There are several schemes for constructing radio receivers. Moreover, it does not matter for what purpose they are used - as a receiver of broadcasting stations or a signal in a control system kit. There are superheterodyne receivers and direct amplification. In the direct amplification receiver circuit, only one type of oscillation converter is used - sometimes even the simplest detector. In fact, this is a detector receiver, only slightly improved. If you pay attention to the design of the radio, you can see that first the high-frequency signal is amplified, and then the low-frequency signal (for output to the speaker).
Features of superheterodynes
Due to the fact that parasitic oscillations can occur, the possibility of amplifying high-frequency oscillations is limited to a small extent. This is especially true when building shortwave receivers. Astreble amplifier is best to use resonant designs. But they need to make a complete reconfiguration of all the oscillatory circuits that are in the design, when changing the frequency.
As a result, the design of the radio receiver becomes much more complicated, as well as its use. But these shortcomings can be eliminated by using the method of converting the received oscillations into one stable and fixed frequency. Moreover, the frequency is usually reduced, this allows you to achieve a high level of gain. It is at this frequency that the resonant amplifier is tuned. This technique is used in modern superheterodyne receivers. Only a fixed frequency is called an intermediate frequency.
Frequency conversion method
And now we need to consider the above-mentioned method of frequency conversion in radio receivers. Suppose there are two types of oscillations, their frequencies are different. When these vibrations are added together, a beat appears. When added, the signal either increases in amplitude, or decreases. If you pay attention to the graph that characterizes this phenomenon, you can see a completely different period. And this is the period of the beats. Moreover, this period is much longer than a similar characteristic of any of the fluctuations that were formed. Accordingly, the opposite is true with frequencies - the sum of oscillations has less.
The beat frequency is easy enough to calculate. It is equal to the difference in the frequencies of the oscillations that were added. And with an increasedifference, the beat frequency increases. It follows that when choosing a relatively large difference in frequency terms, high-frequency beats are obtained. For example, there are two fluctuations - 300 meters (this is 1 MHz) and 205 meters (this is 1.46 MHz). When added, it turns out that the beat frequency will be 460 kHz or 652 meters.
Detection
But superheterodyne type receivers always have a detector. The beats that result from the addition of two different vibrations have a period. And it is fully consistent with the intermediate frequency. But these are not harmonic oscillations of the intermediate frequency; in order to obtain them, it is necessary to carry out the detection procedure. Please note that the detector extracts only oscillations with the modulation frequency from the modulated signal. But in the case of beats, everything is a little different - there is a selection of oscillations of the so-called difference frequency. It is equal to the difference in frequencies that add up. This method of transformation is called the method of heterodyning or mixing.
Implementation of the method when the receiver is running
Let's assume that oscillations from a radio station come into the radio circuit. To carry out transformations, it is necessary to create several auxiliary high-frequency oscillations. Next, the local oscillator frequency is selected. In this case, the difference between the terms of the frequencies should be, for example, 460 kHz. Next, you need to add the oscillations and apply them to the detector lamp (or semiconductor). This results in a difference frequency oscillation (value 460 kHz) in a circuit connected to the anode circuit. Need to pay attention tothe fact that this circuit is tuned to work at the difference frequency.
Using a high-frequency amplifier, you can convert the signal. Its amplitude increases significantly. The amplifier used for this is abbreviated as IF (Intermediate Frequency Amplifier). It can be found in all superheterodyne type receivers.
Practical triode circuit
In order to convert the frequency, you can use the simplest circuit on a single triode lamp. The oscillations that come from the antenna, through the coil, fall on the control grid of the detector lamp. A separate signal comes from the local oscillator, it is superimposed on top of the main one. An oscillatory circuit is installed in the anode circuit of the detector lamp - it is tuned to the difference frequency. When detected, oscillations are obtained, which are further amplified in the IF.
But constructions on radio tubes are used very rarely today - these elements are outdated, it is problematic to get them. But it is convenient to consider all the physical processes that occur in the structure on them. Heptodes, triode-heptodes, and pentodes are often used as detectors. The circuit on a semiconductor triode is very similar to that in which a lamp is used. The supply voltage is less and the winding data of the inductors.
IF on heptodes
Heptode is a lamp with several grids, cathodes and anodes. In fact, these are two radio tubes enclosed in one glass container. The electronic flow of these lamps is also common. ATthe first lamp excites oscillations - this allows you to get rid of the use of a separate local oscillator. But in the second, the oscillations coming from the antenna and the heterodyne ones are mixed. Beats are obtained, oscillations with a difference frequency are separated from them.
