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. It is difficult to find an adequate replacement for the HF superheterodyne, but in the MW range the Q-factor of the circuits can reach 250 or more, then the loop bandwidth 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, it is possible to obtain an improvement in selectivity of up to 20 dB. 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 amplifier 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 that does little to shunt the circuit of the magnetic antenna 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 larger 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.
Fig. 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 to a matched resistance, then its frequency response is not distorted. The ultrasonic frequency response of the receiver is made in fact 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 on schematic diagram... In extreme cases, the KP303A transistors can be replaced with KP303B or KP303I, and KP303E - with KP303G or KP303D. Diodes VD1, VD2 - any high-frequency germanium. A dual air dielectric KPE 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 transient or output transformer from the ultrasound frequency 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 housing, but it is better to make it yourself, for example, from wood, and arrange it the way tuners usually arrange. You can build and 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 small 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.
Establishing the receiver begins with an ultrasonic frequency response. 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 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 rod of the magnetic antenna 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 tuning capacitors C1 and C6. Repeating this operation several times, achieve good matching 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.
So, after drilling new holes in the chassis, the experiments continued further. The ULF remained the same from the Tsyganova scheme (Simple radio). As I wrote in the previous part, the power supply unit was changed and instead of the diode bridge, a bridge consisting of a kenotron and two diodes was installed. After the rework, it was discovered that the ECL82 triode was noisy in heat and to combat this noise, an artificial midpoint of the filament was made, to which a positive voltage of about 20 volts was applied.
The high-frequency part was decided not to repeat the old one, but to make another one. For the UHF and the detector, a 6AM8 lamp was chosen, which I have long wanted to use. This lamp is a diode-pentode with separate cathodes. According to the data that I found, its pentode part is designed to work in the UPCH of TVs, and the diode part is designed to work in a video detector. As far as I know, this lamp had no analogues in the Union, whether it had an analogue among European lamps, I do not know. The data can be seen and. Initially, a cascade with a resistive load was made, which was supposed to work in close to a typical mode. Anode resistor - 4.7K, resistor in the screen mesh circuit - 39K, cathode resistor 120Ω. One of the coils from the previous circuits was supplied to the input circuit. The coil is wound on a cardboard frame with a diameter of 29mm and contains 127 turns of 0.2mm wire, wound turn to turn. A piece of wire about 5m long, stretched outside the window, is used as an antenna. The detector was taken from the circuit of E. Mozzhukhin and V. Fedorenko A simple tube receiver, only a diode from the same 6AM8 was used instead of a semiconductor diode. A tuning indicator on the 6E1P lamp was also added to the circuit. I didn't really believe that he would somehow react to the received signals, but I wanted to try. There were also thoughts to finalize the scheme in the future. The original version of the scheme looked like this:
The scheme started working right away, but I was not happy with the work. More or less normally, only one station was caught, two more were barely audible. In addition to them, a huge amount of noise was caught. At first I thought that the problem was in the coil, which was catching some kind of pickup, but these suspicions did not come true. It turned out that the reason was in the antenna. The receiver was connected to the antenna using a simple wire, which caught a lot of interference. I have not yet found what was the cause of the interference in the room. Turning on the circuit through a surge protector did not help. The amount of interference decreased and the number of received stations increased after I connected the receiver to the antenna using a regular coaxial cable. After that, I started to select a mode for the lamp. I stopped at a 33K anode resistor and a 120K screen mesh resistor. I also tried to change the displacement scheme. I put the gridlik in the form of a 1M resistor and a 22pF capacitor, but did not notice much difference. In the final version, I left both the cathode resistor and the grill. I also tried to use a ferrite antenna with the circuit, but this did not give any normal results. The scheme of the last option looks like this:
The coils are made in one layer. In the process of thinking, the question arose of exactly how to make a coil for the second circuit. The first option is to wind the anode one next to the detector one. The second option is to wind one on top of the other. Well, in addition, the question arose of how many turns the anode coil should have. Thoughts were such that, on the one hand, I would like more turns, as this will increase its inductance and, as a consequence, the amplification of the UHF cascade. On the other hand, if the number of turns of the anode coil is greater than that of the detector coil, you will get a step-down transformer that will reduce the gain.
