A simple radio receiver circuit: description. Old radios. Tuning the high frequency unit How to tune a radio station on the radio

The high-frequency block contains a converter stage, input and heterodyne circuits. In receivers of the first and highest classes, as well as in the VHF range, there is an amplifier in front of the converter high frequency. Checking and adjusting the high-frequency unit can be divided into three stages: 1) checking local oscillator generation; 2) determining the boundaries of the range, often called range laying; 3) pairing of input and heterodyne circuits.

Laying ranges. The tuning of the receiver to the received station is determined by the tuning of the local oscillator circuits. Input and UHF circuits only increase the sensitivity and selectivity of the receiver. When tuning it to different stations, the local oscillator frequency must always differ from the received frequency by an amount equal to the intermediate one. To ensure constant sensitivity and selectivity over the range, it is desirable that this condition be met at all frequencies in the range. However, this is the frequency ratio over the entire range

is ideal. With one-handed setup, it is difficult to obtain such a pairing. Local oscillator circuits used in broadcast receivers provide precise matching of the settings of the input and local oscillator circuits in each band at only three points. In this case, the deviation from ideal conjugation at other points of the range turns out to be quite acceptable (Fig. 82).

For good sensitivity on the KB range, two precise pairing points are sufficient. The necessary relationships between the frequencies of the input and heterodyne circuits are achieved by complicating the circuit of the latter. The heterodyne circuit, in addition to the usual tuning capacitor C 1 and tuning capacitor C2, includes an additional capacitor SZ, called a mating capacitor (Fig. 83). This capacitor (usually a fixed capacitance with a tolerance of ±5%) is connected in series with a variable capacitor. The inductance of the local oscillator coil is less than the inductance of the input circuit coil.

To correctly determine the boundaries of the range, you must remember the following. The local oscillator frequency at the beginning of each range is mainly affected by a change in the capacitance of the tuning capacitor C 2, and at the end of the range - by a change in the position of the inductor core L and the capacitance of the mating capacitor SZ. The beginning of the range can be considered the maximum frequency to which the receiver can be tuned in a given range.

When starting to set up the local oscillator circuits, you should find out the sequence of settings by range. In some receiver circuits, the CB band loop coils are part of the DV band loop coils. In this case, you need to start tuning with medium wave and then tune to long wave.

Most receivers use a band switching scheme that allows each band to be adjusted independently. Therefore, the configuration sequence can be any.

The range is set using the two-point method, the essence of which is to set the limit of the highest frequency (beginning of the range) using a tuning capacitor, and then the lower frequency (end of the range) with the core of the loop coil (Fig. 84). But when setting the limit of the end of the range, the setting of the beginning of the range is somewhat lost. Therefore, you need to check and adjust the beginning of the range again. This operation is performed until both points in the range are in compliance with the scale.

Pairing of input and heterodyne circuits. The setting is made at two points and checked at the third. The exact coupling frequencies in receivers with an intermediate frequency of 465 kHz for the middle of the range (f av) and the ends (f 1 and f 2) can be determined by the formulas:

The circuits are paired at design points, which for standard broadcasting ranges have the following values

In individual radio models, the pairing frequencies may vary slightly. The lower precision coupling frequency is usually selected 5...10% higher than the minimum frequency of the range, and the upper frequency is 2...5% lower than the maximum. Capacitors with variable capacitance allow you to tune the circuits to exact matching frequencies when turning at angles of 20...30, 65...70 and 135...140°, measured from the position of the minimum capacitance.

To configure tube radio receivers and achieve pairing, the output signal of the generator is connected to the input of the radio receiver (Antenna, Ground sockets) through the all-wave equivalent of the antenna (Fig. 85). Transistor radios that have an internal magnetic antenna are tuned!: using a standard field generator, which is a loop antenna connected to the generator through a non-inductive resistor with a resistance of 80 Ohms.

The decade divider at the end of the generator cable is not connected. The antenna frame is made square with a side of 380 mm from copper wire with a diameter of 4...5 mm. The radio receiver is located at a distance of 1 m from the antenna, and the axis of the ferrite rod should be perpendicular to the plane of the frame (Fig. 86). The magnitude of the field strength in μV/m at a distance of 1 m from the frame is equal to the product of the readings of the generator's smooth and step attenuators.

