Broadcasting receivers are currently being built according to a superheterodyne scheme. There are many reasons for this, these are high sensitivity and selectivity, which do not change much when tuning in frequency and changing ranges, and most importantly, ease of assembly and repeatability of parameters during mass production. The direct amplification receiver is a piece of hand-assembled product, characterized by such features as a low level of interference and noise, the absence of interference whistles and false settings. On HF superheterodyne it is difficult to find an adequate replacement, but in the CB range the Q-factor of the circuits can reach 250 or more, then the bandwidth of the circuit is even less than needed to receive AM signals.
The loops can be combined into filters as in the previous design, but there is another way to increase the selectivity of the forward gain receiver, which is rarely used. This is a pseudo-synchronous reception, in which the level of the carrier of the desired station rises in the radio path with a narrow-band high-quality circuit. The amplitude detector of the receiver has the ability to suppress weak signals in the presence of a strong useful one, and the amount of this suppression is proportional to the square of the ratio of the signal amplitudes. Thus, by raising the carrier by only three times, an improvement in selectivity of up to 20 dB can be obtained. Raising the carrier also reduces detection distortion.
But a narrowband loop of, for example, a magnetic antenna that raises the carrier will inevitably weaken the edges of the sidebands of the received signal corresponding to the higher audio frequencies. This disadvantage can be eliminated not only by “modulating” the signal, as was done in the radio receiver, but also by raising the high frequencies in the ultrasonic frequency converter. This is exactly what is done in the described receiver.
The receiver is designed to receive local and powerful long-distance stations in the MW range. In terms of sensitivity, it is not much inferior to class III-TV superheterodyne, but it gives a noticeably better reception quality. Its selectivity, measured by the usual single-signal method, is rather low (10-20 dB with a detuning at 9 kHz), however, the interfering signal in the adjacent channel, equal in amplitude to the useful one, is suppressed due to the described effect by 26-46 dB, which is also comparable to the selectivity of the mentioned superheterodyne.
The output power of the built-in ultrasonic frequency converter does not exceed 0.5 W - with a good speaker, this is more than enough for listening to broadcasts in a living room (the main attention was paid not to the volume, but to the quality). The receiver is powered by any 9-12 V source, the quiescent current consumed does not exceed 10 mA. The schematic diagram of the radio path is shown in Fig. one.
Fig. 1. Schematic diagram of the radio path of the receiver.
The narrow-band loop, emphasizing the carrier of the received signal, is the loop of the L1C1C2 magnetic antenna with a quality factor of at least 250. Its bandwidth at the level of 0.7 with tuning over the range is from 2 to 6 kHz. The signal highlighted by the circuit is fed to the RF amplifier, made according to the cascode circuit on field-effect transistors VT1, VT2. The RF amplifier has a high input impedance, which shunt the magnetic antenna circuit very little, and therefore does not reduce its Q-factor.
The first transistor VT1 was chosen with a low cut-off voltage, and the second VT2 - with a much higher one, about 8 V. This made it possible to connect the gate of the second transistor to the common wire and to get by with a minimum of parts in the amplifier. The total drain current of the transistors is equal to the initial drain current of the first transistor (0.5-2.5 mA), and its automatic drain voltage is equal to the bias voltage of the second transistor (2-4 V).
The load of the cascade amplifier is the second tunable resonant circuit L3C6C7, connected to the amplifier output through the L2 coupling coil. This circuit has a much lower Q factor (no more than 100-120) and passes the AM signal spectrum with only a slight attenuation at the edges of the side bands. The introduction of another loop into the receiver turned out to be useful, because, as practice has shown, if there is a signal from a powerful local station on the air, even far from the frequency of the receiver tuning frequency, the selectivity of one loop may not be enough. In addition, the second loop sharply limits the bandwidth, and, consequently, the power of the noise coming from the RF amplifier to the detector. Structurally, it is easy to introduce the second circuit, since the overwhelming majority of KPIs are produced in the form of double blocks.
The second, aperiodic, URCH cascade is assembled on the ѴТЗ field-effect transistor. It is loaded on the diode detector VD1, VD2, assembled according to the voltage doubling circuit.The AGC signal of negative polarity from the load of the detector, resistor R7, is fed through the filtering chain R4C4 to the gate of the first transistor of the RF amplifier VT1 and locks it when receiving powerful stations. In this case, the total current of the cascade amplifier and its amplification decrease. The capacitance of the blocking capacitor CU, which shunt the detector load, is chosen very small. This is important, since the suppression of interference from neighboring stations in the detector occurs only under the condition that the difference in beat frequency between the carriers of the wanted and interfering stations is not suppressed at the detector load.