In the first version, the anode and detector coils were located side by side on the same frame. The detector coil is one-to-one like the coil of the input circuit. The anode coil was wound with 0.14mm wire for a total of 190 turns. With this coil, the receiver worked well, with some powerful stations it was possible to get very good sound quality, which could well be compared with a factory receiver. The main problem was strong excitations, especially in the high frequency range. The excitement was so strong that constant pressure the detector reached 50V, sometimes even more. I tried to make a screen, and change the position of the coil of the input circuit. Generation was defeated, but not quite. Excitements still appeared. Another method that I have tried is the introduction of AGC. Through a resistor, the negative voltage from the detector was fed to the grid of the UHF lamp. On a part of the range, this helped to get rid of generation altogether, in part to reduce it.
In the second variant, the anode coil was wound over the detector one. It was wound with 0.14-0.15mm wire, 140 turns in total. With it, the receiver also worked, but there was a feeling that the anode coil affected the inductance of the detector. The tuning of the detector circuit did not affect the reception in any way. Then gradually I began to unwind the turns. First I wound 20 turns. It seems that I did not notice any changes. Then he wound up another 60, that is, only 60 turns remained on the coil. There was a feeling that the gain had decreased, but it was still possible to cleanly receive some stations.
The tuning indicator worked somehow with both coils. At strong stations, it was even possible to adjust the contours using it. The excitement was also clearly visible on it, so it turned out that I put it for a reason.
Here are some photos of the finished circuit:
Despite its imperfect performance, this receiver changed my understanding of direct amplification receivers. Previously, I did not think that such a simple direct amplification receiver can sometimes work no worse than a factory super.
Two things remain unclear. How to connect the antenna to the receiver? What is the best way to make the input circuit? It is possible to make inductive coupling of the input circuit with the antenna. And the second more important question is how to make the coil of the detector circuit. How best to position the anode coil and how many turns the anode coil should have.
Superheterodyne radio receiver (superheterodyne) is one of the types of radio receivers based on the principle of converting the received signal into a signal of a fixed intermediate frequency (IF) with its subsequent amplification. The main advantage of a superheterodyne over a direct amplification radio receiver is that the parts of the receiving path that are most critical for the reception quality (narrow-band filter, IF amplifier and demodulator) should not be tuned to different frequencies, which allows them to be performed with significantly better characteristics.
The superheterodyne receiver was invented by the American Edwin Armstrong in 1918.
A simplified block diagram of a superheterodyne is shown in the figure. The radio signal from the antenna is fed to the input of the high-frequency amplifier (in a simplified version, it may not be present), and then to the input of the mixer - a special element with two inputs and one output, which performs the operation of converting the signal by frequency. A signal from a local low-power high-frequency generator - a local oscillator - is fed to the second input of the mixer. The oscillatory circuit of the local oscillator is reconstructed simultaneously with the input circuit of the mixer (and the circuits of the RF amplifier) - usually a variable capacitor (CVC), less often a variable inductance coil (variometer, ferrovariometer). Thus, at the output of the mixer, signals are generated with a frequency equal to the sum and the difference between the frequencies of the local oscillator and the received radio station. The difference signal of a constant intermediate frequency (IF) is extracted using a concentrated selection filter (FSS) and amplified by one or more stages, after which it is fed to a demodulator, which restores a low (audio) frequency signal. Typically, the IF filter is scattered across all stages of the intermediate frequency amplifier, since the FSS greatly attenuates the signal and brings it closer to the noise level. And in receivers with a filter with dispersed selection in each stage, the signal is only slightly attenuated by the filter, and then amplified, which improves the signal-to-noise ratio. Currently, a concentrated selection filter is used only in relatively inexpensive receivers made on integrated circuits (for example, K174XA10), as well as in televisions.
