Microwave circulator Design principle

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Important Design Parameters

This report deals with the design procedure, result analysis and comparison of two routing modules, but before going into the design details, let’s get familiar with some of the important performance parameters which are generally taken into account while designing these routing modules.

Frequency of operation and bandwidth

The desired frequency of operation and the allowed bandwidth is the foremost design consideration any designer will have to deal with. With the rapid growth of wireless applications and systems, frequency spectrum has found its limitations in terms of available frequency bands. In this case, broadband devices have definitely an upper hand over narrow-band devices. Use of broadband devices eliminates the cost of up gradation in case of the requirement for the system to be used for higher frequency range.

Insertion loss

Insertion loss refers to the power loss of the transmitted signal. Insertion loss is the measure of power loss and signal attenuation and it varies with frequency [2]. The transmitted signal power loss, adversely affects the overall performance of the system. Additional power is then required to compensate for the power loss which itself becomes a limitation because of the limited power handling capability of the device [3]. Power loss is suffered by the transmitted signal due to two factors.
• Because of the presence of resistive components like parasitic capacitance, inductance and resistance in the device signal is transmitted through.
• And because of high VSWR created due to mismatch between the ports or transmission lines involved in the design.

Isolation

Isolation in simple words can be defined as ‘very high transmission loss’. It is expressed in decibel (dB). As discussed in ‘introduction’, routing module is supposed to deliver power to one of its ports and block the power from entering the other port. The port where signal is supposed to enter to, is referred to as on-port and port which was blocked for the incoming signal, is referred to as off-port. By this, isolation can be defined as, “the ratio of power level in the off-port to the power level in the on-port”. In other words, isolation is the suppression of the signal in excess of the insertion loss at the off-port [3]. Isolation indicates the ability of the device to prevent stray signals leaking into the off-port. Leaked signal entering the off-port results in reflections and ultimately results in poor performance. Higher the isolation level, better will be the performance. Isolation becomes even more critical consideration in devices which involves the signal entering and leaving more than one port.

Return loss and VSWR

Voltage Standing Wave Ratio ‘VSWR’ is the degree of reflected signals in a device and when it is expressed in dB, it is referred to as ‘Return loss’. Reflections occur in any device due to impedance mismatch between device components or the device itself with other devices. At higher frequencies, any component’s dimensions along with its physical parameters are responsible in determining impedance match or mismatch with other device components. Increased bounced-back signals, resulting from poor matching between the components, reduce the power transfer. Higher values of return loss indicates that reflected signal suffer much obstruction on its way back towards the signal source, so, higher the return loss, better the matching and ultimately efficient the performance.

Reflective or absorptive

Another important design parameter of the routing module is being ‘reflective or absorptive’. If the routing module is reflective, the RF signal is reflected back towards the signal source from the off-port. Reflective designs are relatively simpler and cost economical. On the other hand, the off-port is required to be terminated with a matched termination in order to make the design absorptive. Generally, absorptive designs are bit sophisticated and of course expensive. Microwave circulator design presented in this report is absorptive while RF switch designs are reflective.

Distortion

Distortion due to intermodulation products and harmonics is another key design factor that is taken into account. Harmonics are integral multiples of the fundamental frequency the device is operated at. At these harmonic frequencies, spurious output signals are generated which tend to make the device/system non-linear. Similarly, non-linearity arises by the mixing of harmonics of two or more frequencies. Broadband systems are more prone to be affected by distortion due to harmonics and intermodulation product. Third-order intermodulation product is closest to the fundamental frequency to be easily filtered [4].

Number of throws

Number of throws of the routing module is specified by the topology of the network it is used in. Generally, three-port device for both switch and circulator is used to be able to use them in transceiver systems with one port attached to transmitter, another port to antenna and the third port to the receiver. If routing module is used for applications where diversity technique using multiple antennas is implemented, then routing module needs to have more than three ports according to the requirement. Similarly, in order to increase isolation, microwave circulators are designed sometimes to have four ports with one extra port terminated to reduce reflections. But, on the other hand, the module becomes more prone to losses (insertion loss) as number of throws is increased [2]. So, a balance between isolation and insertion loss must be consider while deciding the number of throws.
After having discussed important design parameters, let’s now have a look at the theoretical working principle of the two important routing modules; Microwave circulator and RF/Microwave switch. Next sections in this report, deals with the study of these two routing modules. This theoretical knowledge will be applied in terms of simulations to be able to verify the behaviour of these devices in real life.

Microwave circulator

Microwave circulator is a passive device which has been used in microwave circuits for decades. The Y-junction microwave circulator is typically having 3 ports (input port, output port and decoupled port). The main reason for using this junction circulator is energy supervision. Circulator allows the maximum power transfer from one port to another port in a way that the power flows from input port to output port and 3rd port will be isolated. It is like the power enters at port 1 is fully transmitted to port 2, power enters at port 2 is completely transmitted to port 3 and power enters at port 3 is completely transmitted to port 1. At the same time the circulation can take place in reverse direction too. That is, the power enters at port 1 is fully transmitted to port 3 and port 2 will be isolated, the power enters at port 3 is completely transmitted to port 2 and port 1 will be isolated and so on. Such device is versatile and it has many applications, it can be used as a switch and like a duplexer in communications and radar systems.

