What are Cavity Filters & How Do They Work?

A cavity filter is a form of a resonant circuit that allows only specific frequencies to go through. Notch filters, in general, are the type of filter discussed here.

"Cavity" is just a name for the space inside of the filter that traps or releases energy.

What are Cavity Filters used for?

Cavity filters are often found in wireless communication devices -- cell phones, walkie-talkies, portable baby monitors, cordless telephones, baby alarms, etc. The cavity filter serves two purposes:

1) to reduce the interference caused by other signals on the same frequency band, and 2) to protect circuit components from power surges due to sudden release of stored energy.

via www.amateur-radio-wiki.net

Most cavity filters consist of 4 basic parts:

  1. 1
    Input line (feeds the signal into the cavity)
  2. 2
    Output line (captures filtered signal coming out of resonant cavity)
  3. 3
    Feedthrough capacitor(s) (allows DC current to flow through cavity)
  4. 4
    A ground connection


A feedthrough capacitor is a capacitor used to connect one part of a circuit with another part, by passing through a conductive post or pin penetrating the insulating material of the circuit board.

Feedthroughs were once commonly found in radios from the 1940s to 1960s. They have been largely phased out of consumer equipment because of cost factors, but they are still mandatory for all medical electronic devices.

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Why do cavity filters block certain frequencies?

A cavity filter manipulates an electromagnetic field of energy around it that can be envisioned as concentric circles emanating from the device when in operation. Signal energy radiates outward in all directions (shown below in green).

The input line captures this signal energy and feeds it into the cavity. The feedthrough capacitor allows DC to flow through while blocking other frequencies (shown below in purple).

The energy inside starts off strong, even though it's radiating outward because the waves are traveling back and forth across each other due to resonance. But as time goes on, the energy weakens because it is bleeding out through the input line.

The signal can also be envisioned like ripples in a pond when you drop something into it (shown below in blue)--the initial splash radiates out but quickly dissipates.

Cavity filters have one or more inductors that function to trap certain frequencies inside of their casing walls by way of an electromagnetic field just like described above.

These cavities usually have two or more cavities that are tuned to different frequency bands.

The cavity closest to the input line traps a range of frequencies, while the other cavity further inside traps another frequency band. Each cavity has a conductive metallic shield around it to shield against radio waves from getting in or out.

Cell phones operate on multiple bands, which can vary from 850MHz to 2100MHz for GSM and UMTS networks, as well as 450MHz to 750MHz for CDMA networks.

In Asia, some cell phones can function anywhere from 900Mhz-1800Mhz, so they would need notch filters centered at each of these bands.

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Cell phone companies encourage customers who travel internationally with their devices (especially those who use prepaid plans) to buy unlocked phones, which allow users to switch out SIM cards.

An unlocked phone does not require a special code from the carrier that originally sold it in order to function with another carrier's SIM card.

Most American carriers sell phones that are "locked" or "network locked."

The term used by different wireless providers can mean slightly different things, but for this article's purposes, all of them refer to phones whose frequency bands cannot be altered by the user -- they will only operate on the bands assigned to them by their respective provider(s).

Unlocking your cell phone provides you more freedom because it allows you to use other SIM cards when traveling abroad without having to worry about switching over all your settings manually--especially if you're using a prepaid plan.

Why Use A Cavity Filter?

Cavity filters are used to block certain frequencies from outside devices. In North America, most cell phones operate on a GSM network which is 1800MHz – 1900Mhz.

That means that your phone emits radio waves inside and around your device at those frequencies along with the ones it receives from the tower. If you live in a building made of metal or near a high voltage power line, for instance, this could cause interference issues.

If you have a phone that operates on CDMA networks (Verizon or Sprint) then your phone would emit radio waves at 700Mhz – 800Mhz instead.

Since these frequency bands overlap with TV channels 2-4, you might experience some signal degradation if you using an analog TV set-top box and live in an area where there is a good amount of TV channels to go around. Not only that but the 700Mhz-800Mhz band is used by weather services to send out storm alerts which could make your phone become unusable during bad weather.

The frequencies at play when using a GSM cell phone are 850MHz - 900MHz for receiving and 824Mhz – 960Mz for sending. In North America, carriers mostly use 850 MHz and 1900 MHz while everywhere else it's usually 900 MHz and 1800/1900 MHz.

This means that if you're traveling abroad with a GSM device that uses any combination of those bands then you'll definitely want a cavity filter installed because these frequencies aren't very common elsewhere and you won't be able to use your phone.

