the complete microwave solution
Ferrite
Basic Description of Operation
The typical junction circulator is basi-
cally made up of a 3-arm stripline circuit
sandwiched between 2 wafers of ferrite
and dielectric material (known as pucks)
and further sandwiched between an upper
and lower ground plane. When a mag-
netic field is applied through the vertical
axis of the stripline assembly, a directivity
or circulation of energy results from one
connector to the other depending on from
which direction the energy approaches the
device as shown in Fig. 1.
Microwave signals entering Port 1 are
directed to Port 2, signals entering Port
2 are directed to Port 3, etc. If Port 3 is
terminated in a 50 Ω load, the device be-
comes an isolator, i.e. a device that passes
signals with low loss in one direction
(Port 1 to Port 2) and with high loss in
the reverse direction (Port 2 to Port 1). As
such, it is used to “isolate” one microwave
device from another.
Isolators and Circulators
Bandwidth vs Performance
Since the characteristic impedance of the
ferrite junction is usually less than 50 Ω,
the circulator must always contain some
type of impedance transformation. In all
Teledyne Microwave circulators, one or
more quarter-wavelength transformers are
used depending on the operating band-
width of the unit. The overall perfor-
mance of the unit is therefore a function
of how well the impedance transformation
is realized in going from 50 Ω to junction
and back to 50 Ω.
Figure 2 Ferrite in Unmagnetized
state
Temperature Effects
The performance of a circulator or isolator
is largely dependent on whether enough
magnetic field is applied to saturate the
ferrite material to its specified saturation
magnetization, or 4πMs. A sample of fer-
rite is made up of many magnetic domains
which, in the unmagnetized state, appears
in a random fashion (See Figure 2).
The 4πMs value of the specific ferrite
coincides with the amount of externally
applied magnetic field needed to align the
domains. When all domains are aligned,
the material is fully saturated (See Figure
3).
Figure 1
Figure 3 Ferrite in Magnetized
state
OFOISR App 06-S-1942
TELEDYNE
MICROWAVE
1274 Terra Bella Avenue, Mountain View, CA 94043
Tel: 1.800.832.6869 or +1.650.962.6944 Fax: +1.650.962.6845
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Ferrite Isolators and Circulators
If the ferrite is only partially magnetized,
the device will exhibit lowfield losses due
to the fact that all the domains are not
perfectly aligned. The 4πMs value of the
ferrite can be plotted as a function of the
applied field, H (See Figure 4).
The variation in the 4πMs can be com-
pensated to some degree, and the ferrite
device can be made relatively stable over
selected temperature ranges. However,
due to the intrinsic composition of the
ferrite material, temperature effects can-
not be totally negated.
Model Numbers
T - 12 S 4 3 U - 30
X - XX X X X X - XX X X
Figure 4
4
π
Ms vs Applied Field
Once saturation has been obtained, Ms
will still vary as a function of tempera-
ture. As the temperature is raised, the
domains will shift position in a random
fashion and will not always realign.
This results in a lowering of the satura-
tion magnetization. If the temperature
is raised even further, the domains will
move faster and be more misaligned. If
this process is continued, the result is
that at some given temperature, the 4π
Ms becomes zero because the thermal
energy within the ferrite is greater than
that supplied by the external field. The
temperature at which this occurs is called
the Curie Temperature (See Figure 5).
Model
T
C
Isolator
Circulator
Customer Mounting Holes
M
A
B
C
U
P
T
S
H
C
Metric threads
Female P1, Male P2
Male P1, Female P2
Male P1, Male P2
Zinc plate and chemical
film
Paint (min. humidity and
no silver)
[3]
Paint (humidity and RFI
up to -50 dB)
[3]
Paint (humidity and RFI
up to -100 dB)
[3]
Paint (100% humidity
and RFI up to -50 dB)
[3]
Custom order
Connectors — Isolators only
[1]
Starting Frequency
(Rounded to lowest GHz)
Example 12.8 GHz = 12
Connectors
S
T
N
SMA
TNC
N
0 = 0-9%
1 = 10-19%
2 = 20-29%
Model Finish
[2]
% of Bandwidth (Approximate)
Example:
Number of Ports
3
4
5
Figure 5 Effect on 4
πMs as
3 port model
4 port model
5 port model
Temperature is raised
[1] Catalog models have SMA female connectors on Port 1 and Port 2.
[2] Standard catalog models have zinc plate and chemical film finish.
[3] Gray epoxy paint color #36231.
OFOISR App 06-S-1942
TELEDYNE
MICROWAVE
1274 Terra Bella Avenue, Mountain View, CA 94043
Tel: 1.800.832.6869 or +1.650.962.6944 Fax: +1.650.962.6845
www.teledynemicrowave.com
microwave@teledyne.com
Ferrite Applications
Above and Below Resonance
Designs
At operating frequencies below 3 GHz,
three distinct design options are avail-
able. The lumped constant design pro-
duces a unit that can operate at very low
frequencies, is temperature stable, and
can be constructed in a small package.