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Working principle

In the Microwave circulator theory part, a brief explanation about the related electromagnetic fields is discussed with the help of Maxwell equations. A brief explanation is presented about the Ferromagnetic materials which includes an active magnetic component’s (YIG) structure, which is necessary in the design and operation of a microwave circulator. A brief explanation is included about how the circulation exactly takes place in a Y-junction circulator when external magnetic field is applied at various stages.

YIG structure

The YIG lattice structure is a cubic, with three sub lattices. The 1st sub lattice is tetrahedral (d-site), 2nd one is octahedral (a-site) and the 3rd one is dodecahedral sites (c-site). The 3+ ions placed on tetrahedral and octahedral sites are surrounded by 2− ions. The 3+ ions placed on the dodecahedral sites and surrounded by 2− ions. The 2− ions can work as an intermediate which makes the interaction between the tetrahedral and octahedral sites possible. The 3+ ions at tetrahedral site has a magnetic moment, which is anti-parallel to both 3+ ions in the octahedral sites and 3+ ions in the dodecahedral sites. All these anti parallel moments are not going to cancel exactly with the parallel moments, so a net magnetization will be there in YIG. YIG has a saturated magnetization at room temperature and it is around 1750G [6].
From the electromagnetic field theory, any magnetic material can be used to obtain the circulation. But in this project case, YIG is used, because it acts like most of the ferrite materials and it is a good insulator. It optimizes all the unwanted eddy current losses and they can be reduced to a better level [7]. In the next section of the theory, a brief discussion about the working principle of the circulator with important magnetic phenomenon is presented.

Characteristics of ferrite

The characteristics of ferrite completely depend on the dipole of the magnet, i.e. the electron spin [8]. The spin of the electron can be seen in the Figure 3 and its magnetic dipole momentum is given in equation (7)

Ferromagnetic resonance (FMR)

From the equation (6b) if = 0, at this particular frequency, the permeability tensor can have a singularity, this is the point where the ferromagnetic resonance (FMR) occurs. Maximum energy transfers occurs, when the driving frequency ( ) of electromagnetic wave reaches to the internal field frequency ( 0) of the ferrite. Applying an electromagnetic wave at resonance frequency gives more losses because the ferrite wouldn’t have the enough saturation magnetization at resonance point. [8].
$The static magnetic field is applied here and it is perpendicular to the internal field and parallel to the internal field. Magnetic field applied perpendicular to the internal field is used in the YIG disk biasing for the good circulation, i.e. maximum power transmission.

Below and above resonance frequency regions

Since it is undesirable to operate the circulator at FMR frequency, the operation of the circulator will be checked at below resonance mode and above resonance mode frequencies.
The selection between these two modes depends on the magnetic loss mechanism. Hysteresis loss can be decreased by applying the higher static magnetic field, so the ferrite will be saturated in excess. The below resonance mode is eliminated because the lower field loss region can merge with the FMR region when higher magnetic field is applied so gives bad circulation. So, the circulator has been designed at above resonance region to overcome the hysteresis losses and also the YIG can be saturated in excess with higher magnetic field biasing.

Microwave circulator Design principle

The junction microwave circulator is the most commonly used circulator and its pictorial view is shown in Figure 5. The micro strip line conductors are covered by the two ferrite disks. These conductors are separated at 120° intervals, will form the 3 ports of the circulator. An external magnetic field is applied normal to the ferrite disks.

Table of contents :

1. Introduction
1.1 Background
1.2 Goal
1.3 Outline
2. Theory
2.1 Important Design Parameters
2.1.1 Frequency of operation and bandwidth
2.1.2 Insertion loss
2.1.3 Isolation
2.1.4 Return loss and VSWR
2.1.5 Reflective or absorptive
2.1.6 Distortion
2.1.7 Number of throws
2.2 Microwave circulator
2.2.1 Working principle
2.2.2 Microwave circulator Design principle
2.3 Microwave/RF Switch
2.3.1 PIN Diode Switch
2.3.2 Field-Effect Transistor Switch
3. Simulations and Results
3.1 Simulations for Microwave Circulator
3.1.1 Experimental Design of Microwave circulator
3.2 Simulations for PIN diode Switch
3.2.1 Series PIN SPDT Switch
3.2.2 Shunt PIN SPDT Switch
3.2.3 Series-shunt PIN SPDT Switch
3.2.4 Very High Isolation Switch
3.3 Simulations for FET Switch
4. Measurements
5. Discussion and Conclusion
6. References
7. Appendix
7.1 Types of transmission lines
7.1.1 Strip line
7.1.2 Micro-strip line
7.1.3 Single ended line and differential line signalling systems
7.1.4 Single ended line signalling system
7.1.5 Differential line signalling system
7.1.6 The circulation approximation
7.1.7 The 3D- view of the circulator & its Electric field distribution

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