The detected audio signal is fed through the correcting circuit R8R9C11 to the gate of the VT4 source follower. By moving the slider of the resistor R8, you can change the amount of boost in the high frequencies of the audio spectrum, attenuated by the narrow-band loop of the magnetic antenna. This variable resistor also successfully serves as a tone control. The source follower matches the high impedance output of the detector with the low impedance low pass filter (LPF) L4C14C15C16. The latter has a bandwidth of about 7 kHz and a pole (i.e., maximum) of attenuation at a frequency of 9 kHz, corresponding to the beat frequency between carrier stations in adjacent frequency channels. The low-pass filter filters this and other beat frequencies of the useful signal with interference and thereby additionally increases the two-signal selectivity of the receiver.
Rice. 2. UZCH receiver.
At the output of the low-pass filter through the matching resistor R12, the volume control R13 is turned on. Resistor R12 is needed so that the output of the low-pass filter is not short-circuited at the lowest volume levels, but is loaded onto a matched resistance, then its frequency response is not distorted. The ultrasonic frequency response of the receiver is actually made according to the same scheme (Fig. 2) as in the receiver-radio-diode (see above), only some ratings of the parts have been changed and the supply voltage has been increased to 9-12 V. Accordingly, the quiescent current has increased to several milliamperes and output power up to hundreds of milliwatts. To further increase the output power in place of VT4, VT5, you can install a complementary pair of more powerful transistors GT402 and GT404.
In the receiver, it is desirable to use transistors of exactly those types that are indicated in the circuit diagram. In an extreme case, the KP303A transistors can be replaced by KP303B or KP303I, and KP303E - by KP303G or KP303D. Diodes VD1, VD2 - any high-frequency germanium. A dual air dielectric KPI unit can be taken from any old broadcasting receiver. Resistors and capacitors can be of any type, adjusted capacitors C1 and C6 are of the KPK-M type. The magnetic antenna is the same as in the previous receiver: a rod with a diameter of 10 and a length of 200 mm made of ferrite 400NN, the L1 coil contains 50 turns of LESHO 21x0.07. For coils L2, L3, standard fittings are used - an armored core with a screen from the IF circuits of portable receivers, for example, the Sokol receiver. The L2 communication coil contains 30, and the L3 loop coil contains 90 turns of PEL 0.1 wire. The location of the coils on the common frame does not really matter.
LPF coil L4 with inductance OD H is wound on a ring with an outer diameter of 16 and a height of 5 mm (K 16x8x5) from ferrite 2000NM. It contains 260 turns of PELSHO OD wire. You can also pick up a ready-made coil, for example, one of the windings of the transitional or output transformer from the ultrasonic frequency converter of old portable receivers. By connecting a 5000 pF capacitor and an oscilloscope in parallel to the coil, the signal from the sound generator is fed to the resulting circuit through a 200 kΩ - 1 MΩ resistor.
Determining the resonant frequency of the circuit based on the maximum voltage across it, a coil is selected so that the resonance is obtained at a frequency of 6.5-7 kHz. This frequency will be the cutoff frequency of the LPF. At the same time, it is useful to check the frequency of the damping pole of 9 kHz by connecting a capacitor C16 in parallel to the coil and specifying its capacitance (1000–1500 pF). In the absence of a suitable coil, it can be replaced (with worse results, of course) with a 2.2 kΩ resistor. Capacitor C16 is excluded in this case.
The recommended arrangement of the receiver boards, controls and magnetic antenna in the receiver housing is shown in Fig. 5. It can be seen that the antenna is as far as possible from the circuit of the amplifier L2 - L3 and the filter coil L4. A suitable plastic box can serve as a case, but it is better to make it yourself, for example, from wood, and arrange it the way tuners usually arrange. It is possible to build a metal case, but without a back wall, so that it less reduces the receiving properties of the magnetic antenna. It is advisable to equip the tuning knob with a vernier with a slight deceleration and a scale of any type.
Fig. 3. The printed circuit board of the radio path.
Fig. 4. UZCH printed circuit board.
Fig. 5. Location of parts in the receiver housing.
The establishment of the receiver begins with an ultrasonic frequency converter. Having applied the supply voltage, the resistance of the resistor R2 is selected so that the voltage across the collectors of transistors VT4 and VT5 is equal to half of the supply voltage. Turning on the milliammeter in the break of the power wire, select the type (D2, D9, D18, etc.) and a copy of the VD1 diode until a quiescent current of about 3-5 mA is obtained. You can turn on several diodes in parallel, but you cannot turn off the diode without removing the power!