In the KB band there is no internal magnetic antenna, so the signal from the generator output is fed to the socket external antenna through a capacitor with a capacity of 20...30 pF or to a whip antenna through a decoupling capacitor with a capacity of 6.8...10 pF.

The receiver is tuned on a scale to the highest frequency of precise coupling, and the signal generator is adjusted to the maximum voltage at the receiver output. By adjusting the tuning capacitor (trimmer) of the input circuit and gradually reducing the generator voltage, we achieve a maximum increase in the output voltage of the receiver. Thus, pairing is carried out at this point in the range.

Then the receiver and generator are tuned to a lower precise coupling frequency. By rotating the core of the input circuit coil, we achieve maximum voltage at the receiver output. For greater accuracy, this operation is repeated until the maximum voltage at the receiver output is reached. After adjusting the contours at the edges of the range, check the accuracy of the pairing at the middle frequency of the range (third point). To reduce the number of tunings of the generator and receiver, the operations of setting the range and pairing the circuits are often performed simultaneously.

Setting up the LW band. Generator standard signals remains connected to the receiver circuit through an equivalent antenna. The generator is set to a lower frequency range of 160 kHz and an output voltage of 200...500 µV with a modulation depth of 30...50%. The lower coupling frequency is set on the receiver scale (the rotation angle of the KPI rotor is approximately 160...170°).

The gain control is moved to the maximum gain position, and the band control is moved to the narrow band position. Then, by rotating the core of the heterodyne circuit coils, the maximum voltage is achieved at the output of the receiver. Without changing the frequencies of the generator and receiver, the coils of the UHF circuits (if any) and input circuits are adjusted in the same way until the maximum voltage is obtained at the output of the receiver. At the same time, the generator output voltage is gradually reduced.

Having adjusted the end of the DV range, set the variable capacitor to the position corresponding to the coupling point at the highest frequency of the range (KPI rotation angle 20...30°). The generator frequency is set to 400 kHz, and the output voltage to 200...600 µV. By rotating the trimming capacitors of the circuits, first the local oscillator, and then the UHF and input circuits, the maximum output voltage of the receiver is achieved.

Tuning the circuits at the highest frequency of the range changes the tuning at the lower frequency. To increase the accuracy of the settings, the described process must be repeated in the same sequence 2...3 times. When re-adjusting the rotor, the KPI should be placed in the previous position, i.e. in the one in which the first adjustment was carried out. Then you need to check the accuracy of the pairing in the middle of the range. The frequency of the exact pairing in the middle of the LW range is 280 kHz. By setting this frequency on the generator and receiver scale respectively, the calibration accuracy and sensitivity of the receiver are checked. If there is a dip in the sensitivity of the receiver in the middle of the range, then it is necessary to change the capacitance of the coupling capacitor and repeat the tuning process.

The final stage is checking that the settings are correct. To do this, a test stick, which is an insulating rod (or tube), is inserted into the tuned circuit first with one end and then with the other end, with a ferrite rod fixed at one end and a copper rod at the other. If the adjustment is made correctly, then when any end of the test stick is brought to the circuit coil field, the signal at the receiver output should decrease. Otherwise, one end of the stick will reduce the signal, and the other will increase it. After the LW band is configured, you can similarly configure the MW and HF bands. However, as already noted, on the HF band it is enough to pair at two points: at the lower and upper frequencies of the range. In most radio receivers, the KB range is divided into several subbands. In this case, the exact pairing frequencies have the following values!

Features of setting the HF range. When tuning the HF band, the signal from the generator can be heard in two places on the tuning scale. One signal is the main one, and the second is the so-called mirror signal. This is explained by the fact that on the HF band the mirror signal is suppressed much worse, and therefore it can be confused with the Main signal. Let us explain this with an example. A voltage with a frequency of 12,100 kHz is applied to the receiver input, i.e., the beginning of the HF range. In order to obtain a frequency equal to the intermediate frequency at the output of the frequency converter, i.e. 465 kHz, it is necessary to adjust the local oscillator to a frequency equal to 12,565 kHz. When the local oscillator is tuned to a frequency of 465 kHz below the received signal, i.e. 11,635 kHz, an intermediate frequency voltage is also provided at the output of the converter. Thus, the intermediate frequency in the receiver will be obtained at two frequencies, the local oscillator, one of which is higher than the signal frequency by the amount of the intermediate frequency (correct), and the other lower (incorrect). In percentage terms, the difference between the correct and incorrect local oscillator frequencies is very small.

