Even in the modern world, radio remains an effective way of receiving and transmitting information, which allows you to bypass borders and unnecessary intermediaries. Simple and extremely reliable, the signal of radio stations can be received regardless of the presence of 5G network towers in your area. How to assemble your receiver from a scattering of microcircuits and parts, you will learn from this material.
The history of radically new receivers began in 1901, when Reginald Fessenden showed the ability to receive a signal on beats. The essence of the revolutionary method was that, in addition to the radio signal from the antenna, an auxiliary signal of a close frequency was supplied to the receiver, as a result of which beats could be detected at the output – a signal with a frequency equal to the frequency difference between the received signal and the output of the auxiliary generator. These beats were heard in telephones, and, as was shown somewhat later, the amplitude of these beats was noticeably higher than the amplitude of the useful signal.
The researcher called the auxiliary generator "heterodyne" (from the Greek ἕτερος – other or external and δύναμις – force), and the receiver itself is "heterodyne". At that time, it was a new method of detection, which made it possible to receive a telegraph radio signal by ear.
Here, the letter O denotes the local oscillator, and the receiver itself consisted of two inductively coupled coils on a common core. In this case, the beat signal made the metal membrane D oscillate (presumably a diffuser). In general, as you know, everything was harsh, quite in the spirit of that distant time. Later, the receiver was upgraded with increased sensitivity.
A careful study of the diagram reveals here crystal diode – yes, imagine, this thing was made already in 1913! However, this design did not gain much success, since at that time the auxiliary signal generator was bulky, complex and very expensive to manufacture. Then the most widespread were mechanical generators, and several years remained before the invention of the first radio tube.
The next iteration was Henry Round heterodyne receiver, created in the same year 1913. In this device, the generator was already on an electronic tube, which performed three functions at once: amplified the received signal, generated an auxiliary one, and also worked as a mixer, multiplying the signals. Because of such abundant functionality, the author gave the receiver the name "autodyne", hinting that the generation of the auxiliary signal here occurs in the receiving and amplifying circuits.
And then a war broke out, which clearly showed how useful radio communication is. But reliable, more sensitive and selective receivers were required, because by that time there were noticeably more radio stations. The then radio receivers had three serious problems: insufficient sensitivity, which is directly related to the communication range, selectivity, that is, the ability to separate the signal of the desired radio station from several received ones, and resistance to atmospheric interference.
By studying these problems, the three researchers independently came up with conceptually similar solutions. The first by a small margin was a Frenchman Lucien Levy, who suggested that if in the receiver the signal of the received station is not immediately converted into an audio frequency, but into some intermediate frequency (above the audible), then at this intermediate frequency it will be easier to get rid of atmospheric interference, after which it can be converted into an audible (sound) …
This solution requires the introduction of an additional local oscillator into the receiver design. The result is a device, in modern terms, with double frequency conversion. Levy called his receiver "superheterodyne," meaning it contained an extra local oscillator. This is probably what explains the origin of such an intricate name.
However, there is another version, which suggests that the prefix "super" migrated from the intermediate frequency, which was higher than the audible one, or, as it was customary to write at that time, supersonic (ultrasound). In any case, one must understand that superheterodyne reception implies the presence of an intermediate frequency.
Here H1 and H2 are the connection points of the first and second local oscillator. Somewhat from the other side, the problem was approached independently of each other Edwin Armstrong and Walter Schottky… They were more interested in the idea of increasing the sensitivity, which required an amplifier on radio tubes. However, one must understand that radio tubes in 1918 were imperfect and capricious devices and it was simply impossible to build an amplifier with a high coefficient, capable of operating at frequencies of the HF band (2-30 MHz).
To solve this problem, the researchers proposed converting a useful high-frequency signal into an intermediate one (at which the lamps could work effectively) and already at this frequency to amplify the signal, which the technologies of that time quite allowed. Moreover, the authors pointed out that such a conversion can be performed in several stages, which will increase the stability of the amplifier.
And if the researches of the German Schottky were theoretical, then the engineer Armstrong in America already in 1918 built a working prototype of his superheterodyne on eight lamps (in fact, an insane amount for that time). It looked something like this.
Nevertheless, then superheterodyne did not find widespread use, and the reason for this was primarily the high price. While just appeared regenerative receivers, which, although inferior to superheterodyne in their characteristics, but made it possible to build an acceptable quality receiver using only one or two lamps. Curiously, the regenerative receiver was also invented by Armstrong and, characteristically, brought him much more income and fame.