By connecting the radio frequency part of the receiver, the modes of the transistors are checked. The voltage at the source of the transistor VT4 should be 2-4 V, at the drain ѴТЗ - 3-5 V and at the junction point of the drain VT1 with the source ѴТ2 - 1.5-3 V. If the voltages are within the specified limits, the receiver is operational and you can try to accept station signals. Listening to the signal at the low-frequency edge of the CB range, the settings of the contours are matched by moving the L1 coil along the magnetic antenna rod and rotating the L2 coil core, achieving maximum reception volume. At the same time, the lower limit of the range is set, focusing, for example, on the frequency of the radio station "Mayak" 549 kHz. Having adopted another station at the upper end of the range, the same is done with trimmer capacitors C1 and C6. Repeating this operation several times, achieve a good pairing of the contour settings throughout the range.
With self-excitation of the RF amplifier, which manifests itself in the form of whistling and distortions when receiving stations, you should reduce the resistance of the resistor R2 and try to rationally arrange the conductors leading to the stator plates of the KPI C2S7 - they should be as short as possible, located further from each other and closer to the "grounded" board surface. As a last resort, these conductors will have to be shielded.
For more accurate tuning to the frequency of the radio station, it is advisable to equip the receiver with a tuning indicator - an LED or a dial gauge connected in series with the resistor R3. Any device with a full deflection current of 1-2 mA will do. It must be shunted with a resistor, the resistance of which is selected so that the arrow deflects to the full scale in the absence of a received signal. When the station signal is received, the AGC system locks the RF amplifier and the deflection of the arrow decreases, indicating the strength of the signal.
The tests of the receiver in Moscow gave quite good results. During the day, almost all local stations were received, listening on any transistor receiver of the superheterodyne type. In the evening and at night, when long-distance transmission opens on the NE, many stations were received several thousand kilometers away. Due to the low single-signal selectivity, several stations can be listened to at the same time, but with fine tuning to a stronger signal, the effect of suppressing the weak is noticeable and the program is listened to cleanly or with little interference.
Schematic diagram of a homemade receiver on five transistors for operation in the SV-DV bands, nostalgic design for a free minute.
Many radio amateurs began their journey by assembling a 4-6 transistor direct amplification receiver. In the USSR, such radio construction kits were sold, as far as I remember, at a price of 6 to 14 rubles. If you have a desire and free time, you can remember your childhood by working with the scheme shown in the figure. Yes, at the same time, and make a "country radio", which is not a pity to leave in a poorly guarded room.
The only condition is that at least one broadcasting station in the range of long or medium waves must operate in your area. However, if there are none, the receiver will be able to receive quite a lot of remote and even "foreign" radio stations at night (there is no factor of "closure" by the signal of a powerful local radio station).
Receiver circuit
As they would have written in the Radio magazine of the 70s, this is a 2-V-2 scheme. That is, two UHF stages, a detector, and two VLF stages.
The signal is received by a magnetic antenna consisting of a ferrite rod 8 mm in diameter and the longer the better, and two coils L1 and L2 on cardboard sleeves. The input circuit is formed by a coil L1 and a variable capacitor C1. Through the communication coil L2, the signal enters the first stage of the RF amplifier on the transistor VT1. Further - the second stage on VT2.
The detector is based on a 1N4148 type VD1 silicon diode. Silicon does not work well as a detector due to a too long linear section with a low I - V characteristic, however, here the diode is under direct current through R4 and R5, which compensates for this disadvantage.
Rice. 1. Schematic diagram of a direct amplification receiver, nostalgia.
Details and installation
Speaker B1 - yes, almost any! The L1 coil for CB contains 90 turns of any winding wire with a diameter of 0.2 to 0.5 mm. L1 for DV - 240 turns in six sections in bulk, any winding wire from 0.1 to 0.3 mm. L2 is approximately 10% of L1.
Installation - by weight by soldering the leads of the parts to each other (or whatever you want).
Establishment
I will not write anything about adjusting and replacing parts, I do not want to spoil your pleasure in reaching everything yourself. I will only hint that the base resistors are responsible for the DC cascade mode.
If there are no powerful local MW and LW radios, and this is for the best, make a direct amplification KB receiver. Wrap L1 and L2 on a frame with a trimmer ferrite core (for example, from a color module or an old TV's IF). L1 - 30 turns, L2 - 10 turns.
And through a 5-10 pF capacitor, connect an external antenna to the top, according to the scheme, plate C1, a long wire stretched from corner to corner under the ceiling.
The block diagram of a direct gain receiver without regeneration (Fig.8.6) includes input circuit, high (radio) frequency amplifier(UHF, URCH), detector(D) and low (audio) frequency amplifier(ULF, UZCH). Sometimes before the RF amplifier include low noise amplifier(LNA).