Therefore, when setting the HF range, you should choose from two local oscillator settings the one that is obtained with a lower capacitance of the circuit capacitor or with a more inverted coil core. The correct setting of the local oscillator is checked at a constant frequency of the generator signal. When increasing the capacitance (or inductance) of the local oscillator circuit, the signal should be heard in one more place on the receiver scale. You can also check the correctness of the local oscillator settings while keeping the receiver settings unchanged. When the frequency of the generator signal changes to a frequency equal to two intermediate ones, i.e., 930 kHz, the signal must also be heard. The higher frequency in this case is called the mirror frequency, and the lower frequency signal is the main one.

Setting up the antenna filter. Setting up the high frequency unit begins with setting up the antenna filter. To do this, the output signal of the generator is connected to the input of the receiver through the equivalent of an antenna. On the frequency scale of the generator, a frequency of 465 kHz and a modulation depth of 30...50% are set. The output voltage of the generator must be such that the output meter connected to monitor the output voltage of the receiver shows a voltage of the order of 0.5... 1 V. Receiver range switch set to the DV position, and the tuning pointer to the frequency of 408 kHz. By rotating the core of the antenna filter circuit, achieve a minimum voltage at the output of the receiver, while increasing the output voltage of the generator as the signal weakens.

After completing the setup, all adjusted cores of the loop coils and the positions of the magnetic antenna coils must be fixed.

You will need just one chip to build a simple and complete FM receiver that is capable of receiving radio stations in the range of 75-120 MHz. The FM receiver contains a minimum of parts, and its configuration, after assembly, is reduced to a minimum. It also has good sensitivity for receiving VHF FM radio stations.
All this thanks to the Philips TDA7000 microcircuit, which can be bought without problems on our favorite Ali Express.

Receiver circuit

FM Receiver Board

Placing elements on the board

Receiver setup

Conclusion

For a long time, radios topped the list of the most significant inventions of mankind. The first such devices have now been reconstructed and changed in a modern way, but little has changed in their assembly circuit - the same antenna, the same grounding and an oscillating circuit for filtering out unnecessary signals. Undoubtedly, circuits have become much more complicated since the time of the creator of radio, Popov. His followers developed transistors and microcircuits to reproduce a higher quality and energy-consuming signal.

Why is it better to start with simple circuits?

If you understand the simple one, you can be sure that most of the path to success in the field of assembly and operation has already been mastered. In this article we will analyze several circuits of such devices, the history of their origin and the main characteristics: frequency, range, etc.

Historical reference

May 7, 1895 is considered the birthday of the radio receiver. On this day, the Russian scientist A.S. Popov demonstrated his apparatus at a meeting of the Russian Physicochemical Society.

In 1899, the first radio communication line, 45 km long, was built between and the city of Kotka. During World War I, direct amplification receivers and vacuum tubes became widespread. During hostilities, the presence of a radio turned out to be strategically necessary.

In 1918, simultaneously in France, Germany and the USA, scientists L. Levvy, L. Schottky and E. Armstrong developed the superheterodyne reception method, but due to weak electron tubes wide use this principle was only received in the 1930s.

Transistor devices emerged and developed in the 50s and 60s. The first widely used four-transistor radio, the Regency TR-1, was created by German physicist Herbert Mathare with the support of industrialist Jakob Michael. It went on sale in the US in 1954. All old radios used transistors.

In the 70s, the study and implementation of integrated circuits began. Receivers are now being developed through greater integration of nodes and digital signal processing.

Device characteristics

Both old and modern radios have certain characteristics:

1. Sensitivity is the ability to receive weak signals.
2. Dynamic range - measured in Hertz.
3. Noise immunity.
4. Selectivity (selectivity) - the ability to suppress extraneous signals.
5. Self-noise level.
6. Stability.

