RF Isolator: Advantages and Disadvantages

This article explores the benefits and drawbacks of using RF Isolators.

What is an Isolator?

Introduction: An RF Isolator is a non-reciprocal device constructed using ferromagnetic materials like ferrites (Mg + Mn) and (Ni + Zn) alloys. When these materials are subjected to a DC magnetic field, the propagation of electromagnetic waves becomes non-reciprocal. This unique characteristic is leveraged in the design of RF Isolators.

Ideally, an RF isolator will absorb signal power in one direction while allowing propagation in the opposite direction. This makes it a unidirectional device, effectively blocking reflections from the reverse direction.

This property finds applications in test and measurement scenarios, as illustrated in Figure 1:

In this setup, reflections from the Device Under Test (DUT) that travel back towards the signal source are effectively blocked. This protects sensitive equipment from potential damage. Furthermore, it helps stabilize the frequency of microwave signal generators or sources.

Types of Isolators:

• Faraday Rotation Isolator
• Microstrip Isolator

Benefits (Advantages) of Isolators

Here are the key advantages of using an RF Isolator:

• As previously mentioned, it’s a two-port non-reciprocal device designed to allow signals to pass with minimal attenuation in one direction, while blocking signals in the opposite direction with high attenuation. When placed between a source and a load, the isolator helps to stabilize the source’s output power by preventing reflections from the load from reaching the source. It also prevents frequency pulling caused by changes in the load.

• RF Isolators are commonly used in Test and Measurement setups to protect equipment from damage during testing, especially when dealing with overload conditions or high reflected power.

• RF Isolators can be incorporated into RF circuit designs, as shown in Figure 2.

  The isolator passes all frequencies from its input to the output port and blocks all frequencies in the opposite direction. This protects delicate RF components, such as amplifiers and mixers.

Drawbacks (Disadvantages) of Isolators

The following are some of the drawbacks to consider when using RF Isolators:

• A major consideration in RF circuit design is that while the RF isolator passes all frequencies within its intended band, it can also pass undesired frequencies along with the desired ones. To address this, appropriate RF filters are often placed after the isolator to selectively allow the desired frequencies to pass through.
• The maximum power handling capability of an RF isolator is limited because reverse power is absorbed by the ferrite material within the isolator and dissipated as heat. To improve heat dissipation, some designs use two ferrite slabs instead of just one.
• Waveguide resonance isolators require a very high steady magnetic field, especially at higher frequencies.