19 GHz Single-Ended-to-Balanced Modified Ladder-Lattice Filters Realized Using Periodically Polarized AlScN BAW Resonators

The Gist

Imagine you are trying to send a secret message across a crowded room. You have a single voice (a standard radio signal), but the person you are talking to needs to hear the message in stereo—two voices speaking in perfect opposite rhythm to cancel out the noise. Usually, to do this, you'd need a bulky, expensive translator box (called a "balun") to convert your single voice into that stereo format.

This paper introduces a new, tiny device that acts as both a filter and a translator all in one. It's a microscopic electronic component designed to work at incredibly high speeds (19 GHz), which is like the speed of light in a fiber optic cable.

Here is the breakdown of what they built and how it works, using simple analogies:

1. The Material: The "Super-Resonant" Drum

At the heart of this device are tiny drums made of a special material called Aluminum Scandium Nitride (AlScN).

• The Analogy: Think of a standard drum skin. If you hit it, it vibrates at a specific pitch. In electronics, these "drums" vibrate at radio frequencies to let certain signals pass and block others.
• The Innovation: The researchers didn't just make a normal drum; they made a "periodically polarized" one. Imagine painting the drum skin with alternating stripes of different materials. This special pattern makes the drum vibrate much more strongly and efficiently than a normal one, allowing it to handle the super-high speeds (19 GHz) required for next-generation wireless tech.

2. The Shape: The "Modified Ladder-Lattice"

The device is shaped like a specific circuit pattern called a "modified ladder-lattice."

• The Analogy: Imagine a ladder. A normal ladder has rungs going up and down. This new design is a ladder where the top and bottom rungs are connected in a special way that creates a "mirror image" effect.
• The Magic: This shape allows the device to take a signal coming in from one side (single-ended) and split it into two outputs that are perfectly opposite to each other (balanced/differential).
• Why it matters: Because of this clever shape, you don't need the bulky "translator box" (balun) mentioned earlier. The filter is the translator. This saves space and reduces signal loss, which is like keeping your voice clear and loud without needing extra amplifiers.

3. The Performance: The "Highway Filter"

The team built two versions of this device, both acting like a toll booth on a highway.

• The Job: Their job is to let only cars (signals) traveling at exactly 19 GHz pass through, while blocking all other cars (noise) that are too fast or too slow.
• The Results:
  • Version 1: Let 1.3 dB of signal through (very little loss) and blocked out-of-band noise by 30 dB. It's like a toll booth that lets 99% of the right cars through and stops 99.9% of the wrong ones.
  • Version 2: A slightly more selective version. It let 1.58 dB through but was even better at blocking the wrong cars (33 dB rejection).
  • Size: These devices are microscopic. They are so small they could fit on the tip of a needle (roughly the size of a grain of sand).

4. How It Handles the "Traffic" (Linearity)

The researchers tested how the device handles heavy traffic (high power).

• The Test: They pushed the device with very strong signals to see if it would get confused or distort the message (intermodulation).
• The Result: The device remained calm and clear even under pressure. It didn't get "confused" by the heavy traffic, meaning it can handle strong signals without breaking down or creating static.

Summary

In short, this paper describes a microscopic, all-in-one traffic cop and translator for radio waves.

1. It uses a special striped drum material to vibrate at super-high speeds.
2. It uses a smart ladder shape to convert a single signal into a balanced stereo signal without needing extra parts.
3. It is tiny, efficient, and strong, making it a perfect candidate for future wireless communication systems that need to move data faster and clearer than ever before.

The authors specifically note that this technology is designed to sit directly between antennas and amplifiers in wireless systems, replacing the need for bulky external components.

Technical Summary

Technical Summary: 19 GHz Single-Ended-to-Balanced Modified Ladder-Lattice Filters Realized Using Periodically Polarized AlScN BAW Resonators

Problem Statement
While aluminum nitride (AlN) bulk acoustic wave (BAW) resonators have successfully replaced bulky ceramic duplexers in mobile radio systems since 2001, their electromechanical coupling coefficient ($k_t^2$) of approximately 7% limits their utility for emerging wideband technologies, such as active electronically scanned array (AESAs) and high-speed data processing systems requiring larger bandwidths. Although substituting scandium (Sc) into AlN or using materials like lithium niobate (LiNbO$_3$) can increase $k_t^2$, these approaches often trade off quality factor ($Q$) or CMOS compatibility. Furthermore, existing acoustic RF filters operating at frequencies above 5 GHz often struggle with performance degradation, characterized by a trade-off between insertion loss and out-of-band (OoB) rejection. This degradation is attributed to reduced $k_t^2$ in overtone modes, Akhiezer damping, reduced film crystallinity, and increased anchor losses as resonator dimensions shrink for higher frequencies. Additionally, conventional filter topologies often require external baluns or passive components to interface single-ended amplifiers with differential antennas, increasing footprint and complexity.