Input circuit and UHF constitute the high-frequency path of the receiver and contain systems of resonant circuits, which serve to obtain maximum signal power from the antenna, and also separate the required signal from many other signals and interference. LNAs (used as needed) are designed to reduce the noise floor of the receiver and determine the sensitivity of the receivers. In some cases, with sufficient received power, UHF may not be available. The modulating function allocated by the detector (demodulator), containing useful information, is amplified and filtered from interference and other combination frequencies in the ULF. Its gain is determined by the voltage (power) that must be supplied to the terminal device for its normal operation.
The receiver is tuned to a useful signal by tuning the frequency of the input circuit, LNA and UHF. Synchronous frequency tuning of all these blocks is not an easy task. In the microwave range, it is technically difficult to match the bandwidth of the receiver with the spectrum width of the useful signal to filter the latter from interference that does not coincide in frequency with the signal. These factors are a disadvantage of direct amplification receivers.
Rice. 8.6. Direct amplification receiver block diagram
Literature: IN AND. Nefedov, “Fundamentals of radio electronics and communications”, Publishing house “Vysshaya shkola”, Moscow, 2002.
Radio receiving devices
22.1. Structural diagrams
Radio- a device connected to the antenna and used for radio reception.
Radio waves emitted by various radio transmitters hit the receiving antenna and create electrical vibrations in it, therefore, for a radio receiver, the antenna is a source of the radio signal. Since a lot of radio waves hit the antenna, the input signal of the receiver
consists of the useful signal s ( t) and interference P(t). Factor k(t) takes into account the change in the transmission coefficient of the communication channel over time and is called multiplicative a hindrance. Hindrance n(t), added to the signal is called additive. In the general case, additive noise consists of harmonic, impulse and fluctuation noise.
Harmonic or lumped in frequency called narrowband interference. The main sources of this interference are other radio transmitters.
Impulse or time-focused is called interference, the shape of which resembles radio pulses. A distinctive feature of impulse noise is the inequality
where t and is the average pulse duration; T is the average distance between impulses.
Impulse noise includes noise generated by atmospheric discharges, industrial plants, and vehicles.
Fluctuation interference - broadband random continuous oscillations. A typical example of fluctuation noise is white noise (see § 2.7). Fluctuation disturbances are generated by the chaotic movement of charge carriers. This interference is one of the main types of interference in space channels and some terrestrial microwave channels. Fluctuation noise also includes the intrinsic noise of the receiver.
The simplest direct gain receiver circuit consists of an input circuit, an RF amplifier, a detector, and an audio frequency amplifier (Figure 22.1). The signal of the desired frequency is allocated by systems of resonant circuits that serve as input circuits and the load of the radio frequency amplifier. Tuning the receiver to the desired frequency is carried out by tuning all resonant circuits.
The simplicity of a direct-amplified radio receiver is only apparent. To obtain a narrow bandwidth, it is necessary to increase the number of resonant circuits and their Q-factor. Consequently, receiver tuning becomes more difficult. Therefore, direct amplification receivers are rarely made.
Currently, superheterodyne radio receivers are in widespread use (Fig. 22.2). In such receivers, the frequency of the received radio signal is converted so that the spectrum concentrated in the vicinity of the frequency ω i, is transferred to the intermediate frequency ω i... Frequency conversion is performed by a converter consisting of a mixer and a local oscillator - a reference oscillator. The principle of operation of such a converter is discussed in § 17.3. Most often intermediate frequency
or 
(22.2)
With the restructuring of the input circuit and the radio frequency amplifier, the frequency of the local oscillator also changes so that the intermediate frequency ω p remains constant. This circumstance allows the use of non-tunable intermediate frequency amplifiers (IFA). Such IFA can be created with good frequency selectivity. Therefore, the main gain and frequency selectivity of the superheterodyne receiver is provided by the IF amplifier. The RF front-end and RF amplifier pre-isolate the signal and attenuate strong interfering RF signals.
The superheterodyne radio receiver, possessing fundamental advantages, is not devoid of disadvantages. The main one is side channels of reception. As is known from the general theory of frequency conversion (see § 17.3), not only a signal, for example, with a frequency ω c = ω g + ω n, but also other signals, whose frequencies ω c ( P, T) satisfy the equality
(22.3)
The main receiving side channel is called mirrored. The frequency of this channel ω SC differs from the frequency of the signal ω c by twice the intermediate frequency: ω SC = ω c ± 2ω p. e. Filters included in the input circuits and RF amplifier. It is useful to bear in mind that the suppression of side reception channels is facilitated with an increase in the intermediate frequency ω p, however, it becomes difficult to obtain a sufficiently narrow IF amplifier bandwidth.
