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Once upon a time, when the computers were large, the prototypes of the first transistors had not yet left the walls of Bell Labs research laboratories, and the MP3 format did not exist at all, the audiophiles were in their analogue paradise, enjoying the warm tube music from radios and vinyl players. How difficult is it today to listen to the immortal hits of Louis Armstrong and Elvis Presley in an "authentic" performance? It's time to find out.
Like any other information, sound is now stored primarily in digital form. Of course, its output quality strongly depends on the characteristics of a particular device – the DAC (digital-to-analog converter) used and the op-amp (operational amplifier). But in general, there is no getting anywhere from discreteness and quantization of the signal.
Here, of course, you can recall the cassettes, records and players – turntables. They have their own connoisseurs, and even today it’s not difficult to get copies of such equipment in good condition. However, to assemble something similar "on the knee" will not work: here you need quite complex mechanics. What to do?
There is an exit! The signal can be taken from the radio. Especially since earlier it was completely trivial: you open a book for radio amateurs and collect a 1V1 or 1V2 DV / SV receiver – the circuits there are very simple. And after a couple of hours you can listen to the radio “Ma
Direct gain receivers are classified by the number of gain stages before and after the detector. Thus, 1V1 means that the receiver contains one cascade of UHF (high frequency amplifier), a detector and one cascade of VLF (low frequency amplifier). See pages for more details. Wikipedia.
But this was earlier, and since 2014, broadcasting in the Far East and Far East bands on the territory of Russia has been completely stopped (the air is noisy, and unprofitable). However, in fairness it can be noted that there has never been a high-quality sound at long and medium waves. This is explained by a narrow broadcast band (about 10 kHz), and its width is directly related to the width of the range of the transmitted audio signal. Thus, only FM band will satisfy our requests.
Things are a bit more complicated here, as direct gain receivers are already ineffective. Although, of course, they are also sometimes collected, but it is rather exotic. More or less acceptable results can be achieved only by collecting supergenerator. The super-regenerative receiver, perhaps, has the best ratio of simplicity of design and efficiency. Literally from a dozen details you can assemble a working circuit. However, the sound quality leaves much to be desired, and there is practically nothing to be done about it.
In other words, in order to achieve good results, we are forced to opt for superheterodyne. A modern FM receiver can be implemented on a single RDA5807 chip, which contains a complete digitally controlled superheterodyne path. It supports stereo and RDS, but more about that some other time.
The most simple to implement is a superheterodyne with a low intermediate frequency and a pulse-frequency detector. Such a receiver can contain only one tunable circuit, which greatly simplifies the design. Let us examine the principle of its work in more detail.
A superheterodyne receiver, in contrast to a direct gain receiver, involves converting the received signal into an intermediate frequency at which the selection is performed. This solution reduces the number of tunable elements, which greatly simplifies the task.
The diagram clearly shows that the received signal is amplified and fed into the mixer, and the output from the local oscillator (auxiliary generator) is also fed there. As a result, the mixer signal contains beats, the frequency of which is equal to the difference between the received signal and the local oscillator signal. From the mixer, the flow enters a band-pass filter that emits an intermediate frequency signal.
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