Methodology
The authors present a novel filter topology and fabrication process to address these challenges:

1. Topology Design: The paper introduces a single-ended-to-balanced modified ladder-lattice (mLL) filter. Unlike conventional balanced ladder-lattice filters that employ mirrored segments, the mLL topology utilizes a terminal single-ended ladder segment for the input/output port and a modified lattice segment that performs single-ended-to-differential conversion. This design eliminates the need for external baluns. Two designs were implemented:

   • Design Type-1: Consists of a single ladder unit and one modified lattice unit.
   • Design Type-2: Incorporates two ladder units and one modified lattice unit, with the return port of the post-lattice ladder unit connecting to a balanced port rather than ground, enhancing selectivity.
     The working principle relies on the phase relationships between series and shunt resonators. In the passband, the modified lattice segment creates a 180° phase difference between the two output ports, while outside the passband, the signals remain in-phase, resulting in cancellation at the differential output.

2. Material and Fabrication: The filters were realized using 3-layer periodically polarized piezoelectric (P3F) Aluminum Scandium Nitride (AlScN) resonators. The material stack includes two polycrystalline Al$_{0.7}$Sc$_{0.3}$N layers (nitrogen-polar) and one single-crystal Al$_{0.8}$Sc$_{0.2}$N layer (metal-polar). This P3F structure increases the effective piezoelectric thickness to stabilize resonator impedances at high frequencies (19 GHz) and compensate for increased BAW capacitance. The devices were fabricated using the Akoustis XBAW™ process with molybdenum electrodes and aluminum interconnects.

3. Simulation and Optimization: Filters were designed and optimized in the AWR Design Environment using a Process Design Kit (PDK) containing fixed resonator models. Optimization focused on minimizing insertion loss by balancing resonator sizes to maximize $Q_p$ for shunt resonators and $Q_s$ for series resonators, while ensuring impedance matching for high OoB rejection.

Key Contributions and Results
The paper reports the first-time demonstration of two mLL filters operating near 19 GHz with the following performance metrics:

• Filter Type-1:

  • Center Frequency: 19.03 GHz
  • Minimum Insertion Loss (IL): 1.29 dB
  • Fractional 3-dB Bandwidth (FBW): 6.26% (1.91 GHz)
  • Average Out-of-Band Rejection: ~30 dB
  • Footprint: 348 μm × 476 μm
  • Input Third-Order Intercept Point (IIP3): 43.52 dBm

• Filter Type-2 (Higher Selectivity):

  • Center Frequency: 18.82 GHz
  • Minimum Insertion Loss (IL): 1.58 dB
  • Fractional 3-dB Bandwidth (FBW): 5.11% (961 MHz)
  • Average Out-of-Band Rejection: ~33 dB
  • Footprint: 392 μm × 476 μm
  • Input Third-Order Intercept Point (IIP3): 43.06 dBm

Both filters demonstrated correct single-ended-to-differential conversion, with phase differences between output ports exceeding 100° within the passband and remaining near 0° (or low values) outside the passband. The authors note that spurious responses observed in the transmission were attributed to the asymmetric P3F structure not perfectly rejecting certain thickness extensional modes. Electromagnetic (EM) simulations correlated well with measurements in spurious-free regions.

Significance
The authors claim that these filters represent a competitive solution for wireless communications by integrating single-ended-to-differential conversion directly into the filter structure, thereby eliminating the need for external baluns, capacitors, or inductors. This results in a compact footprint suitable for direct interfacing between single-ended power/low-noise amplifiers and differential antennas or converters. The work demonstrates that the modified ladder-lattice topology can effectively utilize resonators with lower quality factors and limited impedance contrast in the PDK to achieve high selectivity and low insertion loss at millimeter-wave frequencies (19 GHz), addressing the performance decline typically seen in high-frequency acoustic filters.