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RF Components: Ferrites

Operating Characteristics of stripline isolators and circulators

The following is a summary to simplify the operating characteristics of Mica's various stripline isolators and circulators.

STRIPLINE JUNCTION EXPLAINED: We break our stripline junction circulators and isolators into two types, drop-in and coaxial (coax). Both the drop-in and coax use the same basic construction as shown, where the center conductor (called the circuit) is sandwiched between two ferrite disks or triangles (called the pucks). This circuit and puck assembly is then further sandwiched between two ground planes and magnetically biased using permanent magnets.

When the ferrite material is properly magnetized, ferrimagetic resonance occurs when a rotating RF magnetic field has the same direction and frequency as the electrons in the ferrite material. The magnetic biasing of the junction circulator must be set so the operating region is either above or below the ferrimagnetic resonance. Extremely high insertion loss will occur at resonance. High insertion loss can also occur at very low biasing fields.

The ferrite and the intersection of the three transmission lines from the Y-junction circuit is where the actual circulation takes place. A simple model can explain the way this works as shown. When an RF signal is applied at port 1 two counter-rotating waves are generated that will rotate at different velocities w+ and w-. The velocity of the wave as it propagates through the magnetically biased ferrite material will depend on its direction if rotation. With the proper ferrite material and biasing field the phase velocity of the wave traveling in one direction will be greater than the wave velocity traveling in the other direction. With the signal applied to port 1 the signals will arrive in phase at port 2 for maximum power transfer and cancel at port 3.

DROP-IN TYPE: The drop-in isolator or circulator is a non-connectorized unit, allowing the center conductor to be soldered or ribbon bonded directly to a stripline circuit board assembly, by the use of tabs. The tabs on the drop-in units are usually 5 mil thick beryllium copper and can be gold plated for ribbon bonding, silver or solder tin plated for better solderability or unplanted for cost savings. The tabs may also be configured for flush mount or stress relief. Careful consideration should be given to the mounting and grounding of the unit housing as well as the geometry of the mating substrate. It is essential that MICA is supplied with as much information as possible on how the unit is to be integrated into the final assembly. The drop-in type isolators and circulators are of an open construction and are not recommended for use in high humidity conditions where water condensation may occur, where salt atmosphere is of concern or where EMI or RFI limitations may apply.

COAX TYPE: The coax isolators and circulator are connectorized units and can be supplied with various connector types. SMA female or male connectors are to most popular, cost effective and easiest to install. Other connector configurations can be used such as K or 3.5mm, right angle, TYPE N, TNC, GPO and OSP to name just a few. Some connectors can, however cause limitations on size, electrical, or environmental performance. The coax type isolators and circulators can be built to withstand high humidity up to 100%, salt atmospheres and pass EMI or RFI specifications through the use of sealing epoxies and paints.

Other connector configurations may be used such as, mixed connector types, removable connectors, where one or more connections can have removable shells allowing the center conductor to be soldered to a circuit board much like the drop-in units. Waveguide adapters, called isoadapters, are a circulator or isolator mounted to a waveguide flange and is particularly useful for connecting directly to a wave antenna with the waveguide port while the SMA connector port can be fed directly into a solid stale amplifier.

OPERATING PARAMETERS: The three main operating parameters for a circulator or isolator are Isolation, Insertion Loss and VSWR: Isolation: This is a measure of the signal in the high loss direction. In the case of the isolator, which has an internally terminated 3rd port, the isolation will be the ratio of the signal applied to the output port to the signal measured at the input port, expressed in dB. In the case of the circulator, this parameter is not applicable; however, a precision termination may be used on the 3rd port to measure isolation.

Insertion loss: This is a measure of the signal in the low loss direction. In the case of the isolator the insertion loss will be the ratio of the output signal to the input signal expressed in dB. In the case of the circulator a precision termination will be used on the 3rd port.

VSWR: The measurement of the reflected signal back toward the source from any give port.

Breakdown: The peak power breakdown value of a stripline junction is reduced by an increase in load mismatch, altitude, temperature or pulse width. A mismatch on the output port will reflect a percentage of the signal back into the junction causing a high internal voltage level. The peak power rating can be increased be filling the internal volume of the device with a high dielectric strength material. Hermetically sealed modules can be used to maintain pressurization for high altitude operation.

Limiting: Another effect related to the peak power rating of a circulator is known as non-linearity or peak power threshold. As the peak power level increases beyond a critical value, the loss versus magnetic field curve will show considerable changes in the B/R region. The A/R region will show little affect.

Average Power: The power dissipation in the junction is in proportion to the insertion loss. If the average power level is significant, the dissipated power will cause heating of the ferrite and degradation in performance. The average power rating is also dependent on the mismatch at the output port. For example if a signal of 100 W average power were applied at the input with a mismatch of 6.00:1 on the output, 51 watts would be reflected, requiring the junction to handle 151 watts total. Note that this will also affect the termination power rating requirement for isolators.

OPERATING FREQUENCY There are two types of stripline junction circulator/isolator designs, above resonance (AR) and below resonance (BR).
AR designs fall into the 150 MHz to approximately 2.5 GHz range. Although operation above and below these frequencies are achievable, impractical magnetic circuits would be required in order to bias the ferrite material.
BR designs are generally limited to frequencies above 500 MHz to approximately 40 GHz

SIZE: The size of the circulator or isolator is a function of the frequency and/or bandwidth. Lower operating frequencies or units with greater operating bandwidths are usually larger than higher frequency or narrower bandwidth units.

BANDWIDTH: The A/R junction is generally limited to 35% maximum bandwidth. The B/R junctions allow for broader bandwidth designs up to 100% or more.

TEMPERATURE: Operating temperatures from -54°C to +95°C can be obtained on most models depending on the bandwidth and level of performance desired.
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