 3. Schematic drawing of a stripline (triplate) circulator for above resonance applications. 

Since these designs feature a steel housing and a cover, they are typically mechanically robust. For military and aerospace applications, these devices will need to withstand g-forces and mechanical shock so the mechanical strength of the ceramic ferrite material (or ferrite assembly) and possibly the permanent magnet (if it is a ceramic material such as barium hexaferrite BaFe12O19) become important considerations. To make certain the ferrite is completely saturated by the field from the permanent magnet and to adjust the frequency of operation, the ferrite is surrounded by a non-magnetic dielectric ceramic ring. The dielectric ring may be bonded to the ferrite material using a polymer based or ceramic based adhesive. With some material combinations, the dielectric material may be “co-fired” onto a pre-sintered ferrite to form a strong ceramic bond. Figure 4 shows the co-fire process in comparison to the ceramic gluing process. Co-firing has the advantage of creating a ceramic-ceramic bond without the need for polymer adhesives which increase the insertion loss of the device. In addition, metals such as silver can be directly applied over the ceramic-ceramic bond – an impossibility when low temperature polymer-based adhesives are used. 

Figure 4:  Representation of co-fire process to produce ferromagnetic dielectric composites (D. Firor Skyworks RF Ceramics).

The mechanical strength of the dielectric ring material becomes a critical parameter for the ruggedness of the ferrite-dielectric assemblies and the stripline circulator in general. Table 1 shows the mechanical properties of various Skyworks dielectric materials which may be used as the ring for a ferrite-based assembly. 

Table 1:  Mechanical properties of Skyworks dielectric materials commonly used for ferrite-based assemblies for circulator applications.

 In addition to the brittle ceramic components, it is critical to design the housing, the leads and transitions of the circulator to withstand mechanical forces as well. Humidity resistance, vibration control and electromagnetic (EM) hardening are other parameters which may be taken into consideration for military applications. 

3. High Power Operation:

A third area where circulators for military applications may differ from those for commercial applications is in the power levels required for operation. Whereas commercial circulators operate at peak power levels under 100 W, it is not unusual for devices for military applications to require peak levels well above 1 kW. This has serious implications in the design of the circulator. For example, with increased power levels comes Joule heating and therefore it is important to provide thermal pathways or heat sinks to dissipate this excess heat. An important factor in the design of a circulator for high power levels is to spec the performance at higher temperature. High temperature application favors the use of a high temperature magnet such as Sm-Co alloys rather than a lower cost ceramic magnet based on barium hexaferrite. The change in the saturation magnetization of the ferrite with temperature relative to the change in the applied field from the permanent magnet with temperature becomes an important parameter impacting device performance. Figure 5 shows curves of the saturation magnetization with temperature for several Skyworks ferrite products. In addition, transitions and circuit components need to be larger or thicker to accommodate the higher power levels.

Figure 5. Curves of saturation magnetization with temperature for several Skyworks ferrite materials.

Most ferrite materials have a critical power level of operation before non-linearity sets in and the device exhibits extremely high insertion losses. The onset of non-linearity corresponds to the RF power through a ferrite being converted to magnons or spin waves. The presence of spin waves increases the loss of the ferrite and will considerably degrade the performance of the device. The power threshold for the onset of non-linearity may be increased by doping the ferrite with atoms such as Ho3+ or Dy3+ which convert the excess RF energy to lattice vibrations rather than spin waves. Doping with these atoms however increases the device insertion loss at power levels below the onset of non-linearity. Another method of increasing the non-linearity threshold is by reducing the grain size of the ceramic so that the grain boundaries will scatter spin waves, but this is extremely difficult to control in a manufacturing setting. 

Skyworks extensive product offerings and global reach:

In addition, Skyworks has the advantage of being a large company, manufacturing many components in the wireless space. This leads to the potential of custom fitting the circulator with these other components, such as amplifiers, to create an integrated solution. This can be done, for example, by matching components at impedance values less than 50 ohms. 

Skyworks also has a global reach with circulators being designed in Ireland and manufactured in several Asian nations. Skyworks is one of the few circulator manufacturers in the world with a choice of manufacturing paths through multiple nations, making it an excellent secure choice for military applications in many countries.

In summary, although the operation of circulators for military applications is similar conceptually to those used for commercial applications, the wider range of frequencies, the increased mechanical ruggedness requirements, and the need to operate at high power levels offer additional design challenges for developing products for military applications. With the circulator design and ceramic processing expertise within Skyworks, we are in an excellent position to succeed in this exciting market.

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