Truly an era superheterodyne receivers began only in the 1930s, when lamps became much more widely available and the corresponding patents expired. As a result, by the end of World War II, superheterodyne devices practically supplanted all other types of receivers. Superheterodyne receivers are now considered the standard. The main advantage of the superheterodyne is that you can select the received signal by tuning the local oscillator itself.
In this case, the intermediate frequency remains constant, so that high-performance crystal filters can be used in the intermediate frequency amplifier. This makes it easy to obtain the desired adjacent channel selectivity.
Sensitivity, selectivity and bandwidth
Among all the characteristics of any receiver, it is useful to highlight a number of key ones: sensitivity, selectivity, and bandwidth. Sensitivity is the minimum level of a radio signal in microvolts that allows a signal with a given signal-to-noise ratio to be obtained at the output. Or, to put it simply, this is the minimum signal level at which the station can still be heard. Good modern receivers have a sensitivity of about 1 μV.
Adjacent channel selectivity characterizes the ability of the receiver to select the desired signal in the presence of closely spaced interfering signals, measured in decibels. Let's say there are two stations of equal power, 10 kHz apart (typical channel width on HF broadcast bands). Selectivity will show how much weaker the signal of the neighboring station will be received when tuning to the desired one.
Finally, bandwidth is a parameter closely related to selectivity, which shows the deviation of the signal frequency from the tuning frequency when the signal is attenuated by 3 dB (this is about 0.7 for voltage and 0.5 for power).
What is the profit?
Of course, now assembling your own radio is devoid of economic feasibility. Moreover, with the development of the Internet, radio broadcasting has lost its former relevance today. Even the FM range has noticeably thinned, not to mention the short waves. And yet, radio reception at short wavelengths, as it is now customary to express it, gives a feeling of "warm lamp-like". Moreover, the very idea of “freely” transmitting information, bypassing borders and intermediaries, still looks very topical.
So, in fact, without getting up from a chair, you can run, if not around the world, then at least along your own continent: thousands of kilometers for short waves is not a problem at all, even in large cities where the radio air is very noisy. While in Moscow, you can easily hear China, India, Qatar and other countries. There is even such a phenomenon as DXing – "hunting" for distant radio stations, a kind of competition. By accepting the radio station and sending the appropriate response, you can receive a QSL card with the radio station's logo.
On the Internet, some forums have separate themesdedicated to such cards. According to participants, the Chinese willingly send cards. However, personally I am more interested in the very creation and configuration of the receiver. Further I will talk about a relatively simple receiver with a digital scale and quartz frequency stabilization, which is quite suitable for receiving signals from distant stations.
Of course, much simpler solutions can be used for shortwave reception. For example, regenerative receivers, the most famous of which is perhaps the "Mohican" MFJ-8100. It can be purchased ready-made (for a hundred dollars on popular online sites) or in the form of an assembly kit, or you can even assemble it yourself – since the circuit is open. But the regenerator is more like "for pampering", because listening to the station, you will constantly have to adjust the regeneration and attenuator. This is due to the fact that the HF signal almost constantly changes its intensity over a wide range. This is due to atmospheric phenomena affecting the passage. And this is precisely what the regenerator does not like very much.
So, the essence of the work of local oscillators in such a receiver is that the input "high-frequency" signal is converted into an intermediate frequency (we will use 455 kHz), at which the main selection and amplification of the signal will be performed. This is followed by a detector that extracts an audio frequency signal, and an amplifier necessary for loudspeaker reception. Let's consider a block diagram of a superheterodyne.
The design that I had already used in the SDR receiver was taken as a basis, but in this case I thought that using the STM32F103 microcontroller was redundant, and ported some pieces of code to STM32F030. The latter is weaker in performance, but somewhat cheaper and, moreover, is available in the LQFP32 case, which is more convenient for homemade products. This is one of the few MCUs with a Cortex-M core and a 0.8 mm pin pitch. However, at SI5351 the pitch is still 0.5 mm, so it won't be possible to completely get rid of the little things in the project.
I added a power regulator and an operational amplifier to the circuit to display the received signal level. The op-amp operates in the repeater mode, and there is a voltage divider at its output, which makes it possible to measure the voltage of the AGC control signal (varies in the range from 0.5 to 4.7 V). Since the control voltage of the AGC is close to the supply voltage, a rail-to-rail operational amplifier MV358 is used. It can be replaced here with the more common LM358, but then the upper limit of the measured voltage will drop to 4 V (when powered by 5 V).
Also, the circuit contains the ability to control varicaps for autotuning the input circuits, but I did not find suitable varicaps, so I did not implement such a function. The synthesizer circuit is shown in the figure.