How to Specify Isolators & Circulators

by Tony Edridge
M2 Global Technology, Ltd.

M2 GLOBAL’S standard and high power isolator and circulator products are available in Coax, Waveguide, Drop-in, Puck, and Dual Junction configurations, over the frequency range 300 MHz to 40 GHz.  All models have been optimized to meet the following parameters for most popular applications: bandwidth, VSWR, isolation, insertion loss, temperature, and size.  These and other parameters can be selectively optimized for your specific application.  The following is a brief description of the various parameters and available options.

VSWR, or Voltage Standing Wave Ratio, is a measure of the signal reflected from a given port when a signal is applied to that port.  For critical applications, a Smith Chart (with an impedance plot recorded at a specified reference plane), can be provided with each device.  A typical specification for VSWR is 1.25; however, values of 1.10 can be achieved for some device configurations.

This parameter is used to specify the reverse loss characteristic of an isolator, between the output and input ports.  All isolators described in this catalog consist of a circulator with an internal termination.  The three parameters, isolation, VSWR, and insertion loss, are required to specify electrical performance of an isolator, whereas a circulator is completely defined by its VSWR and insertion loss.  Although a circulator can be made into an isolator by terminating one port, it does not have an intrinsic isolation value.  With a termination on the third port, the isolation measured would depend on the VSWR of both the termination and the circulator port.  Most isolators are specified at 20 dB, but values of 26 dB can be obtained for narrow band applications.

Example:  A circulator has a measured VSWR of 1.2 for all three ports.  If a perfect test termination with a VSWR equal to 1.00 were placed on Port 3, the resulting isolation from Port 2 to Port 1 would be the return loss equivalent to the circulator VSWR, in this case 20.8 dB.  If a test termination with a VSWR of 1.05 were placed on Port 3, the isolation from Port 2 to Port 1 would vary between 18.2 and 22.5 dB, depending on the phase difference between the two VSWRs.

This parameter is used to specify the forward loss characteristics of an isolator or circulator.  Most of our catalog models have an insertion loss specification between 0.2 to 0.4 dB. Many low noise systems require an isolator with as low an insertion loss as possible.  For these applications, the insertion loss can be minimized by using low loss ferrite and dielectric materials, and by silver plating circuit elements.  Insertion loss of .10 dB is routinely achieved in production for certain device configurations.

The operating temperature range of an isolator or circulator is limited by the properties of magnets and ferrite materials.  In general, as the operating frequencies decrease, isolator temperature sensitivity increases.  Catalog units make use of temperature compensation maaterials where possible.  Operating temperatures from -20 to +65°C or from -40°C to 100°C are typical, although some models are limited to 0 to 50°C.  Special temperature compensation can be provided for most units to operate from -55 to +125°C.

Catalog units all have sufficient magnetic shielding for general handling and mounting, and can be mounted within 1/2 inch of one another (or from other magnetic materials) without degrading electrical performance.  For more stringent applications (mounting in direct contact with a magnetic plate), additional shielding may be required, usually increasing package size.

Standard Models have an RFI leakage specification at close proximity of -40 dB.  Special packaging and sealing methods are available to improve RFI shielding.  Leakage values up to 100 dB can be provided at a nominal cost.  RFI leakage is usually not specified for Puck configurations.

The termination is designed to safely dissipate reverse power into the isolator heat sink.  The termination power rating should be specified to exceed power levels that might occur under normal or anticipated fault conditions.  Maximum reverse power depends on the customer application, but may be as high as the power applied to the input port.

Isolators are rated for reverse power levels between 1 and 500 Watts, depending on device configuration and termination capabilities.  Special design considerations are required for pulsed signals with high peak power.

The input power to an isolator or circulator can be supplied from a continuous wave (CW) or a pulsed source.  In the case of a pulsed source, both the peak and average power components of the pulse train should be specified in order to determine adequate safety margins.

CW (or average) power ratings depend on frequency and on device configuration.  Low frequency waveguide devices generally have the highest power ratings.

Isolators and circulators for high peak power applications have special design features to avoid breakdown or arcing, which would otherwise cause permanent degradation in performance.  Proper connector selection, optimized internal geometry, and encapsulation are required to maximize the peak power capability of a particular model.  Peak power levels up to 5 kW are possible on certain models.  Contingent on the peak power level and other parameters, units can be provided that will operate to altitudes of over 100,000 feet.

High peak powers can cause an increase in the insertion loss in below-resonance designs, due to non-linearity effects of the ferrite material.  This increase can occur at peak power levels considerably lower than that required for breakdown or arcing.  The increased insertion loss would cause more power to be dissipated in the ferrite region of the device, which could result in overheating.  Special ferrite materials are used to avoid this situation.  Such non-linearity effects do not occur in above resonance models.

The CW power rating of an isolator or circulator is determined by its insertion loss, the internal geometry of the ferrite region, and the type of cooling available.  The insertion loss of an isolator or circulator causes a small fraction of the input power to be absorbed and dissipated in the ferrite region and the conductor surfaces as heat.  Adequate cooling techniques are necessary to insure the ferrite material does not reach an excessive temperature.  Mounting the device to a heat sink is sufficient in many cases if the average power is moderate.

In high power applications, a component with a high VSWR connected to the output port of an isolator will reflect a substantial amount of power.  The temperature of the ferrite region as well as the internal voltage will increase, causing performance to deteriorate or arcing to occur below the rated power level.

Isolators and circulators that meet stringent peak and average power levels require design considerations for many parameters.  These include normal and worst-case load VSWR conditions and the cooling that would be required under worst case conditions.

The connectors used on coaxial models are N-Type or SMA female.  Other connectors can be provided based on operating frequency and package size; however, certain types may cause some electrical degradation.

Many applications require isolators and circulators to be supplied as phase matched sets.  Although our catalog models are not phase matched, this feature can be provided on a specified basis.  The tolerance in phase matching will depend on the particular model and size of the lot to be matched.  Phase matched pairs can usually be provided to within ±5 degrees.  Linearity of the insertion phase also can be specified.  It is usually defined as a deviation from a best fit straight line of insertion phase versus frequency.