Usually the lamps on the diagrams are separated by a dotted line. The two lower grids are connected to the cathode through several elements - a classic feedback circuit is obtained. But the control grid directly of the local oscillator is connected to the oscillatory circuit. With feedback, current and oscillation occur.
The current penetrates through the second grid and the oscillations are transferred to the second lamp. All signals that come from the antenna go to the fourth grid. Grids No. 3 and No. 5 are interconnected inside the base and have a constant voltage on them. These are peculiar screens located between two lamps. The result is that the second lamp is completely shielded. Tuning a superheterodyne receiver is usually not required. The main thing is to adjust the bandpass filters.
Processes taking place in the scheme
The current oscillates, they are created by the first lamp. In this case, all parameters of the second radio tube change. It is in it that all vibrations are mixed - from the antenna and the local oscillator. Oscillations are generated with a difference frequency. An oscillatory circuit is included in the anode circuit - it is tuned to this particular frequency. Next comes the selection fromoscillation anode current. And after these processes, a signal is sent to the input of the IF.
With the help of special converting lamps, a significant simplification of the superheterodyne design occurs. The number of tubes is reduced, eliminating several difficulties that may arise when operating a circuit using a separate local oscillator. Everything discussed above refers to the transformations of an unmodulated waveform (without speech and music). This makes it much easier to consider the principle of operation of the device.
Modulated signals
In the case when the modulated wave is converted, everything is done a little differently. The oscillations of the local oscillator have a constant amplitude. The IF oscillation and beat are modulated, as is the carrier. To convert the modulated signal into sound, one more detection must be performed. It is for this reason that in superheterodyne HF receivers, after amplification, a signal is applied to the second detector. And only after it, the modulation signal is fed to the headphone or the ULF input (low frequency amplifier).
In the design of the IF there is one or two cascades of the resonant type. As a rule, tuned transformers are used. Moreover, two windings are configured at once, and not one. As a result, a more advantageous shape of the resonance curve can be achieved. The sensitivity and selectivity of the receiving device is increased. These transformers, which have their windings tuned, are called bandpass filters. They are configured usingadjustable core or trimmer capacitor. They are configured once and do not need to be touched during the operation of the receiver.
LO frequency
Now let's look at a simple superheterodyne receiver using a tube or a transistor. You can change the local oscillator frequencies in the required range. And it must be selected in such a way that with any frequency oscillations that come from the antenna, the same value of the intermediate frequency is obtained. When the superheterodyne is tuned, the frequency of the amplified oscillation is adjusted to a specific resonant amplifier. It turns out a clear advantage - there is no need to configure a large number of inter-tube oscillatory circuits. It is enough to adjust the heterodyne circuit and the input. There is a significant simplification of the setup.
Intermediate frequency
To obtain a fixed IF when operating at any frequency that is in the operating range of the receiver, it is necessary to shift the oscillations of the local oscillator. Typically, superheterodyne radios use an IF of 460 kHz. Much less commonly used is 110 kHz. This frequency indicates how much the ranges of the local oscillator and the input circuit differ by.
With the help of resonant amplification, the sensitivity and selectivity of the device is increased. And thanks to the use of the transformation of the incoming oscillation, it is possible to improve the selectivity index. Very often, two radio stations operating relatively close (according tofrequency), interfere with each other. Such properties must be taken into account if you plan to assemble a homemade superheterodyne receiver.
How stations are received
Now we can look at a specific example to understand how a superheterodyne receiver works. Let's say an IF equal to 460 kHz is used. And the station operates at a frequency of 1 MHz (1000 kHz). And she is hindered by a weak station that broadcasts at a frequency of 1010 kHz. Their frequency difference is 1%. In order to achieve an IF equal to 460 kHz, it is necessary to tune the local oscillator to 1.46 MHz. In this case, the interfering radio will output an IF of only 450 kHz.
And now you can see that the signals of the two stations differ by more than 2%. Two signals fled, this happened with the help of frequency converters. Reception of the main station has been simplified, and the selectivity of the radio has improved.
Now you know all the principles of superheterodyne receivers. In modern radios, everything is much simpler - you need to use only one chip to build. And in it, several devices are assembled on a semiconductor crystal - detectors, local oscillators, RF, LF, IF amplifiers. It remains only to add an oscillatory circuit and a few capacitors, resistors. And a complete receiver is assembled.