In conventional receivers of long, medium and short waves the intermediate frequency, as a rule, is 465 or 455 kHz, in VHF - 6.5 or 10.7 MHz. TVs use an intermediate frequency of 38 MHz. Since the superheterodyne receiver is well tuned to the signal with an intermediate frequency, even a weak signal at this frequency is received. Therefore, the intermediate frequency is used to transmit SOS signals. Any radio stations in the world are prohibited from operating on these frequencies.
disadvantages
The most significant drawback is the presence of the so-called mirror receive channel - the second input frequency, which gives the same difference with the local oscillator frequency as the operating frequency. The signal transmitted at this frequency can pass through the IF filters along with the operating signal.
For example, if the input is tuned to a radio station transmitting at 70 MHz and the LO is 76.5 MHz, the IF filter will output a normal 6.5 MHz signal. However, if there is another powerful radio station at a frequency of 83 MHz, its signal can also leak to the input of the mixer, and the difference signal with a frequency of also 83 - 76.5 = 6.5 MHz will not be suppressed. In this case, the reception is accompanied by various interference. The image selectivity depends on the quality factor and the number of input circuits. With two tunable input circuits, a three-section variable capacitor (CVC) is required, which is expensive.
To reduce interference from the image channel, the method of double (or even triple) frequency conversion is often used. Such receivers, despite the rather high complexity of construction and adjustment, have become the de facto standard in professional and amateur radio communications.
In modern receivers, a digital frequency synthesizer with quartz stabilization is used as a local oscillator.
Regenerative radio receiver (regenerator)- a radio receiver with positive feedback in one of the RF amplification stages. Usually direct amplification, but superheterodyne with regeneration both in the amplifier and in the amplifier are known.
It differs from direct amplification receivers in higher sensitivity (limited by noise) and selectivity (limited by the stability of parameters), reduced stability of operation.
Regenerative radio receiver circuit
History
Invented by E. Armstrong while he was in college, patented in 1914, then also patented by Lee de Forest in 1916. This led to a 12-year lawsuit in the US Supreme Court in favor of Lee de Forest.
The regenerator allows you to get the most out of one reinforcing element. Therefore, in the early years of the development of radio engineering, when lamps, passive parts and power supplies were expensive, it was widely used in professional, amateur and household receivers, successfully competing with the superheterodyne invented in 1918 by the same Armstrong.
The absolute record of radio communication range before the space age was set on January 12, 1930 by the Soviet radio operator E.T. Krenkel with the Antarctic expedition of R.E. Bird is on a regenerative receiver.
With wide distribution in the late 1930s. mixing tube-heptode and quartz IF filters, the advantage of the superheterodyne in stability and selectivity became decisive, and by the end of the 1940s the regenerator was completely replaced from serious applications, remaining only in amateur radio assembly kits.
Advantages and disadvantages
Advantages:
Disadvantages:
Theoretical basis
In a regenerative receiver, the quality factor (Q) of the oscillatory circuit is increased by compensating for part of the losses due to the amplifier energy, i.e. introducing positive feedback.
Q = resonant resistance / loss resistance, i.e. Q = Z / R
Positive feedback, compensating for some of the losses, introduces some negative resistance: Qreg = Z / (R - Rneg)
Regeneration rate: M = Qreg / Q = R / (R - Rneg)
Hence, it can be seen that with an increase in the feedback, the regeneration coefficient M and the quality factor can tend to infinity, but their practical growth is limited by the stability of the circuit parameters - if the change in the gain is greater than 1 / M, then the regenerator will either break into generation (if the gain has increased), or will lose half of the sensitivity and selectivity (if the gain has dropped).
