How to Choose an Isolator for aMillimeter-Wave System
Millimeter-wave (MMW) systems often employ isolators to suppress
standing waves and achieve optimal performance. Isolators pass an RF
signal traveling in the forward direction while absorbing signals travelingin the reverse direction. The absorbed signal is converted to heat energy
in a resistive load internal to the isolator.
There are a number of ways to implement the isolator function. For
example, an isolator can be formed by terminating one port ofa circulator. This discussion is restricted to the most common type ofisolator at the higher MMW frequencies, which is the Faraday rotation
isolator in rectangular waveguide. A further delineation is made between
isolators that operate in magnetic saturation and those that do not. Thisdistinction is helpful in understanding the unique performance
characteristics of the two types. The majority of isolators on thecommercial market are the unsaturated type while those producedby Micro Harmonics Corporation (MHC) are the saturated type.
It is common to think of isolators simply in terms of how well they isolate(how well they absorb the reverse traveling signal). But there are manyother important attributes which should be taken into consideration. We
will look at nine distinct characteristics.
1: Bandwidth – Faraday rotation isolators are inherently broadband since
the rate of signal rotation in the ferrite rod is largely independent offrequency. Both types of isolators, saturated and unsaturated, arecapable of bandwidths well in excess of a standard waveguide band(40%). For example, the MHC WR-10 isolator gives very good
performance over the extended waveguide band from 67-118 GHz, afractional bandwidth of 55%.
2: Isolation – MMW isolators typically have more than 20 dB isolation(and often more than 30 dB isolation) across full waveguidebands. Maximum isolation occurs when the signal polarity is rotated
45° as it propagates through the ferrite rod. This puts the electric field
vector into the plane of an internal resistive layer. The rotation must be
precisely 45° to obtain 30 dB levels. Even small rotational errors of ± 2°
can reduce the isolation from 30 dB to near 20 dB as indicated in thegraph.

The unsaturated isolators have a slight advantage here. They use a much
longer ferrite rod and a permanent magnet to produce the magnetic bias.
When the magnet is perfectly aligned to the ferrite rod, the signal rotation
is much greater than 45°. The magnet is then rotated off-axis and locked
into place when the optimal isolation is achieved. This tuning procedure
can even overcome small misalignments of the resistive layer to thewaveguide. Conversely, precise control over both the ferrite rod lengthand alignment of the resistive layer are critical for the saturated type. But
there are inherent advantages to using a saturating magnetic bias which
are discussed in the next section.
3: Insertion Loss – This is a measure of how much loss is incurred as theRF signal traverses through the isolator in the forward direction. No loss
is ideal. This is perhaps the most important measure for MMW isolators
because of the difficulty and expense of generating MMW signal power.
A primary concern of MMW system designers is minimizing loss andpreserving signal power. Using an isolator with the lowest possibleinsertion loss helps to ensure that the maximum output is achieved.

The predominate loss mechanism in isolators is loss in the ferritematerial. Saturated isolators like those made by Micro Harmonics (MHC)
use the shortest possible ferrite rod length to achieve the required
45° rotation. Since the ferrite rod is significantly shorter, the insertion loss
is minimized. The table below shows RF signal transmission for bothtypes of isolators (50% transmission equates to an insertion loss of 3dB). In the WR-4.3 band 170-260 GHz, an MHC isolator has atransmission of more than 70% (1.4 dB insertion loss). In the same band,the unsaturated competitor products are closer to 25% (6 dB). By using
the unsaturated isolator in your system, you are throwing away 75% ofyour RF signal power.

In the case where there is signal power to spare, the lowest loss isolator
is still the best choice. The RF source can simply be “turned down”. This
lowers the operating temperature, increases the mean time to failure, anddecreases the power consumption. Conversely, when an isolator withhigh insertion loss is used, the signal source must be driven harder toovercome the loss. The system runs hotter, fails more quickly, uses morepower, has higher noise, and generates more harmonic content.

There are other costs to using a high insertion loss isolator. Because theRF signal power is reduced, it may be more difficult to meet the system
specification. This can lead to many additional hours of tuning andtesting and missed deadlines.
4: Frequency Coverage – Traditional style, unsaturated isolators arecommercially available from over a dozen vendors at frequencies up tothe WR-10 band (75-110 GHz). A few vendors have models up to the WR6.5 band 110-170 GHz. Fewer still offer models up to WR-4.3 band. MicroHarmonics produces and stocks isolators up to the WR-2.8 band 260-400
GHz. It is no mystery why few vendors offer the traditional style isolators
at the higher MMW bands. Who can afford that type of loss in theirsystem?
5: Power Handling – MHC isolators employ a unique diamondheatsink that substantially lowers the internal temperature. This meansthat they can handle more power while operating at much lower internal
temperatures when compared to competitor products. No other vendor
offers this. The graph below shows the maximum reverse power ratingsof the MHC isolators and the average of other vendors. The MHC power
ratings are conservative to ensure low internal temperatures and long life.

6: Port Reflections – The primary function of an isolator is to absorb
reflected signals and suppress standing waves. But if an isolator has highreflections at the ports, you are simply trading one reflection foranother. Port reflections are often expressed in terms of VSWR. The
lower the VSWR, the better. The VSWRs of saturated and unsaturated
isolators are similar.
7: RF Test Data – MMW isolators have unique signatures that arise fromsmall misalignments and variations in the internal parts. The differencesin the performance between one isolator and the next can be substantial.
Max, min, and typical specs are helpful, but what you need to see is theactual test data for the isolator you are buying. Micro Harmonics tests
every isolator across the full waveguide band on a vector network
analyzer. The RF test data is supplied to the customer at no additionalcost.
8: Stray Magnetic Fields – Small external magnetic fields can alter theoperating point of an isolator and substantially degrade the performance.
Traditional style, unsaturated isolators are sensitive to even small straymagnetic fields. This is contrary to what many engineers assume since
the traditional isolators employ a magnetic sheath around the main body.
But this sheath does not protect against axial magnetic fields which are
precisely the ones that alter the Faraday rotation and degrade the isolator
performance. MHC isolators are not affected by external magnetic fields
because the ferrites are biased hard into magnetic saturation. It would
require a very high magnetic field in exactly the opposite polarity to pull
the MHC isolator out of saturation and perturb the performance.
9: Size & Weight – Size and weight are often important considerations for
MMW system designers. Micro Harmonics produces the most compact
and lightest weight isolators on the global market. For example, in theWR-10 band 75-110 GHz, an MHC isolator has external dimensions of 19
x 19 x 16.7 mm and weighs only 16 grams. A typical competitor isolator
in the same band measures 25 x 25 x 70 mm and weighs 60 grams. The
difference is a factor of four in both size and weight.

Conclusion
A variety of manufacturers produce MMW Faraday rotation isolators thatexhibit high isolation and low VSWRs across full waveguide bands. But
there is a wide gap in performance when it comes to insertion loss. The
isolators made by Micro Harmonics have the lowest insertion loss by awide margin. This is one reason why their isolators are offered at muchhigher frequency bands.
Additionally, Micro Harmonics isolators have unique diamondheatsinks which channel heat away from the internal resistive load. This
substantially lowers the internal operating temperature leading to
reduced failure rates and higher power ratings. The MHC isolators arealso much smaller and lighter weight than the traditional style isolators.
Lastly, MMW isolators have unique signatures across their operating
bands which cannot be sufficiently characterized using “typical”
specifications. If the manufacturer tested your isolator, they can easily
provide the data. If you have any additional questions, feel free to contact
us!