These characteristics do not change in new generations of receivers and determine their performance and ease of use.

The principle of operation of radio receivers

In the most general form, USSR radio receivers worked according to the following scheme:

1. Due to fluctuations in the electromagnetic field, alternating current appears in the antenna.
2. The oscillations are filtered (selectivity) to separate information from noise, i.e., the important component of the signal is isolated.
3. The received signal is converted into sound (in the case of radio receivers).

Using a similar principle, an image appears on a TV, digital data is transmitted, and radio-controlled equipment (children’s helicopters, cars) operates.

The first receiver was more like a glass tube with two electrodes and sawdust inside. The work was carried out according to the principle of the action of charges on metal powder. The receiver had a huge resistance by modern standards (up to 1000 Ohms) due to the fact that the sawdust had poor contact with each other, and part of the charge slipped into the air space, where it was dissipated. Over time, these filings were replaced by an oscillating circuit and transistors to store and transmit energy.

Depending on the individual receiver circuit, the signal in it may undergo additional amplitude and frequency filtering, amplification, digitization for further software processing, etc. A simple radio receiver circuit provides for single signal processing.

Terminology

An oscillating circuit in its simplest form is a coil and a capacitor closed in a circuit. With their help, you can select the one you need from all the incoming signals due to the circuit’s own frequency of oscillation. USSR radios, as well as modern devices, are based on this segment. How does it all work?

As a rule, radios are powered by batteries, the number of which varies from 1 to 9. For transistor devices, 7D-0.1 and Krona batteries with a voltage of up to 9 V are widely used. The more batteries required simple circuit radio receiver, the longer it will work.

Based on the frequency of received signals, devices are divided into the following types:

1. Long-wave (LW) - from 150 to 450 kHz (easily scattered in the ionosphere). What matters are ground waves, the intensity of which decreases with distance.
2. Medium wave (MV) - from 500 to 1500 kHz (easily scattered in the ionosphere during the day, but reflected at night). During daylight hours, the radius of action is determined by grounded waves, at night - by reflected ones.
3. Shortwave (HF) - from 3 to 30 MHz (do not land, are exclusively reflected by the ionosphere, so there is a radio silence zone around the receiver). At low power transmitter, short waves can travel long distances.
4. Ultrashortwave (UHF) - from 30 to 300 MHz (have a high penetrating ability, are usually reflected by the ionosphere and easily bend around obstacles).
5. - from 300 MHz to 3 GHz (used in cellular communication and Wi-Fi, operate within visual range, do not go around obstacles and propagate in a straight line).
6. Extremely high frequency (EHF) - from 3 to 30 GHz (used for satellite communications, are reflected from obstacles and operate within line of sight).
7. Hyper-high frequency (HHF) - from 30 GHz to 300 GHz (they do not bend around obstacles and are reflected like light, they are used extremely limited).

When using HF, MF and DV radio broadcasting can be carried out while being far from the station. The VHF band receives signals more specifically, but if a station only supports it, then you won’t be able to listen on other frequencies. The receiver can be equipped with a player for listening to music, a projector for displaying on remote surfaces, a clock and an alarm clock. The description of the radio receiver circuit with such additions will become more complicated.

The introduction of microcircuits into radio receivers made it possible to significantly increase the reception radius and frequency of signals. Their main advantage is relatively low energy consumption and small size, which is convenient for carrying. The microcircuit contains all the necessary parameters for downsampling the signal and making the output data easier to read. Digital signal processing dominates modern devices. were intended only for transmitting an audio signal, only in recent decades the design of receivers has developed and become more complex.

Circuits of the simplest receivers

The circuit of the simplest radio receiver for assembling a house was developed back in Soviet times. Then, as now, devices were divided into detector, direct amplification, direct conversion, superheterodyne, reflex, regenerative and super-regenerative. Detector receivers are considered the simplest to understand and assemble, from which the development of radio can be considered to have begun at the beginning of the 20th century. The most difficult devices to build were those based on microcircuits and several transistors. However, once you understand one pattern, others will no longer pose a problem.