Another disadvantage of a superheterodyne receiver is the possibility of combinational whistles. Such whistles appear at some frequencies of the received signal ω c "= ω g - ω p, at which ω p", approximately equal to the frequency ω p, is obtained in accordance with (22.3) and by a more complex transformation. Under this condition, the IF amplifier amplifies two signals with close frequencies. Due to the beats of the carriers of these signals, a low-frequency envelope appears with a frequency | ω p - ω p "|, which is allocated by an amplitude detector, then amplified and listened to in the form of a whistle. The third disadvantage of a superheterodyne receiver is that it can cause radio interference to other receivers if the oscillator hits the antenna.
All of the enumerated drawbacks in modern superheterodyne receivers are eliminated by the rational choice of an intermediate frequency or two intermediate frequencies in double conversion receivers, the use of mixers that perform almost perfectly accurate voltage multiplication, and reliable isolation of the local oscillator from the input circuits.
In addition to the main functional units, such as input circuits, amplifiers of radio, intermediate and audio frequencies, a frequency converter and a detector, circuits of modern radio receivers are supplemented with devices and systems that qualitatively improve technical and operational indicators. These are automatic gain control and automatic frequency control systems.
Structural and circuitry features, design and element base of a radio receiver are determined by its purpose, operating conditions, and the range of received waves.
Receivers are divided into broadcasting, television, communication, radar, navigation, and other receivers according to their intended purpose. The properties of the received signals are determined by the purpose of the receiver. For example, broadcast receivers are designed to receive voice and music signals; television - for receiving image and sound signals; communicators - for receiving telephone and telegraph signals, digital control signals, etc.
According to the operating conditions, stationary and non-stationary receivers are distinguished. Receivers for various purposes can be both stationary and non-stationary. Stationary receivers are considered to be not intended for use on moving objects. Non-stationary receivers include all receivers installed on mobile objects, for example, space, aircraft, ship, automobile, portable, etc.
To implement receivers, the industry produces specialized ICs that perform the functions of one or more functional units. Such examples of IP are given in the previous chapters. So, as an intermediate and radio frequency amplifier, the K175UV4 IC can be used (see Fig. 14.17), the frequency conversion is performed by the 219PS1 IC (see Fig. 17.9). The audio frequency amplifier can be the K174UN5 IC (see Fig. 15.7). Specialized series of ICs are also produced. For broadcasting receivers, ICs of the 235 series are intended, for television - ICs of the K174 series, etc.
Structural diagrams of receivers, depending on their purpose, are supplemented with specific functional units. Complex communication receivers are supplied with software setting devices. Receivers designed to receive digital information are equipped with post-detector processing devices that filter and decode the received signal. These devices are often based on MP. In television receivers, the signal from the output of the detector is divided into an image and sound signal. Pulse sequences are extracted from the image signal, which are necessary for synchronizing the horizontal and vertical scan generators. All these transformations are performed by specialized ICs.
In the middle of the 20th century, the broadcast medium wave band was very popular. Its attractiveness was explained not only by the presence of a large number of broadcasting radio stations, but also by the ability to listen to the work of numerous radio hooligans, sometimes broadcasting popular music of that time. At the beginning of the 21st century, the situation on this range has changed dramatically, and there are much fewer broadcasting radio stations, interest in it has disappeared, and the fleet of receiving equipment has become obsolete.
So many people think now, they write about it on the Internet, and so did I. But suddenly I discovered that there are few broadcasting radio stations (especially Russian-speaking ones) in Central Asia in this range, but there are still a lot of them in Europe, and the interest of radio amateurs in this range is gradually increasing. Is it nostalgia or the reason for the simplicity of the design of this class of receivers? Most likely, both! When I assembled this receiver and began to regularly listen to the mid-wave range, I again discovered that we still have broadcasting stations on this range. It seems to me that something has clearly changed on the air. Maybe due to the fact that I began to regularly listen to this range, and the stations appeared?
The direct amplification radio receiver, the description of which is given below, despite the apparent complexity of the circuit, is quite suitable for repetition even by novice radio amateurs. The receiver circuit is shown in the figure. The RF signal from the magnetic antenna WA1 is fed to the gate of the transistor VT1, on which the paraphase stage is assembled. Its gain is less than one, but its task is to receive at the outputs two signals of the same amplitude, but opposite in phase. The use of a field-effect transistor makes it possible to obtain signals of greater identity in comparison with a similar stage on a bipolar transistor (the currents through the source and drain resistors are equal, in contrast to the currents of a bipolar transistor). The high input impedance of the transistor shunts the magnetic antenna circuit a little, allowing the gate of the transistor to be directly connected to it. In this case, the Q-factor of the antenna contour practically does not deteriorate, which ensures better selectivity. In this stage, the gain of the RF signal is also controlled through the gate circuit using the AGC system.
