A Key Player in 5G Base Stations: SAW Filters

A Guide to SAW Filters – Essential Components for 5G Base Station Performance

Deployment of 5G networks is accelerating globally, and its promise of ultra-high speeds, ultra-low latency, and massive connectivity capabilities is posing unprecedented challenges to wireless infrastructure-especially base stations. In the complex RF front-end (RFFE) system of a base station, filters play a critical role as “traffic cops,” responsible for accurately filtering out signals in the desired frequency band while strongly suppressing out-of-band interference and noise. Among many filter technologies, surface acoustic wave (SAW) filters have found an important place in 5G base stations, especially in mid-band deployments, due to their excellent performance in specific frequency bands (e.g., Sub-6GHz), miniaturization, and mature processes.

SAW filters in 5G base stations:
#Band Isolation: Separates closely neighboring 5G NR bands (e.g., n77, n78, n79) to prevent mutual interference.
#Receive Channel Protection: Suppresses leakage of transmit signals into highly sensitive receivers (Tx Leakage) to avoid receiver blockage or saturation.
#Anti-interference: Filters out strong, unintended interference signals from other systems (e.g., 4G LTE, Wi-Fi) to ensure the purity of the 5G signal.
#Meet Spectrum Template Requirements: Ensure that the out-of-band spurious emissions of the transmitted signal meet stringent regulatory standards.

Why are SAW filters suitable for 5G base stations, especially in the mid-band?
1. High-frequency performance and miniaturization: SAW technology can effectively operate in the Sub-6 GHz band, with a particular performance advantage in the 1 GHz to around 2.5 GHz band. Modern SAW filters are fabricated using precision semiconductor lithography processes, allowing for very small dimensions and precise frequency control, which is critical for Massive MIMO antenna units and compact micro-base stations/RRUs that require the integration of a large number of filters.
2. Steep out-of-band rejection: SAW filters provide very steep transition bands (high roll-off characteristics), which is critical for isolating closely neighboring 5G bands and preventing neighboring channels from interfering.
3. Low Insertion Loss: Maintaining low insertion loss in the target passband is critical to the efficiency of the base station amplification link. A good SAW design can achieve good loss performance in this band, helping to reduce the overall power consumption and thermal design difficulty of the base station.
4. Mature, Reliable and Cost Effective: SAW technology is a mature process that has been proven over decades, offering high reliability and the cost benefits of mass production.

Spotlight on Key Parameters: 2400MHz Band as an Example

According to the relevant technical documents, examples of key performance parameters for a typical SAW filter suitable for 5G base stations (especially small base stations or band-specific filter banks) in the frequency band around 2400 MHz may include:

1. Center Frequency: around 2400 MHz (or as designed for a specific band, e.g. 2405 MHz, 2420 MHz, etc.).
2. Bandwidth: Based on the bandwidth of the carried 5G NR channel, e.g. 20MHz, 40MHz, 60MHz, 80MHz, 100MHz, etc. The document will specify its passband. The document will specify the passband range (e.g. 2390 MHz – 2410 MHz).
3. Insertion Loss (IL): Typically in the range of 1.5 dB to 3.0 dB (or less). This is a key measure of the energy loss in the passband of a signal as it passes through the filter, and low loss is critical to system sensitivity. Typical and maximum values are given in the documentation.
4. Out-of-Band Rejection: Rejection at specific offset frequencies (e.g., center frequency ±50MHz, ±100MHz) is critical. Documentation typically demonstrates rejection levels of 30 dB, 40 dB or more at the target rejection frequency (e.g., the edge of the Wi-Fi 2.4GHz ISM band or the adjacent 5G NR band) to ensure effective isolation of interference.
5. Passband Ripple: Low (e.g. < 0.5 dB) to ensure flatness of the signal in the passband.
6. Power Handling: For filters in close proximity to the transmitter path (e.g. Tx filters), they need to be able to withstand a certain amount of RF power without distortion or damage. The documentation will indicate the power rating (e.g. +23dBm, +26dBm).

Conclusion:
In the context of 5G base stations pursuing higher performance, larger capacity, and denser deployment, SAW filters have become a key component for optimizing the performance of the RF front-end of the base station and guaranteeing the network quality and efficiency by virtue of their excellent frequency selectivity in the Sub-6GHz mid-frequency band, miniaturized dimensions, low insertion loss, and mature mass production capabilities. Especially when dealing with applications in bands near 2.4 GHz (either specific 5G NR bands or anti-Wi-Fi interference), selecting a SAW filter solution with excellent 2400 MHz band parameters (e.g., low insertion loss, high rejection) is indispensable for building a high-performance and reliable 5G network infrastructure. Engineers should refer to the device documentation to ensure that the performance parameters fully meet the stringent requirements of the specific 5G band and base station architecture.