Advantages of the Hybrid Circulator
Introduction
Circulators are 3 port components that couple power in a circulatingfashion: power from port 1 is coupled to port 2, from port 2 to port 3, andfrom port 3 to port 1. Circulators are widely deployed in communications
and radar systems because they enable a system architecture whereby asingle antenna (or array of antennas) can be used fortransmission and reception. Figure 1 illustrates a possible transmitreceive system realized with a traditional Y-junction circulator. An isolator
is sometimes used in front of the receiver to block reflected signals frompropagating to the transmitter.

Figure 1: Example of a transmit-receive system realized with a Y-junction circulator.

The Y-junction circulator has been the dominant technology at highfrequencies for many decades. Unfortunately, the Y-junction circulator
bandwidth decreases as frequency increases. A well-tuned Y-junction
circulator has a 20 dB fractional bandwidth near 10% in the WR-15 band
(50–75 GHz) but the fractional bandwidth drops to 2% in the WR-5.1 band
(140–220 GHz). Y-junction circulators are very difficult to tune at thehigher mm-wave frequencies because the fractional bandwidth is small,and the center frequency is highly sensitive to the ferrite core dimensions.

Micro Harmonics recently developed a new type of circulator that has
significantly greater bandwidth and higher frequency coverage than the
traditional Y-junction. Their “hybrid” circulator comprises a Faraday
rotator and an orthomode transducer which work together to achieve thecirculator function. Their initial line of hybrid circulators employs
asymmetric OMTs which enables an astounding 24% fractional
bandwidth. In theory, a symmetric OMT can be used to achieve fullrectangular waveguide band operation (40% bandwidth). Anotherimportant advantage of the hybrid circulator is that the bandwidth does
not degrade at higher frequencies like the Y-junction.

Bandwidth and Frequency Coverage
When it comes to bandwidth, more is better. In communication systems
the theoretical capacity for a given channel is directly proportional to thechannel’s bandwidth because the capacity is governed by Shannon’s
theorem:

C=B log2(1+S/N)
where C is the channel capacity (maximum theoretical data rate), B is thechannel bandwidth, S is the received signal power, and N is the totalchannel noise power across B. So, in communications systems the wider
the bandwidth, the higher the available data rate.
In test and measurement, the bandwidth specification can be limited andconstrained by the most-narrow band component in the system.
Wideband test equipment can be used in more scenarios and therefore
wide bandwidth subcomponents are often advantageous. Similararguments could be made for other technologies such as radar andremote sensing.
A comparison between the fractional bandwidths of the hybrid
circulator and a Y-junction circulator will clearly demonstrate thesuperiority of the hybrid circulator’s bandwidth. Fractional bandwidth isexpressed as a percentage and defined as:BWfractional=Fhigh–FlowFcenter
where Fhigh is the upper cutoff frequency of the component, Flow is thelower cutoff frequency of the component, and Fcenter is the center
frequency of the component—the arithmetic mean Fcenter=Fhigh+Flow2. For
reference, full waveguide bands are defined at around 40% fractionalbandwidth. Please note, at the time of this writing, the author could not
find any extant Y-junction circulators in WR-6.5, WR-4.3, or WR-3.4. (MicroHarmonics designed a WR-5.1 Y-junction circulator for reference, butmanufacturing commercial Y-junction circulators at these frequencies is
not feasible). Figure 2 below shows the fractional bandwidth of thetraditional Y-junction circulator and the hybrid circulator for comparison.

Figure 2: Comparison of the fractional bandwidths for hybrid circulators and Y-junction
circulators.

Not only does Figure 2 demonstrate the superior bandwidth of the hybrid
circulator, but it also shows that the hybrid circulator is an enablingtechnology for higher frequency systems. Until the release of the hybrid
circulator, there was no component on the market at WR-6.5 or above thatprovided the 3-port circulating functionality. Now there is.

Architectural Simplicity
Another feature of the hybrid circulator is that it can be configured sothat the transmitter port is internally isolated from the receiver port. To
understand why this is helpful, first look back at the transmit-receive
system in Figure 1. Isolators are placed on both the transmitter and thereceiver ports to reduce the signal power reflected to the transmitter. In aY-junction circulator, signals reflected at the receiver are coupled to thetransmitter port which is why a receiver port isolator is sometimes
required. Figure 3 below illustrates a possible transmit-receive system
realized with the hybrid circulator, rather than a traditional Y-junction
circulator.

Figure 3: Example of a transmit-receive system realized with a hybrid circulator.

Comparing this setup with the setup in Figure 1, it can be observed thatthe isolator in the receiver chain was removed. The isolator is no longer
necessary since port 3 does not couple power to port 1. This example
illustrates one way that the hybrid circulator can act as a 2-for-1
component. In these scenarios realizing a system with the hybrid
circulator will save space and simplify the system architecture. It should

also be noted that the ability to remove an isolator before the receiverchain will reduce the total insertion loss thereby improving the noise
figure and sensitivity of the receiver.
Sometimes the wider bandwidth provided by the hybrid circulator results
in a simplified system architecture. One example of this would be apoint-to-point communication system in WR-12 which is required to cover
the two popular unlicensed sub-bands, 71–76 GHz and 81–86 GHz. A Yjunction circulator would struggle to cover any one of those sub-bands,
whereas the hybrid circulator can cover 70—86 GHz—encompassing bothsub-bands. Not only does this reduce the number of total circulators
required from 2 to 1, but it will have repercussions on the rest of thesystem design, saving space and cost elsewhere.
You can find more details about the hybrid circulator here. You can find asummary of the performance of our hybrid circulators and their spec
sheets here. Feel free to reach out to Micro Harmonics
at sales@mhc1.com if you have any questions.