How Do Cavity Filters Work?

GSM frequencies used in North America: 850MHz and 1900Mhz

If the cavity filters were tuned to specific frequencies, they would work by blocking out any unwanted interference from cellular devices operating on the same frequency.

They're designed to basically block anything that is low enough in frequency not to register as a TV channel or AM/FM radio station which means that it's still safe to use an analog TV with this type of filter installed.

This will prevent your smartphone from overheating and damaging internal components during periods where you can't move around while using it. There has been a number of issues reported concerning smartphones heating up while being used for extended periods of time in areas where there are lots of TV channels to go around.

It's important to note that if the cavity filters were tuned to specific frequencies, they would work by blocking out any unwanted interference from cellular devices operating on the same frequency which means that it might not be able to eliminate all interference when used with non-North American devices.

Is Cavity a High Pass Filter?

A high-pass filter (HPF) is an electronic filter that passes higher frequencies well but attenuates signals with frequencies lower than the cutoff frequency.

Low pass filters (LPFs) are the opposite of HPFs and block out high frequencies while passing low ones.

But that's not all, cavity filters also work as speaker protection because they prevent the audio amplifier from overloading which might even cause permanent damage to internal components.

They can't be used for cellular devices operating on GSM 900 MHz (used in Europe) since those phones don't use this type of filter or 1.8 GHz (used in Russia) since those frequencies aren't available everywhere, but there are plenty of other options out there depending on what kind of devices you're using and where you're traveling to.

How do you tune a cavity filter?

Before installing the cavity filters, you need to know what frequency range they will be working within. Go to your TV and tune it to a channel that doesn't exist since they often run on frequencies assigned for mobile phones or digital cable which will allow you to find where exactly in the spectrum is located.

Once you do that, use a meter that can read these types of signals and locate all of the available channels above the one that your phone operates on.

Once this is done, attach a capacitor with a value equal to half of the lowest channel's frequency (with the same polarity) and solder its leads straight onto an exposed section of electrical contacts from each cavity filter package. You need only solder one lead from each capacitor unless there are two or more cavities available.

After doing this, solder the other lead from each capacitor to the other part of electrical contacts and install your newly tuned cavity filter package onto your device and properly secure it with heat-shrink tubing.

Note: for best results, you can also attach a resistor to the ground (with a value equal to half of the lowest channel's frequency) using an insulated wire.

This will allow all signals below that frequency to get through since they're not strong enough to cause any damage.

You should then connect one end of that resistor into your mobile phone and attach another end into the ground via an electrical contact which means you don't need additional wires connecting them; everything is done right on the same circuit board where cavities are installed.

If you don't want to go through all of these steps, there's an easier way to do it using a cable tester that can easily spot where exactly in the spectrum is located and what capacitors should be used for each cavity filter package since they're already tuned.

What Is A Duplexer?

What Is A Duplexer?

A duplexer is a filter that removes all frequency signals except for those coming from the antenna which allows the use of two devices at once. It's also known as "transceiver diplexer", "diplex filter" or simply an "antenna coupler".

Duplexers are responsible for allowing your phone to communicate with baseband processors so that you have neither over-compression nor under-compression of audio during hands-free operation.

All of this can be done without any additional components but will require the use of an external device connected between your mobile phone and car radio via Bluetooth or auxiliary input.

You can't just plug in some headphones into the headphone jack since that won't send out signals capable of reaching the antenna.

A cable with a microphone is required and it usually comes with volume control and an on/off button so that you can answer calls, adjust volume, and hang up when finished since your car's radio doesn't have such capabilities simply because there's no need for them.

Can Other Filters Be Used For Duplexers?

Yes, but it's not recommended to do that unless you have a meter that can read all possible overheating issues.

For example, if your phone uses an 800 MHz frequency for communicating with baseband processors and runs on 1.8 GHz or 900 MHz (depending on cellular band) to send/receive audio signals, the best way is to use one cavity filter package tuned at 1 - 2 GHz because this will eliminate any signal interference without increasing chances of overheating.

The safest option would be to use three different cavity filters each made for one specific range of frequencies since then you don't have to worry about overheating or about any other electrical components getting damaged due to defective tuning or anything else.

It should also be noted that any t-connectors, couplers, or any other kinds of filter/splitters are not recommended due to the fact that they could actually cause overheating issues if defective and will make the life of an audio system technician very difficult.

After all, it's simple physics - signal interference leads to increased power usage which means energy loss which means overheating; no matter how many parts you try to use for this purpose - there's always going to be some form of signal