To improve stability and achieve smooth control near the generation threshold, the regenerator should have a negative feedback by signal level or AGC. In the above circuit, such an OOS is provided by the R1C2 circuit (grid leak, from the English grid leak) - the signal is detected by a diode consisting of a grid and a lamp cathode, and is isolated on a resistor R1. The variable component is amplified and sounds in the headphones, while the constant component locks the lamp and reduces its gain.
Without such an AGC, the feedback control will be very "sharp", and if the regenerator breaks into generation, then the oscillation range will be limited only by the power supply, and it can be stopped only by significantly reducing the feedback (the phenomenon of hysteresis). Such an amplifier is not suitable for use as a regenerator.
Direct amplification radio is one of the simplest types of radio receivers.
Direct gain receiver block diagram
A direct amplification radio receiver (heradeaus) consists of an oscillatory circuit, several high-frequency amplification stages, a square-law amplitude detector, and several low-frequency amplification stages.
The oscillating circuit serves to isolate the signal of the desired radio station. As a rule, the tuning frequency of the oscillatory circuit is changed by a variable capacitor. An antenna is connected to the oscillatory circuit, sometimes grounding.
The signal selected by the oscillating circuit is fed to a high-frequency amplifier. A high frequency amplifier (UHF), as a rule, consists of several stages of a selective transistor amplifier. From the UHF signal is fed to the diode detector, the signal is removed from the detector audio frequency, which is amplified by several more stages of a low-frequency amplifier (ULF), from where it is fed to a speaker or headphones.
In the literature, direct amplification receivers are classified according to the number of stages of low and high frequency amplifiers. A receiver with n high gain and m low gain stages is designated n-V-m, where V stands for detector. For example, a receiver with one UHF stage and one ULF stage is designated 1-V-1. The detector receiver, which can be considered as a special case of the forward amplification receiver, is denoted 0-V-0.
Advantages and disadvantages
The main disadvantage of a direct amplification receiver is low selectivity (selectivity), that is, a small attenuation of signals from neighboring radio stations in comparison with the signal of the station to which the receiver is tuned (this does not apply to a regenerative receiver, which is a type of direct amplification receiver). Therefore, this type of receiver is convenient to use only for receiving powerful radio stations operating in the long-wave or medium-wave range (due to the peculiarities of wave propagation in the ionosphere, long-wave and medium-wave signals cannot propagate too far, therefore the receiver "sees" only a limited number of local stations). Because of this drawback, direct amplification receivers are not manufactured by industry and are mainly used today only in amateur radio practice.
Typically, this type of radio can only receive amplitude modulated radio broadcasts. It is also usually necessary to connect an external antenna and ground, due to their low sensitivity, limited by gain.
Direct conversion radio- a kind of radio receiver, in which the received high-frequency signal is converted directly into the output low-frequency by mixing the local oscillator signal with the received signal. The local oscillator frequency is equal to (almost equal to) or a multiple of the signal frequency. Also called homodyne or heterodyne - not to be confused with superheterodyne.
History
The first direct conversion receivers appeared at the dawn of radio, when there were no radio tubes yet, communications were carried out at long and ultra-long waves, transmitters were spark and arc, and receivers, even communication ones, were detector ones.
It was noticed that the sensitivity of the detector receiver to weak signals increases significantly if the receiver was connected to its own low-power generator operating at a frequency close to the frequency of the received signal. When receiving a telegraph signal, beats were heard with an audio frequency equal to the difference between the local oscillator frequency and the signal frequency. The first heterodynes were machine electric generators, then they were replaced by vacuum tube generators.
By the 1940s, direct conversion receivers had been supplanted by superheterodyne and direct gain receivers. This was due to the fact that the main amplification and selection of the direct conversion receiver was carried out at a low frequency. It is difficult to build a tube amplifier with high sensitivity and low noise figure. The revival of direct conversion receivers began in the 60s with the use of a new element base - operational amplifiers, transistors. It became possible to use high-Q active filters on operational amplifiers. It turned out that with comparative simplicity, direct conversion receivers show characteristics comparable to superheterodyne. In addition, since the frequency of the local oscillator of direct conversion receivers can be two times lower than the signal frequency, it is convenient to use them for receiving EHF and microwave signals.
