Cavity Resonant Frequency Filter Explanation and Its Significance

Cavity Resonant Frequency Filter Explanation and Its Significance

With the global deployment of LoRaWAN® and Sigfox IoT Low-Power Wide-Area Networks (LPWAN) operating in 868 MHz and 915 MHz unlicensed spectrum, there has been much discussion on whether Cavity RF Filters are necessary, their primary purpose, and why alternative filters like SAW, BAW, Ceramic, and others cannot replace them in LoRaWAN® Gateways.

In a recent analysis, this topic is addressed in simple terms and answered practical questions about the type and specifications of RF Cavity Filters typically used in outdoor LoRaWAN® Gateways for optimal system performance and lasting radio lifespan.

Cavity RF Filters serve multiple purposes:

1. Improving LoRaWAN® system performance in noisy radio frequency (RF) environments.
2. Protecting the LoRaWAN® receiver, particularly the Low Noise Amplifier (LNA), from saturation and failure, which can be caused by strong interfering signals.
3. Shielding the LoRaWAN® transmitter from generating harmful Cross Intermodulation Products (CIPs).
4. Lessening the impact of LoRaWAN® transmit emissions on neighboring cellular and other networks, as regulated by FCC and CE.

It is universally recognized that any outdoor base station, radio, or gateway operating above 200 MHz should incorporate RF Filters at its antenna output for both received and transmitted signals.

About Cavity RF Filters

Typically, Cavity RF Filters consist of large aluminum blocks with 2-3 RF connectors. These filters have several screws along their body for securing and tuning purposes. The aluminum casing is often silver-plated, copper-plated, or even gold-plated (in space applications) to reduce RF losses and achieve the necessary high Q factor or filter selectivity.

Cavity RF Filters have diverse practical applications, with passbands ranging from less than 0.5% to up to 20% of the operating frequency, although most common applications feature passbands between 1% and 10% of the operating frequency. These filters are indispensable for the input signal bandlimiting of the LNA, reducing the effects of other wireless systems and delivering the best receiver performance possible. Furthermore, RF Cavity Filters also minimize the effects of transmitted signals on other wireless systems.

Benefits of Using Cavity RF Filters in LoRaWAN® Gateways

Here's why specifically Cavity RF Filters should be utilized in outdoor LoRaWAN® Gateways rather than other types of filters:

1. Industry-leading low insertion loss, typically 1 dB or less.
2. Exceptional selectivity of 50 dB to 100 dB, ensuring steep and large rejection outside the filter passband.
3. Ability to handle high transmit power signals from other wireless systems, unlike other filters with high selectivity but limited capacity to handle high power.

Cavity filters are bulky and heavy, but their mass and volume can be beneficial in dissipating heat generated by the radio and power amplifier, reducing the need for larger chassis and expensive heat-syncs. Additionally, Cavity filters offer superior protection against lightning strikes compared to standard lightning arrestors.

Cavity Filter performance is influenced by the shape, size, and Q factor of the cavity housing, the number of resonators, their physical layout, and the Q factor of the resonator and cavity. A larger diameter cavity offers better selectivity due to its higher Q factor resulting from the higher volume. RF Cavity Filters are custom-designed to achieve optimal performance at the lowest cost.

The performance of RF Cavity Filters can be easily simulated before manufacturing using proprietary software. Upon achieving satisfactory simulation results and designer agreement, CAD files are generated for precise CNC machining, followed by optimization and testing to ensure compliance with relevant specifications.

Outdoor LoRaWAN® Gateways Requirements

Since outdoor LoRaWAN® Gateways are often mounted near or close to other high-power wireless systems, they must incorporate RF Cavity Filters between the LNAs, power amplifier (PA), and antenna to ensure optimal performance, protect the radio from damage, and comply with FCC and CE regulations on transmit emissions.

Here's a list of necessary RF Cavity Filter specifications for outdoor LoRaWAN® Gateways:

1. High-Q resonant structures for low passband insertion loss and high out-of-band rejection.
2. 33 dBm (2W) and 39 dBm (8W) RMS power capability, accommodating signal peak-to-average ratio variations and drifts over temperature and altitude.
3. Less than 1 dB average insertion loss within the passband (868 MHz or 915 MHz, depending on deployment).
4. Rx Port should provide 40 dB rejection within 5 MHz and 85 dB rejection within 10-15 MHz of its passband to protect the gateway receiver from high-power interfering signals from nearby wireless systems.
5. Tx Port should offer 30 dB rejection within 5 MHz and 75 dB rejection within 10-15 MHz of its passband to:a. Reject gateway transmit spurious signals and harmonics that affect the performance of 3G-5G wireless systems.b. Eliminate Cross Modulation products from unwanted signals mixing with the gateway transmit signal and producing 2nd, 3rd, 5th intermodulation products, which can fall into the gateway receiver band and degrade its performance.

It is crucial to select the right RF Cavity Filters to ensure the best LoRaWAN® network performance and deployment, as different geographic regions, countries, and cities can have distinct wireless system deployments that affect gateway performance.

In conclusion, Cavity RF Filters provide superior selectivity, low insertion loss, and robustness, making them suitable for outdoor LoRaWAN® Gateways. Their customizability, compact design, and resistance to various environmental factors add to their suitability in these applications. Although Cavity filters may be more expensive compared to other filter types, their unparalleled performance and durability justify their use in critical outdoor LoRaWAN® Gateway applications.

For further information, please refer to our website: Innowaverf

[1] [Innowaverf]

1. Cavity RF Filters, crucial in LoRaWAN® Gateways, improve system performance in noisy RF environments.
2. They protect LoRaWAN® receivers from saturation and failure due to strong interfering signals.
3. Cavity RF Filters shield the LoRaWAN® transmitter from generating harmful Cross InterModulation Products (CIPs).
4. These filters lessen the impact of LoRaWAN® transmit emissions on neighboring cellular and other networks.
5. In outdoor base stations, radio, or gateways operating above 200 MHz, RF Filters at the antenna output are necessary for both received and transmitted signals.
6. Science and technology, particularly environmental and data-and-cloud-computing, have advanced the design and performance of Cavity RF Filters.
7. These filters have industry-leading low insertion loss, exceptional selectivity, and the ability to handle high transmit power signals.
8. CBD, a component of plant-based therapies and treatments, has generated significant interest in personal finance and wealth management circles, reflecting its potential growth as a viable business in the health and wellness industry.
9. The sports industry, particularly football, generates billions of dollars in revenue and creates numerous business opportunities in finance, sponsorships, and merchandising, contributing to global economic growth.

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