SAW filters from TV and phone. How it works

In TVs or cell phones, where there is a radio receiving or transmitting board, you can see a part in a metal or other case on the board. This part is called a SAW filter or a filter on surface acoustic waves. I will briefly tell you how it works in this article.

To make it clearer, I disassembled one SAW filter, on the body of which there is an inscription KYOSERA 38.0. This is a bandpass SAW filter from a TV to an intermediate frequency.

Inside the filter you can see a transmitter and receiver, which are located on a substrate. By sending an electrical signal to the transmitter, it is converted into mechanical or acoustic energy, which spreads along the surface of the piezoelectric material (substrate). This energy is sent to the receiver and converted back into an electrical signal. What is this for? It is necessary to filter out unnecessary frequencies and pass the necessary ones. For example, a SAW filter from a TV (CRT TV) will pass frequencies from 34 to 39 MHz, and suppress the rest of the frequencies. Filters in a cell phone work in much the same way. The EPCOS 4109 filter at 1747 MHz from the NOKIA 3210 phone will pass the necessary frequencies and suppress the unnecessary ones.SAW filters can operate at very high frequencies, unlike quartz and other filters.In a quartz filter there are vibrations of a quartz plate, but in a SAW filter there will be a surface wave.

I fed a signal from a generator, the frequency of which I will change, to the input of the SAW filter from the TV. I connected an oscilloscope to the output of the filter to observe the signal.

At the filter input the frequency is 32 MHz, at the output the signal is suppressed

At the input of the filter there is a signal with a frequency of 35 MHz and the filter passes this signal and it is visible on the oscilloscope

Now the filter input is 41 MHz and the signal at the output is suppressed. The filter passed the necessary frequencies and suppressed the unnecessary ones.

Let's look at a SAW filter from a cell phone under a microscope

The size of SAW filters in phones is small compared to filters in TVs, since such filters operate at even higher frequencies.