The Evolution of Ferrite-based Microwave Circulators from Connectorized to True Surface Mount Devices
Abstract
Microwave Circulators are RF devices that are leveraged in a vast range of applications from duplexers in military and commercial radar, to general protective circuitry as isolators. In these high performance RF and microwave applications, they are often ferrite-based devices, taking up blocks of real estate on boards and in assemblies due to the length of the transmission line and inherent packaging required. A few microwave circulator companies have attempted to create surface mount, tape and reel oriented replacements of these traditionally bulky devices. But some have been more successful than others in overcoming the challenge in achieving true automated assembly and solder reflow compatibility. This article explores how TRAK Microwave, a Smiths Interconnect brand, has accepted and successfully completed the challenge and recently announced a new line of microwave circulators starting with X-band models.
What does an RF or microwave circulator do?
A circulator in an RF or microwave application is a passive device that is often leveraged for it’s non-reciprocal properties to permit the flow of a signal in one direction only. This component is often used either as an isolator, or, a duplexer. An isolator is essentially a 3-port circulator where one port is terminated with a matched load to send signals down a 2-port path with little to no reflections back. Duplexers are typically used in radar applications to send signals from the transmit path to the antenna, or, from the antenna to the receive path—all while isolating the transmit and receive paths. In other words, a 3-port circulator will allow a signal to transmit from port-1 to port-2 but not port-3, or from port-2 to port-3 but not port-1, and from port-3 to port-1 but not port-2.
How are microwave circulators made?
Circulators are often created from ferrite materials, or, a type of dielectric material that can be magnetized. The rotation and direction of the signal transmission is based on the interaction of the electromagnetic wave with the ferrite. Typically, a circulator is made with two disc-shaped magnets and ferrites that sandwich a stripline ‘y-junction’, or a y-shaped conductor, to generate an electromagnetic pattern with identical counter-rotating waves for non-reciprocal properties. It is a challenge to keep circulators thin and small as it typically requires a long transmission line for impedance matching, and ferrite materials tend to be bulky.
Typical Circulator Packaging
There are several different packages for a traditional circulator: coaxial, drop-in, microstrip or, surface mounted (SMD). Coaxial circulators simply have coaxial connectors to connect to the ports while microstrip-based circulators have ferrite material deposited on a ground plane with y-shaped metallization pattern and a magnetic disc placed on top. Drop-in packaging typically includes four tapped holes to secure the circulator to the circuit board with screws as well as a number of leads to solder down the component. SMD circulators are similar to drop in circulators in packaging with the difference being that SMD circulators do not require the tapped holes to secure the circulator to the board.
Steps in mounting a circulator
Attaching a circulator typically requires custom mounting, particularly for the SMD and drop-in models of circulators. Firstly, drop-in models require a mounting area to be cut-out on the PCB as well as corresponding tapped holes to screw down the component. It is important that the mounting area is minimally cut out to ensure that the trace is as close as possible to the edges of the circulator, then, the circuit tabs have to be carefully bent to align with the trace pad to prevent any short circuiting. Soldering down the circulator must be rapid and precise as excess heat from the soldering iron can demagnetize the magnets thereby degrading performance of the device. This custom mounting step is why engineers and program managers have been requesting a surface mount, automated assembly solution.
A true surface mount device would allow for automated assembly, for example, tape-and-reel compatibility and solder reflow compatibility. Most current circulators will not fit into standard tape-and-reel packaging and some may even require mounting screws for a secure fitting. Since most circulators require ribbon bonding and the manual bending of leads to solder down the component properly, it is difficult to allow for a solder reflow profile as that requires the trace to be flush with the lead. TRAK eliminates the ribbon bonding techniques and manual installations with standard SMT packaging.
Future models will include performance from 2-4 GHz in 0.75” square base at 0.066” high. These new devices are optimal for radar phased array antenna applications, communications applications, military applications, and cellular applications as they not only allow for automated assembly they further lower costs by eliminating a commonly required ferrite junction in the transmit/receive chain.
So where might you get duped?
Some manufacturers, as you'll see upon close inspection, are attempting to pass off quasi-stripline devices using a wrap-around ground plane to achieve “surface mount” status. While mountable directly to a ground plane, these devices are far from automation capable.To get a glimpse at the diference, watch our video "The back story on achieving the world's first TRUE SMT microwave circulator". Jump to 2:40 to see a "quasi" SMT example.