Simple detector receiver

The circuit of the simplest radio receiver contains two parts: a germanium diode (D8 and D9 are suitable) and a main telephone with high resistance (TON1 or TON2). Since there is no oscillatory circuit in the circuit, it will not be able to catch signals from a specific radio station broadcast in a given area, but it will cope with its main task.

To work, you will need a good antenna that can be thrown onto a tree, and a ground wire. To be sure, it is enough to attach it to a massive piece of metal (for example, to a bucket) and bury it a few centimeters into the ground.

Option with oscillating circuit

To introduce selectivity, you can add an inductor and a capacitor to the previous circuit, creating an oscillatory circuit. Now, if you wish, you can catch the signal of a specific radio station and even amplify it.

Tube regenerative shortwave receiver

Tube radio receivers, the circuit of which is quite simple, are made to receive signals from amateur stations on short distances- for ranges from VHF (ultra-short wave) to LW (long wave). Finger battery lamps work on this circuit. They generate best on VHF. And the resistance of the anode load is removed by low frequency. All details are shown in the diagram; only the coils and inductor can be considered homemade. If you want to receive television signals, then the L2 coil (EBF11) is made up of 7 turns with a diameter of 15 mm and a 1.5 mm wire. 5 turns are suitable.

Direct amplification radio receiver with two transistors

The circuit also contains a two-stage low-frequency amplifier - this is a tunable input oscillatory circuit of the radio receiver. The first stage is an RF modulated signal detector. The inductor coil is wound in 80 turns with PEV-0.25 wire (from the sixth turn there is a tap from below according to the diagram) on a ferrite rod with a diameter of 10 mm and a length of 40.

This simple radio receiver circuit is designed to recognize powerful signals from nearby stations.

Supergenerative device for FM bands

The FM receiver, assembled according to E. Solodovnikov’s model, is easy to assemble, but has high sensitivity(up to 1 µV). Such devices are used for high frequency signals(more than 1 MHz) with amplitude modulation. Thanks to the strong positive feedback the coefficient increases to infinity, and the circuit goes into generation mode. For this reason, self-excitation occurs. To avoid it and use the receiver as a high-frequency amplifier, set the coefficient level and, when it reaches this value, sharply reduce it to a minimum. For continuous gain monitoring, you can use a sawtooth pulse generator, or you can do it simpler.

In practice, the amplifier itself often acts as a generator. Using filters (R6C7) that highlight low-frequency signals, the passage of ultrasonic vibrations to the input of the subsequent ULF cascade is limited. For FM signals 100-108 MHz, coil L1 is converted into a half-turn with a cross-section of 30 mm and a linear part of 20 mm with a wire diameter of 1 mm. And coil L2 contains 2-3 turns with a diameter of 15 mm and a wire with a cross-section of 0.7 mm inside a half-turn. Receiver amplification is possible for signals from 87.5 MHz.

Device on a chip

The HF radio receiver, whose circuit was developed in the 70s, is now considered the prototype of the Internet. Shortwave signals (3-30 MHz) travel vast distances. It is not difficult to set up a receiver to listen to broadcasts in another country. For this, the prototype received the name world radio.

Simple HF receiver

A simpler radio receiver circuit lacks a microcircuit. Covers the range from 4 to 13 MHz in frequency and up to 75 meters in length. Power supply - 9 V from the Krona battery. The installation wire can serve as an antenna. The receiver works with headphones from the player. The high-frequency treatise is built on transistors VT1 and VT2. Due to capacitor C3, a positive reverse charge arises, regulated by resistor R5.

Modern radios

Modern devices are very similar to radio receivers in the USSR: they use the same antenna, which produces weak electromagnetic oscillations. High-frequency vibrations from different radio stations appear in the antenna. They are not used directly to transmit a signal, but carry out the operation of the subsequent circuit. Now this effect is achieved using semiconductor devices.

Receivers were widely developed in the middle of the 20th century and have been continuously improved since then, despite their replacement mobile phones, tablets and TVs.

The general design of radio receivers has changed slightly since Popov's time. We can say that the circuits have become much more complicated, microcircuits and transistors have been added, and it has become possible to receive not only an audio signal, but also to build in a projector. This is how receivers evolved into televisions. Now, if you wish, you can build whatever your heart desires into the device.