Piezoelectric bandpass filter in a radio receiver. How it works and why it is needed

On the boards of superheterodyne radios you can see a yellow part with three leads, on the body of which is indicated a number from 455 to 465. This part is called a piezoelectric bandpass filter, and the number indicates the frequency in kilohertz at which it operates. In this article, I will talk in detail about this part and show how this filter works

To understand the principle of operation of this filter, I will give a simple structural diagram of a superheterodyne radio receiver. The radio station signal comes from the antenna to the mixer input, and the mixer receives a signal from the heterodyne, the frequency of which is higher or lower than the frequency of the radio station signal by 455 kHz. As a result, at the mixer output there is a difference between the two signals equal to 455 kHz, but this signal also contains other frequencies and various unnecessary interference. To eliminate these unnecessary frequencies and interference, a filter is installed at the mixer output, which passes the useful signal with a frequency of about +/- 455 kHz, and does not pass all other frequencies further to the input of the intermediate frequency amplifier (IFA)

Why did you choose an intermediate frequency of about 455-465 kHz? There is no radio broadcasting at these frequencies, and in many countries this has long been the practice so as not to interfere with radio receivers.

The characteristics of the filters indicate their bandwidth, usually about 10 kHz, and the center frequency of the filter.

To demonstrate the filter, I fed a signal from a generator to its input, the frequency of which I will change, and connected an oscilloscope to the filter output to observe the signal at the output

The maximum signal at the filter output will be at a frequency of 460 kHz, as indicated in the datasheet. And if there is a 400 kHz signal at the filter input, then you will see a very weak signal at the output. The filter suppresses this signal

If you disassemble the filter, you can see a plate possibly made of piezoceramics

Such filters are also available in black and other colors, but are most often found in yellow. The letters next to the number indicate the filter's bandwidth