Can a circulator be used as an isolator? If so, how? Difference between circulator and isolator

Circulator can be separated into two classes, 4 port waveguide circulators based on Faraday rotation of waves propagating in a magnetized material and 3 port Y junction circulators based on cancellation of waves propagating over two different paths near a magnetized material. A circulator can also be used as an isolator.

When one port of the 3 port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports.

Circulator:

A circulator is a multiport non – reciprocal device with port arranged such that the EM energy entering a certain port is coupled to an adjacent port and not coupled to other port. The most commonly used circulators are three port symmetrical devices. The important specification of a circulator is its insertion loss, which is the loss in the signal level as it travels in the direction that is supposed to go. Is all port of circulator are matched, and then a signal applied to one port. For insertion to port 1 will emerge from port 2 with a loss called the insertion loss.

Fig. 19 Three port circulator

For three port circulator the scattering matrix is

\(\left[S\right]=\left[\begin{matrix}0&0&S_{13}\\S_{21}&0&0\\0&S_{32}&0\\\end{matrix}\right]\)

Insertion loss I.L \(\left(in\ dB\right)=-10\log{\left(\frac{P_2}{P_1}\right)}\)

A small part of the input signal also emerges from port 3 and the ratio of this signal to the input signal is called isolation.\(\ I\ \left(in\ dB\right)=10\log{\left(\frac{P_3}{P_1}\right)}\)

Other important circulator parameters are VSWR, frequency range, number of ports and configurations, continuous wave and pulse power handling capabilities, temperature range, phase linearity, type of transmission media, size weight and Environmental factor. The circulator separates the input and output ports in negative resistance amplifier. Circulators are also used to couple a transmitter and receiver to a common antenna.

Isolator:

Isolator is a non – reciprocal two port device. It provides very high attenuation to microwaves signals passing from port 2 to port 1, when the signal passing from port 1 to port 2 are transmitted with negligible attenuation. The scattering matrix for the isolator is

\(\left[S\right]=\left[\begin{matrix}0&0\\S_{21}&0\\\end{matrix}\right]\)

Basically an isolator is of two types,

1. Faraday Rotation isolator
2. Resonance abortion isolator

In Faraday Rotation Isolator a section of the waveguide 45° twist and 45° Faraday rotation is take place. The thin resistive cards are placed in the input and output guides to absorb the polarized field with electric vector parallel to the wide side of the guide.

Fig. 20 Faraday rotation isolator

In Resonance isolator is designed to overcome the limited power handling capability of Faraday rotation isolator. A rectangular waveguide and the ferrite slab placed in the transverse direction parallel to the E – field. The C- shaped magnet is used to magnetize the ferrite. The slab is positioned such that the H – field inside the ferrite should be circularly polarized. Due to resonance absorption, R.F wave propagating in one direction travels without attenuation due to small value of μ_-¹¹ and there is a strong absorption for the oppositely travelling wave due to high value of μ₊¹¹. The ferrite is located at a distance of \(\left(\frac{a}{\pi}\tan^{-1}{\frac{\pi}{\beta a}}\right)\) away from one of the sidewall. Proper design makes the forward loss below 0.5 dB/in a t λ₀=3 cm and reverse loss can be as high as 6 to 10 dB/ in or more.

Therefore we can conclude that

1. Circulators are devices that transmit microwave frequencies between ports, or electrical connections whereas isolator refers to a device that sends microwave frequencies in one direction.
2. Circulator may be three, four or multi port networks. Power in a circulator inputted via in any port and it is coupled to the any adjacent port and not coupled to other port in any particular direction whereas isolators are two port devices and are primarily used to protect equipment on the input side from power transmitted on the output side. This prevents the microwave frequency source from being detuned.
3. The circulator terminates one of its three of four ports with a power load, which absorbs the power that enters it. In isolator transmit the frequency in unidirectional way from input to output and the power is absorbed by the second port. This absorption is possible via a ferrite material, which is contained in the isolator and enables the microwave signal to produce a magnetic field. This field allows power in the second port to be displaced by the ferrite.