Rice. Receiver circuit
Antiphase signals are fed to the inputs of a symmetrical amplifier (V. Rubtsov. An intermediate frequency amplifier with improved symmetry. - Radio, 2005, No. 12, p. 67), assembled on VT2-VT5 transistors. This amplifier has a high gain (up to 6000), is stable and generates two antiphase signals at the output. These signals are fed to a push-pull detector of AM signals, assembled on diodes VD1-VD4. A feature of such a detector is that a voltage with a doubled frequency of the input signal is generated at its output, and a signal with an input frequency is significantly suppressed. In addition, the RF signal is suppressed by the smoothing capacitor C11. As a result, the RF part of the receiver has an increased resistance to self-excitation. A constant voltage of negative polarity from the output of the detector through the low-pass filter R4C4 is fed to the gate of the transistor VT1. With an increase in the level of the received signal, the constant voltage at the output of the detector increases (in absolute value), which leads to a decrease in the gain of the RF path. This is how the AGC system works. Despite the fact that the work of the AGC leads to a change in the operating modes of the paraphase cascade, this practically does not affect the quality of reception.
The AF signal through the capacitor C10 is fed to the volume control R14 and then to the input of the UMZCH, assembled on VT6-VT10 transistors according to the well-known scheme. The maximum output power of the amplifier is 150 mW.
Fixed resistors MLT, S2-23, VS are used, variable resistors - SP, SPO, SP3. The KP302B transistor can be replaced with the KP302V, KP303E, KP307A transistor. It is desirable to select transistors in a symmetrical amplifier with close base current transfer ratios. D311 diodes can be replaced with diodes of the D9 series with any letter index. Oxide capacitors - K50-35 or imported, the rest - KT, KM, K10-7V, K73. Variable capacitor - with air dielectric. The ULF uses a 3GDSH-8-8 dynamic head with a voice coil resistance of 8 ohms, but any small-sized 0.5 ... 1 W power with the same resistance will do.
The magnetic antenna is wound on a round or flat ferrite magnetic core of the 400NN or 600NN brand with a length of 100 ... 140 mm. The coil for the SV range contains 70 ... 80 turns of PEV or PELSHO wire with a diameter of 0.2 ... 0.25 mm or 250 ... 280 turns of a thinner wire if it is supposed to use a receiver in the DV range. Coil winding type CB - turn to turn, DV - sectional (5 ... 6 sections). You can use any other magnetic antenna from pocket radios.
If the constant resistor R13 is replaced with a trimmer and the lower terminal of the resistor R4 is connected to its engine, then using the trimmer it is possible to vary the response threshold and the AGC depth within wide limits. This can be done by ear while receiving a powerful radio station. The speed (time constant) of the AGC system can be changed by selecting the capacitor C4. The ULF is adjusted by a selection of the resistor R20, with its help, the quiescent current of 1.5 ... 3 mA of the transistor VT10 is set (in the collector circuit). With a selection of resistor R16, half the supply voltage (+6 ... 7 V) is set at the junction point of the collector of the transistor VT9 and the emitter of the transistor VT10.
The receiver is tuned to the station with the variable capacitor C1 and the rotation of the magnetic antenna (this way you can tune out the interference). To improve the sensitivity of the receiver, an external antenna drop wire can be placed near the MA coil (1 ... 2 cm). The receiver was wired on a breadboard PCB and showed good performance. It is desirable that the connecting wires be kept to a minimum.
Date of publication: 22.10.2017
Readers' opinions
- sergey vladimirovich goryachev / 12.03.2018 - 17:21
Thank you for your publications. Excellent and understandable developments. I wish you creative success! RA9YV 73!
For a long time, radio receivers occupied one of the first places in popularity among other radio-electronic designs. The emergence of new sound-reproducing devices, CD-players, tape recorders and the rapid development of computer technology have pushed radio reception technology out of the leading positions without diminishing its importance.
Receivers are subdivided into detector, direct amplification, superheterodyne type, direct conversion, with positive feedback (regenerative, superregenerative), etc.
Simple two-transistor direct amplification radio receiver
A simple forward-gain receiver is shown in Fig. 1 [MK 10 / 83-11]. It contains a tunable input oscillatory circuit - a magnetic antenna and a two-stage LF amplifier.
The first stage of the amplifier is also an RF modulated signal detector. Like many similar simple direct amplification receivers, this receiver is capable of receiving signals from powerful, not so distant radio stations.
The inductor is wound on a ferrite rod 40 mm long and 10 mm in diameter. It contains 80 turns of wire PEV-0.25 mm with a branch from the 6th turn from the bottom (according to the diagram).
Rice. 1. Scheme of a simple radio receiver on two transistors.
Reflex receiver Y. Prokoptsov
The radio receiver, designed by Yu. Prokoptsev (Fig. 3), is intended for reception in the medium-wave range [R 9 / 99-52]. The receiver is also assembled according to the reflex scheme.