The limitations of this type of design
are bandwidth (approximately 10°/o
maximum) and power handling capabil-
ity (low power only). The above resonance
approach, so named because the ferrite
resonant frequency is above the operating
band, can be made in a small to medium
size, and is temperature stable. This unit
is also limited by bandwidth, which can
approach 35%. The final design is the
below resonance configuration, where the
ferrite resonant frequency is below the
operating band. This unit offers extremely
wide bandwidths (in excess of 70%). The
only disadvantages of units of this type
are larger size and decreased tempera-
ture stability. Above 3 GHz, however,
the below resonance approach results in
a highly dependable and efficient unit.
Teledyne Microwave evaluates individual
requirements and chooses the design best
tailored to satisfy them.
Bandwidth
Up to 100%
Up to 10%
Up to 25%
Up to 30%
30% to 40%
Typ Frequency Range
500 MHz to 1.5 GHz
1.5 GHz to 18 GHz
500 MHz to 2.5 GHz
2.0 GHz to 26.5 GHz
1.5 GHz to 26.5 GHz
Four Port Type
Figure 8
Single Junction Tee Type
Figure 9
Several mounting options are provided. For specific models, use these drawings
for reference and consult the factory, or ask for a mechanical outline drawing.
Design Approach and Basic Characteristics
Lumped Constant Small size, temperature stable.
Below Resonance Small size, low insertion loss, temperature stable.
Above Resonance Medium size, excellent temperature stability.
Below Resonance Medium size, good insertion loss, average tempera-
ture stability.
Below Resonance Maximally flat tuned, excellent return loss and isola-
tion, high performance unit good for communication band and high
ratio applications.
Below Resonance Standard octave band design, medium size.
Below Resonance Standard octave-plus design provides extended
band coverage. Higher loss and larger size at lower frequencies.
Up to 67%
Up to 90%
500 MHz to 26.5 GHz
1.0 GHz to 18.0 GHz
OFOISR App 06-S-1942
TELEDYNE
MICROWAVE
1274 Terra Bella Avenue, Mountain View, CA 94043
Tel: 1.800.832.6869 or +1.650.962.6944 Fax: +1.650.962.6845
www.teledynemicrowave.com
microwave@teledyne.com
Ferrite Applications
Isolator Input VSWR
The effective input VSWR of an isolator
will vary as a function of the load VSWR.
Considering a typical 20 dB isolator, the
basic input VSWR with a 50 Ω load on
the output will be 1.22:1. As the output
load mismatch is increased, energy is
reflected to the termination port, attenu-
ated by 20 dB, and the balance is reflected
back to the input. This will increase the
total input VSWR seen at the input. The
obvious worst case is when the output is
terminated in a short circuit and the input
sees two 1.22:1 VSWRs adding up in
some phase. If the designer must maintain
a certain maximum VSWR under any
load condition, he may have to specify a
higher isolation unit which could be either
a higher performance single junction
design or a multiple junction unit. Curves
illustrating the inter-relationship of input
VSWR, isolation required, and load
VSWR are shown below (Figure 6).
To determine the value of isolation
needed to reduce the input VSWR to a
desired value.
1. Divide the desired reduced input
VSWR by the isolator’s input VSWR to
obtain the normalized VSWR.
2. Determine the intersection of the nor-
malized VSWR and the device VSWR
terminating the isolator. Read the
required isolation from the abscissa.
Example: You have a device with an
input VSWR of 5.0:1 and you would like
to reduce this to 1.50:1 using a ferrite
isolator with a 1.20:1 input VSWR. The
normalized VSWR is 1.50/1.20 = 1.25,
the terminating VSWR is 5.0:1 and the
resultant required isolation is 17 dB.
VSWR Reduction using a
Ferrite Isolator
Notes:
To determine the worst case
resultant VSWR with a
known value of isolation:
1. Determine the intersec-
tion of isolation and the
device VSWR terminat-
ing the isolator. Read the
normalized VSWR from
the ordinate.
2. Multiply the normal-
ized VSWR by the isolators
input VSWR to obtain the
resultant reduced input
VSWR.
Example: You have an isola-
tor with an input VSWR
of 1.25:1 and isolation of
20 dB. You are terminating
this unit with a device that
has a 4.0:1 VSWR. The re-
sultant normalized VSWR
is 1.15:1 and the resultant
reduced input VSWR is
1.25 x 1.15 = 1.44:1.
Stripline Circulators and
Isolators
Virtually all of the standard Teledyne
Microwave ferrite designs can be provided
with outputs that mate with stripline.
Consult our engineers to coordinate the
most desirable interface configuration.
Discussion of 4-Port Circulator/Isolator
Parameters
A 4-port circulator is shown in Figure 7.
The isolator equivalent would have both
ports 3 and 4 terminated. The given inser-
tion loss spec would be valid for a signal
traveling from ports 1-2 and the isolation
valid for a signal traveling from ports 2-1.
These numbers are indicative of the isola-
tor only (i.e. ports 3 and 4 terminated).
When specifications on a circulator are
discussed, some guide-lines should be
followed.
Figure 6
OFOISR App 06-S-1942
TELEDYNE
MICROWAVE
1274 Terra Bella Avenue, Mountain View, CA 94043
Tel: 1.800.832.6869 or +1.650.962.6944 Fax: +1.650.962.6845
www.teledynemicrowave.com
microwave@teledyne.com
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