What is Cavity RF Filter and Why is it Important?

With the global deployment of the LoRaWAN® and Sigfox IoT LPWAN networks that operate in 868 MHz and 915 MHz global unlicensed spectrum, there have been many questions if Cavity RF Filters are required, what is their main purpose, and why they can’t be replaced with other type filters such as SAW, BAW, Ceramic filters and others that are mounted on the PCB after the Low Noise Amplifier (LNA).

This short paper addresses these and other questions in simple terms and gives practical answers on the type and specifications RF Cavity Filters typical outdoor LoRaWAN® Gateways should incorporate to ensure good or excellent LoRaWAN® system performance and no damage to its Radio LNA and even PA (what are still common today) over time.

The Band Pass filters are used to reduce the interference outside of the LoRaWAN® Gateway operating band from high Tx power 3G-5G Base stations, Fixed Mobile and Paging systems, TV and Radio stations, GPS repeaters, and many others operating from few MHz to 500 MHz away.

Cavity RF Filters help to:

• improve LoRaWAN® system performance in noisy RF environment,
• protects the LoRaWAN® Receiver and specifically LNA from saturation and outright failures (burn-outs),
• protect LoRaWAN® Transmitter from generating Cross Intermodulation Products
• reduces the effect of LoRaWAN® Tx emissions on collocated cellular and other type networks that is regulated by FCC and CE.

It is universally accepted and practiced that any outdoor Base station, Radio or Gateway operating above 200 MHz should always incorporate RF Filters at its Antenna output for Rx and Tx signals.

What Cavity RF Filters are

Generally, they are larger aluminum blocks with few RF connectors (2 if it is a Filter and 3 if it is a Duplexer that filters Tx and Rx signals and combines them into a single Antenna port). These filters have many screws on one or more sides of their body as per the below picture – some screws are used to secure the top plate to the chassis and others are tuning screws.

The aluminum body is always plated (silver, copper, or even gold but only for space applications) to reduce the RF losses and achieve the high Q or filter selectivity that is needed to obtain low losses over the filter passband and steep rejection outside of the filter passband.

The RF Cavity Filters were and still are the wireless industry workhorse and are used in all wireless systems from 1G to 5 G as well as private and military communication systems. They cover a very broad spectrum from 50 MHz all the way up to 20+ GHz. As the frequency increases their size is reduced because of shorter wavelengths (speed of light is constant and defined as the product of RF signal frequency times its wavelengths).

Figure 1: A typical RF Cavity Filter or Duplexer 

RF Cavity Filters have a broad range of practical applications as their passband can vary from less than 0.5% to up to 20% of the operating frequency, although for most common applications the passband is between 1% to 10% of the operating frequency. Most if not all wireless systems use RF Cavity Filters between the Antenna and Radio to reduce the effects of other wireless systems on its receiver (bandlimited the LNA input signal to reject lower and upper frequencies outside of the system performance) and obtain the best receiver performance in a real RF environment. Furthermore, the RF Cavity Filters also used on the Tx signal to ensure the PA noise and emissions are bandlimited and do not impact another type of wireless system and themselves as Tx signals are much stronger than any receiver signals by 120 dB to 150 dB.

What are RF Cavity Filter’s main benefits?

Now let’s talk about why specifically RF Cavity Filter should be used for outdoor LoRaWAN® Gateways instead of other types of filters.

The RF Cavity Filters undisputed high value, when compared to other type filters:

1. very low insertion loss, typically 1 dB or much less
2. very high selectivity typically 50 dB to 100 dB (steep and large rejection outside the filter passband)
3. ability to handle very high Tx power signals of its system and other wireless systems signals appearing at its Antenna or Rx input. There are many types of filters that provide high selectivity, but they have very high insertion loss and can’t handle high Tx power.

RF Cavity filters are large and heavy, but in many cases, these are not major concerns as experienced radio design engineers use their volume and mass to dissipate heat generated by the Radio and Power Amplifier, thus reducing the need for larger chassis and expensive heat-syncs. Furthermore, RF Cavity Filter provides the best protection from lightning strikes (50 kVA or double of typical lightning arrestors), and if designed correctly they come for free with a proper RF Cavity Filter design.

RF Cavity Filter performance, in simple terms, is based on the shape and size of the cavity housing, the number of resonators, the physical layout of resonators, and the Q or selectivity of the resonator and cavity. A larger diameter cavity provides better selectivity because of its higher Q resulting from the higher volume. The RF Cavity Filters are custom designed to the exact RF and mechanical specifications to ensure the wireless system will have the optimum performance at the lowest cost.

These days the filter design and performance are easily simulated before they are manufactured using vendor proprietary software. Once the simulation results are satisfactory and agreed upon among the designers, the CAD files are generated to machine the filter housing and resonators with high precision CNC machined. Once the machine filter is performance-optimized and proven to meet all relevant specifications, a steel tooling produced to reduce the filter housing cost and time it takes to produce them and increase the production volume

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Figure 2: A typical RF Cavity Filter housing with Cavities and Resonators

Standard cavity filters generally are designed using aluminum as the base metal. As most raw metals are inherently lossy, filter housings are silver-plated for improved electrical characteristics and current flow. Brass, copper, aluminum, or bi-metal resonators are used to minimize frequency drift over temperature. The Quarter-wavelengths resonant RF Cavity Filters are most common. It is important to understand how the various specifications impact the practical use of the cavities.