Rice. 3. Scheme of a reflex radio receiver for the MW range.
The antenna is made of a piece of a 400NN ferrite rod with a length of 50 and a diameter of 8 mm. Coil L1 contains 120 turns of PELSHO-0.15 mm single-layer winding, and L2 contains 15 ... 20 turns of the same wire. Establishing the receiver is reduced to setting the collector current of the transistor VT2, equal to 8 ... 10 mA, using the resistor R2. Then the collector current of the transistor VT3 is adjusted within 0.3 ... 0.5 mA by selecting the resistor R4.
We will not consider superheterodyne receivers in this review. However, if desired, they can be obtained by combining a direct gain receiver (Fig. 1 - 3) and a converter (Fig. 10), or from a direct conversion receiver (Fig. 11).
Super-regenerative FM radio
A super-regenerative radio receiver has a high sensitivity (up to μV units) with sufficient simplicity. In fig. 4 shows a fragment of a diagram of a super-regenerative radio receiver by E. Solodovnikov (without ULF, which can be made according to one of the previously given schemes -) [Rl 3 / 99-19].
Rice. 4. Diagram of a super-regenerative radio receiver E. Solodovnikov.
The high sensitivity of the receiver is due to the presence of deep positive feedback, due to which the gain of the stage, after turning on the radio receiver, rather quickly increases to infinity, the circuit goes into the generation mode.
So that self-excitation does not occur, and the circuit can work as a highly sensitive high-frequency amplifier, a very original technique is used. As soon as the gain of the amplification stage rises above a certain predetermined level, it is sharply reduced to a minimum.
The graph of the change in the gain over time resembles a saw. It is according to this law that the gain of the amplifier is changed. The average gain can be up to a million. The gain can be controlled using a special additional sawtooth pulse generator.
In practice, they act more simply: the high-frequency amplifier itself is used as such a generator for a dual purpose. The generation of sawtooth pulses occurs at an ultrasonic frequency inaudible to the ear, usually tens of kHz. In order to prevent ultrasonic vibrations from penetrating to the input of the next ULF cascade, use the simplest filters that separate audio frequency signals (R6C7, Fig. 4).
Superregenerative receivers are usually used to receive high frequency (over 10 MHz) amplitude modulated signals. Receiving signals with frequency modulation is possible by converting frequency modulation into amplitude modulation and subsequent detection by the emitter junction of the transistor of the thus obtained amplitude-modulated signal.
Conversion of frequency modulation to amplitude modulation occurs when the receiver intended for receiving amplitude-modulated signals is not tuned accurately to the frequency of receiving a frequency-modulated signal.
With such a setting, a change in the frequency of the received signal of constant amplitude will cause a change in the amplitude of the signal taken from the oscillatory circuit: as the frequency of the received signal approaches the resonance frequency of the oscillatory circuit, the amplitude of the output signal increases, with distance from the resonant one, it decreases.
Along with the indisputable advantages, the "super-regenerator" circuit has a lot of disadvantages. These are low selectivity, increased noise level, dependence of the generation threshold on the receiving frequency, on the supply voltage, etc.
When receiving FM broadcasting signals in the FM range - 100 ... 108 MHz or sound signals from television, the L1 coil is a half-turn with a diameter of 30 mm with a linear part of 20 mm. Wire diameter - 1 mm. L2 has 2 ... 3 turns with a diameter of 15 mm from a wire with a diameter of 0.7 mm, located inside a half-turn.
For the 66 ... 74 MHz range, the L1 coil contains 5 turns with a diameter of 5 mm from a 0.7 mm wire with a pitch of 1 ... 2 mm. L2 has 2 ... 3 turns of the same wire. Both coils have no bobbins and are parallel to each other. The antenna is made of a piece of mounting wire 50 ... 100 cm long. The device is adjusted with potentiometer R2.
Regenerative transistor radios KP303
Regenerative receivers, or receivers that use positive feedbacks to increase the sensitivity, are not found in industrial designs. However, to master all possible options for the implementation of the receiving technique, it is possible to recommend that you familiarize yourself with the operation of two such devices designed by I. Grigoriev (Fig. 5 and 6) [Rl 9 / 95-12; 10 / 95-12].
Rice. 5. Scheme of a receiver for receiving AM signals in the HF, MW and LW ranges.
The receiver (Fig. 5) is designed to receive AM signals in the range of short, medium and long waves. Its sensitivity at a frequency of 20 MHz reaches 10 μV. For comparison: the sensitivity of the most advanced direct amplification receiver is about 100 times lower.
Rice. 6. Scheme of a simple regenerative radio receiver for the frequency ranges 1.5 ... 40 MHz.
