APPLICATION NOTE
Factors That Influence the Power Handling Capability of
Circulators
Introduction
This Application Note describes the factors that influence the
power handling capability of circulators. The most common
circulators are the junction type, available in stripline (coaxial,
drop-in, and surface mount), microstrip, and waveguide forms.
Circulator power handling depends on several factors: power
handling of coaxial connectors, power handing of the basic
circulator, and power threshold of the ferrite material used. The
power handling capability of circulators can be exceeded in two
ways: peak power and average power. Excessive peak power
leads to corona and arcing due to the high voltages that can be
present. Average power failure is typically due to overheating.
Stripline and microstrip circulators operate in one of two magnetic
bias regions: above resonance and below resonance.
Above resonance circulators are typically limited to an upper
operating frequency of 2.5 GHz. However, with careful material
selection and optimized magnetic circuit design, the upper
operating frequency limit can be extended to 3.6 GHz. Below
resonance circulators can operate up to frequencies of
approximately 30 GHz.
Circulators That Operate Above Resonance
(Magnetic)
Stripline circulators are constructed using a center conductor
sandwiched between two ferrite discs or triangles. These ferrites
are placed between ground planes and magnetically biased by
permanent magnets outside the ground planes.
The average power rating for an above resonance circulator is a
function of the circulator insertion loss, the thermal conductivities
of the materials used in the construction of the circulator, and the
temperature of the heat sink on which the circulator is mounted.
The power dissipated in the circulator junction is calculated from
the average input power and the insertion loss in dB, using the
following equation:
(
Power dissipated = PIN 1 − 10 (−loss / 10 )
)
The peak power rating for an above-resonance circulator is a
factor of the ground plane spacing and the proximity of the center
conductor to the circulator housing. At excessive peak powers,
voltage breakdown can occur. Maintaining a sufficient air gap
between the center conductor and ground, and the use of high
dielectric strength materials ensures that the peak power rating is
achieved.
The peak power rating is important in pulsed power applications.
Bursts (pulses) of RF with no RF present between bursts is
referred to as “pulsed RF.” The most general case of pulsed RF
consists of pulses of a fixed pulse width (PW) that come at a fixed
time interval or period (T). The ratio of the pulse width to the pulse
period is the duty cycle:
Pulse Width ( PW )
Average Power
= Duty Cycle =
Pulse Period (T )
Peak Power
The peak power rating for an above-resonance circulator is a
factor of the ground plane spacing and the proximity of the center
conductor to the circulator housing. At high peak powers, the
main concern is voltage breakdown.
Circulators That Operate Below Resonance
(Magnetic)
The average and peak power ratings of a below resonance
circulator are a function of the ferrite material used. High power
levels in below resonance circulators result in the onset of nonlinear absorption in ferrites.
The generally accepted model for explaining the phenomenon of
non-linear absorption is that of spin wave build up in the material.
Spin waves are excited when the RF H-field exceeds a critical
value. The critical field is dependent on many factors, such as
geometry, magnetization, RF power, main resonance line width,
the relevant spin wave line widths, gyromagnetic ratio, and
operating frequency. The theory provides for a spectrum of spin
waves, some having lower critical fields than others. This
accounts for the fact that as RF power is increased, absorption
continues to increase as more spin waves are excited.
Average power failure in a circulator leads to overheating, which
results in degraded performance that exceeds the specified
circulator limits.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201543A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • May 18, 2011
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APPLICATION NOTE • POWER HANDLING OF CIRCULATORS
The insertion loss of the circulator is effectively non-linear and
increases as the power level increases. The power threshold in
below resonance circulators can be increased by doping the
ferrite material with elements such as cobalt and holmium, which
results in a marginal increase in insertion loss at low power
levels.
Reduced grain size of polycrystalline ferrites and garnets
increases the high power threshold without increasing the low
power insertion loss. When the spin wave wavelength and the
grain size are similar, the spin waves are effectively broken up
and the peak power threshold is increased. The disadvantage of
smaller grain size is a higher cost.
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Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
May 18, 2011 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • 201543A