When designing RF filters, the same three main criteria why they are used in the first place must be met (it is worth repeating it): 

a) minimum insertion loss across the filter passband,

b) maximum rejection or attenuation at frequency band edge or certain frequencies outside of the band edge

c) maximum Tx power and heat dissipation.

The term Insertion Loss is defined as the unintentional loss at the desired pass frequency or band, while Rejection is the desired loss at the undesired frequency or band. The goal is to minimize the insertion loss at the desired frequency and maximize the attenuation or rejection at the undesired frequency. A general simulation of a Chebyshev bandpass RF Cavity Filter is provided below.

Figure 3: A general simulation of a Chebyshev bandpass RF Cavity Filter

The S21 parameter is the filter attenuation between the filter Port 2 and 1 (Antenna poor and Rx Port, or Antenna port and Tx port), or a filter rejection across the frequencies it supports.

The S11 parameter, is reflected signal across different frequencies it supports. As can be seen from the above figure the filter insertion loss, or its S21 parameter, is minimal across the filter passband (close to zero dB, but in reality, it is between 0.5 dB to 1.5 dB). The filter Reflected Power or S11 is 20 dB down, or 100 times lower, meaning only 1% of RF signal power that enters Port 1 is reflected and the rest of the power is passed through the filter.

The other important observation is that the filter order or a number of resonators defines how steep the filter rejection is. As the number of its resonators is increased from N = 5, 7, and 9, the filter becomes sharper and rejects the collocated frequency bands on both ends much faster.

Let’s now discuss what type of RF Cavity Filters and their key specifications the outdoor LoRaWAN Gateways must incorporate. Unlike the indoor LoRaWAN Gateways that are deployed far away from any high-power wireless system antennas, the outdoor LoRaWAN® Gateways are mounted on telecom and microwave towers, rooftops, and light poles, and in many cases, there are other types of wireless systems mounted already present or in close proximity (less than 50 meters) or TV towers that could be 1 km away. For practical reasons, let’s assume there are some 3G and 4G systems at 880 MHz to 894 MHz with typical 20W and 40W per carrier (or 43 dBm and 46 dBm) and Fixed Mobiles or Paging Systems at 931 MHz with typical 60W per carrier (48 dBm).

Having established the Outdoor LoRaWAN®, Sigfox and other technology IoT Gateways operating in the 868 MH and 915 MH unlicensed spectrum must incorporate RF Cavity Filters between their LNAs, PA, and Antenna, let us now cover the RF Cavity Filter key specifications. Given most of these parameters were discussed in the previous sections, we will list them there in a bullet for people to remember or reference when considering procuring or deploying LoRaWAN or other types of LoRaWAN Gateways.

What Should the RF Cavity Filters Support?

• High-Q resonant structures to ensure low Passband Insertion Loss and high Out-of-Band Rejection.
• Minimum 33 dBm (2W) RMS power and at least 39 dBm (8W) Peak power to compensate for RF Signal Peak to Average Ratio (low for LoRaWAN signal), up to 3,000 m altitude and drift over the operating temperature.
• Less than 1 dB typical (2 dB worst case) Insertion Loss less its Passband (868 MHz or 915 MHz depending on the deployment).
• The higher loss will directly increase the Gateway Noise Figure (reduce its coverage) and require higher PA Tx power to overcome the Insertion Loss.
• Rx Port should provide at least 40 dB rejection within 5 MHz and 85 dB rejection within 10-15 MHz of its Passband to protect the Gateway Receiver from all Out-of-Band high power Interference Signals (3G-5G Cellular, Fixed Mobile, Paging, Digital TV, GPS Repeater and other high power wireless systems that could be up to 500 MHz away as most LNA are broadband devices).

• It is important to note that different geographic regions, countries, and even cities have multiple wireless systems deployed and it is important to ensure the right RF Cavity Filters are used ensure the most effective and best LoRaWAN® network operation and deployment.
• Tx Port should provide at least 30 dB rejection within 5 MHz and 75 dB rejection within 10-15 MHz of its Passband to:

• Rejects Gateway Tx spurious signals and harmonics impacting 3G-5G wireless system Receiver performance.
• Eliminated Cross Modulations products (the result of Out-of-Band high power Interference Signals mixing with the Gateway Tx signal and producing 2nd, 3rd, 5th intermodulation products) falling in the Gateway Receiver band and degrading its performance.
• Support -40C to +75C Ambient Temperature range.
• Built-in EST and Lighting Protection (25 KVA to 50 kVA) at Antenna Port.
• Water and Humidity are tight to ensure its tuning and performance are not impacted over its lifetime
• Robust Ground connection and very low Impedance to the Gateway Chassis Ground to eliminate any ESD surge and inrush current from entering the Gateway.
• Any ESD Surge or Inrush Current must be terminated / leave via the Filter Ground or the Gateway most likely will be damaged.

There are many specifications one should consider when performing the Gateway RF Block and Level Calculations and deriving the RF Cavity filter specifications, however the above specifications would get one very close to the optimal filter and would ensure the LoRaWAN® Gateway and network performance would increase substantially (require less Gateways to cover the same geographic area, often 2-5 times less Gateways), ensure the Gateways do not fail prematurely, do not degrade the performance of collocated 3G-5G Base stations, and substantially reduce the site visits.

All these factors will also result in much lower LoRaWAN® operator deployment and operating costs. In summary, the RF Cavity filers not only substantially improve the performance of LoRaWAN® Gateways and networks, ensure they do not fail prematurely, but also reduce the operator deployment cost and operating cost.

References:

https://www.innowaverf.com/900mhz-ism_filter-anti-paging_model_501_903m_923m