The receiver (Fig. 6) is capable of operating in the 1.5 ... 40 MHz range. For the range of 1.5 ... 3.7 MHz, the L1 coil has an inductance of 23 μH and contains 39 turns of wire with a diameter of 0.5 mm on a frame with a diameter of 20 mm with a winding width of 30 mm. The L2 coil has 10 turns of the same wire and is wound on the same frame.
For the range 3 ... 24 MHz, the L1 coil with an inductance of 1.4 μH contains 10 turns of a wire with a diameter of 2 mm, wound on a frame with a diameter of 20 mm, with a winding width of 40 mm. The L2 coil has 3 turns with a wire diameter of 1.0 mm.
In the range 24 ... 40 MHz, L1 (0.5 μH) contains 5 turns, the winding width is 30 mm, and L2 has 2 turns. The operating point of the receivers (Fig. 5, 6) is set with the potentiometer R4.
VHF FM transistor radio receiver GT311
To receive FM signals, you can use VHF direct conversion receivers with phase-locked frequency control. Such receivers contain a frequency converter with a combined local oscillator, which simultaneously performs the functions of a sync detector.
Rice. 7. Scheme of VHF FM radio receiver A. Zakharov for the frequency range 66 ... 74 MHz.
The input circuit of the device is tuned to the receive frequency, the local oscillator circuit is tuned to the receive frequency, halved. Signal conversion occurs at the second harmonic of the local oscillator, so the intermediate frequency is in the audio range. The diagram of A. Zakharov's receiver is shown in Fig. 7 [P 12 / 85-28]. For the frequency range 66 ... 74 MHz, frameless coils with an inner diameter of 5 mm and a winding pitch of 1 mm contain, respectively, 6 turns with a tap from the middle (I) and 20 turns (L2) of a PEV-0.56 mm wire.
Simple Direct Amplified Receiver with Loop Antenna
A simple medium-wave direct-amplified radio receiver assembled according to the traditional scheme by G. Shulgin (Fig. 8) has a loop antenna [R 12 / 81-49]. It is wound on a workpiece: a plywood plate with dimensions of 56x56x5 mm. The inductor L1 (350 μH) has 39 turns of wire PEV-0.15 mm with a tap from 4 turns from the bottom (according to the diagram).
Rice. 8. Diagram of a radio receiver with a loop antenna for the MW range.
Simple radio receiver with a field-effect transistor input stage
In fig. 9 shows a simple radio receiver G. Shulga (without ULF) with an input stage on a field-effect transistor [R 6 / 82-52]. The magnetic antenna and variable capacitor are used from an old radio.
Rice. 9. Simple radio receiver G. Shulga.
FM frequency converter circuit
Frequency converter-converter E. Rodionova, fig. 10, allows you to "transfer" signals from one frequency band to another frequency region: from 88 ... 108 MHz to 66 ... 73 MHz [Rl 4 / 99-24].
Rice. 10. Scheme of a converter from 88 ... 108 MHz to 66 ... 73 MHz.
The heterodyne (generator) of the converter is assembled on a VT2 transistor and operates at a frequency of approximately 30 ... 35 MHz. Coil I is made of 40 cm long winding wire wound on a mandrel with a diameter of 4 mm. The converter is adjusted by stretching or squeezing the turns of the coil L1.
Superheterodyne and Direct Conversion Receiver Input Circuits
Finally, in Fig. 11 shows a diagram of the input circuit of the simplest superheterodyne receiver, and Fig. 12 receiver with zero intermediate frequency - direct conversion receiver.
Rice. 11. Scheme of the converter V. Besedin.
Converter V. Besedin (Fig. 11) "transfers" the input signal from the frequency band 2 ... 30 MHz to a lower "intermediate" frequency, for example, 1 MHz [R 4 / 95-19]. If a signal with a frequency of 0.5 ... 18 MHz from the HHF is applied to the diodes VD1 and VD2, then at the output of the LC filter L2C3 a signal will be allocated, the frequency of which f3 is equal to the difference between the frequency of the input signal f1 and the doubled frequency of the local oscillator f2: f3 = f1-2f2 or Af3 = Af1-2f2.
And if these frequencies are multiples of each other (f1 = 2f2), Fig. 2, then an ULF can be connected to the output of the device and receive telegraph signals and signals with single-sideband modulation.
Rice. 12. Scheme of the converter on transistors.
Note that the diagram in Fig. 12 easily converts to the circuit in fig. 11 by replacing diode-connected transistors directly with diodes, and vice versa.
The sensitivity of even simple direct conversion circuits can be as high as 1 μV. Coil L1 (Fig. 11, 12) contains 9 turns of PEV wire 0.51 mm, wound a turn to a turn on a frame with a diameter of 10 mm. Branch from the 3rd turn from the bottom.
Literature: Shustov M.A. Practical Circuitry (Book 1), 2003.
