mmWave Attenuator Limitations Spark Renewed Interest in Faraday Rotation Solutions

The move up the electromagnetic spectrum (EM) into millimeter waves (mmWaves) is proving to be a double-edged sword. System designers eager to leverage wider bandwidths and incredibly high data throughputs must also contend with a host of new challenges. Of amplified importance – particularly between 75-330 GHz – is the issue of attenuation.

When the strength or magnitude of a radio frequency (RF) signal must be reduced, attenuators are traditionally relied upon. They are especially useful for signal leveling or switching applications. At frequencies above 50 GHz, two electronically tunable variable attenuator technologies are most often employed: PIN diode and resistive vane attenuators. However, both come with substantial drawbacks which have led engineers in search of alternatives.

As part of that pursuit, component manufacturers returned to research published in 1961 on Faraday rotation. In a paper entitled Broad-band Isolators and Variable Attenuators for Millimeter Wavelengths, C.E. Barnes laid out a theory for utilizing Faraday rotation in mmWave applications. At the time, several RF engineers pursued the theory, but no one was able to achieve the desired results. Now, with the demand for alternative mmWave attenuators increasing, manufacturers are finding ways to successfully deploy a third option for mmWave voltage variable attenuators.

Writing off the PIN diode  

PIN diode attenuators are a popular choice for signal attenuation at lower frequencies due to their relatively low cost, compact size, and fast switching speeds – often as quick as 100 ns. These devices operate by applying a variable bias voltage to the PIN diodes, which controls the level of signal attenuation.

However, as the operating frequency increases beyond 60 GHz, several performance issues become apparent:

• High Insertion Loss: PIN diode attenuators experience significant signal loss, with insertion losses in the WR-10 band (75-110 GHz) reaching up to 5 dB when combined with isolators. This loss can severely impact system performance in high-frequency applications.
• Limited Dynamic Range: The dynamic range of PIN attenuators at higher frequencies is often limited to around 20 dB, which constrains their effectiveness in applications that require a wide range of signal control.
• Port Reflections and Return Loss: High port reflections, with return losses nearing 10 dB, are common in PIN diode attenuators. These reflections can cause signal degradation and interference, reducing overall system efficiency.
• Susceptibility to ESD and Noise: PIN diode attenuators are highly sensitive to electrostatic discharge, which can lead to device failure. Additionally, they can introduce noise and intermodulation distortion (IMD), further complicating signal integrity at higher frequencies.

These limitations make PIN diode attenuators increasingly ineffective for mm-wave applications, prompting the need for alternative solutions that can handle the demands of these high-frequency environments.

Moving away from resistive vane attenuators

On the other hand, resistive vane attenuators have long been the go-to technology for high-frequency applications, offering a much flatter frequency response and significantly higher dynamic range compared to PIN diode attenuators. These devices utilize a mechanical actuator to insert a resistive vane into a waveguide, gradually attenuating the signal. 

While this approach works well in controlled lab settings, it poses several challenges in real-world applications:

• Size and Weight: Resistive vane attenuators are large and heavy, making them unsuitable for compact or portable systems where space and weight are critical considerations.
• Slow Switching Speeds: With switching speeds that can take up to two seconds, resistive vane attenuators are far slower than their electronic counterparts. This delay can be problematic in applications that require rapid signal adjustments.
• Mechanical Complexity and Cost: The use of calibrated control circuits and motor-driven actuators increases both the complexity and cost of these attenuators, making them a less attractive option for budget-conscious projects.
• Field Use Limitations: Their mechanical nature and large footprint make resistive vane attenuators impractical for deployment in field environments, where robustness and portability are essential.

Given these disadvantages, there is a clear need for a solution that combines the high-frequency performance of resistive vane attenuators with the compact size and speed of PIN diode devices.

Comparison table of variable attenuators

Faraday Rotation Solutions Resurface

Faraday rotation attenuators utilize a magnetic coil to produce a variable magnetic bias field in a ferrite rod. A Faraday voltage variable attenuator could, in theory, offer full waveguide band operation and high-power handling compared to other technologies. 

As an example of what is possible, Micro Harmonics Corporation (MHC) recently developed the first two lines of mm-wave attenuators in both the W band (WR 10, 75-110 GHz) and D band (WR 6.5, 110-170 GHz) based on the Faraday rotation principles. Their approach leverages the Faraday effect to rotate the RF signal's polarity, directing it into a fixed resistive layer embedded in a ceramic cone.

Unlike resistive vane attenuators, Faraday voltage variable attenuators achieve signal attenuation without any moving parts, leading to several key advantages:

• Superior Frequency Response: The Faraday rotation attenuators provide a frequency response that is flatter than PIN diode devices, although not as flat as resistive vane attenuators. This makes them well-suited for a wide range of high-frequency applications.
• Higher Dynamic Range: With a dynamic range of up to 35 dB in the WR-10 band, Faraday attenuators outperform PIN diode devices (20 dB), providing more precise signal control in mm-wave systems.
• Reduced Insertion Loss: Faraday attenuators have an average insertion loss of 1.2 dB in the WR-10 band, significantly better than both resistive vane attenuators (1.5-3 dB) and PIN diode attenuators (3-5 dB).
• Robustness and ESD Resistance: Being passive devices, Faraday rotation attenuators are inherently immune to ESD damage, a common failure mode in PIN diode attenuators.
• Compact and Lightweight: Faraday technology can be much smaller and lighter than resistive vane attenuators, making it ideal for both lab and field use in compact systems.
• Improved Power Handling: With power ratings up to 2.3 W, the MHC attenuators are on par with high-performance resistive vane attenuators and vastly superior to the 100-mW limit of PIN diode devices.

Overcoming Faraday Issues

One potential drawback when using magnets and ferrite in a Faraday attenuator is the issue of repeatability due to the ferrite's magnetic memory. This means that simply shifting from one current level to another does not always guarantee the same attenuation when returning to the initial level. However, extensive studies have shown that this can be mitigated effectively by forcing “a reset” on the attenuator by simply bringing the current down to zero before adjusting it back to a desired voltage. This process can be executed rapidly, maximizing repeatability without significant delay.

Stray magnetic fields can also affect the performance of Faraday technologies. To overcome this, manufacturers have found that incorporating external magnetic shields around the attenuator ensures stable operation even in magnetically noisy environments.

Practical Applications 

Faraday rotation-based attenuators are particularly well-suited for applications in telecommunications, radar systems, and test and measurement equipment operating at mm-wave frequencies. Their combination of high dynamic range, low insertion loss, and compact size makes them ideal for field use in aerospace and defense systems, where both performance and portability are critical.

Faraday attenuators are also beneficial in research and development environments, where precise signal control and rapid adjustments are necessary for experimental setups involving high-frequency components.