What could be your first constructively finished straight-line receiver? This is a question you have undoubtedly asked yourself more than once.
In the magazine "Radio", in radio technical brochures and books published, for example, by the publishing houses DOSAAF, "Radio and Communication", "Children's Literature", many amateur direct amplification receivers are described. Different in complexity, they are all similar in principle of operation, and in each of them you can easily consider those elements and nodes with which you have already experimented in previous workshops.
In this workshop, I offer a choice of two options for a direct amplification receiver. 2- V-3, one of them is reflex, both are with a push-pull power amplifier, but the bass amplifier of one of the receivers is transformer, and the other is transformerless.
Reflexive 2-V-3. On the shelves of shops selling radio goods, there are sets of parts and materials intended for self-assembly of small-sized direct amplification receivers. One of these sets called "Cricket" is offered to you as the first version of the receiver.
The Cricket Kit contains all the parts and materials, including even solder and rosin, required to assemble a 2-V-3 reflex receiver with an internal magnetic antenna. A properly mounted and tuned receiver provides loud reception of local and most powerful remote broadcasting stations operating in the wavelength range from about 250 to 1500 m. The output power of the receiver is about 100 mW. A Krona battery can be used to power it. battery 7D-0.1, two 3336L batteries connected in series, and at home - a power supply unit mounted at the tenth workshop.
The schematic diagram of this receiver is shown in Fig. 76. As you can see, the receiver is a latitarameter, transistors V1 and V2 work in a two-stage HF amplifier, and in a three-stage LF amplifier - that all transistor V 2 and transistors V4 — V6. Transistor stage V-2, thus, it is reflex, the role of the detector is performed by the diode V Z,
How does the receiver work? Input tunable magnetic antenna circuit W1 form a coil L1 with a flat ferrite core and a variable capacitor C1. Through capacitor C2 an external antenna can be connected to the loop ( X1), which increases the volume of the receiver. Modulated high-frequency signal of the station, to which the input circuit is tuned, through the communication coil L2 enters the base of the transistor VI. Transistor Amplified Signal Through Coil L4, inductively coupled to the collector coil £ D is fed to the base of the transistor V2 the second stage of the RF amplifier. From the choke L5, which is the high-frequency load of this transistor, the amplified signal enters through the capacitor C7 on diode V3, is detected by it and further, being already a low-frequency signal, through a resistor R6 and coil L4 high frequency transformer LSL4 hits the base of the transistor V2, working now as a preamplifier of the LF voltage.
For low frequency signal transistor V2 connected according to the scheme with a common collector and its low-frequency load is a resistor R7. The low frequency voltage created on this resistor through an electrolytic capacitor C9 and variable resistor R10, acting as a volume control, enters the base of the transistor V4 the second stage of the bass amplifier. Interstage transformer T1, included in the collector circuit of this transistor, provides the transistors V5 and V6 the output stage is a push-pull mode of operation.
Let's take a closer look at the transistor circuits. V1 andV2. Here are the resistors R5 and R3 form a voltage divider from which it is removed and through the coil L4 fed to the base of the transistor V2 (with respect to its emitter) a small (about 0.1 V) negative bias voltage. From the same divider through a resistor R6 negative voltage is applied to the diode V3, opening it somewhat and thereby increasing its efficiency as a detector. Simultaneous resistor R6, diode V3 and resistor R7, transistor load V2, form another divider, from which to the base of the transistor VI through a resistor R4 and a coil of communication L2 a bias voltage is applied equal to the voltage drop across the resistor R7. In this case, between the emitter of the transistor V2 and the base of the transistor VI negative DC feedback is created, stabilizing the operation of these receiver transistors. During the reception of signals from powerful stations on the resistor R7, the low-frequency voltage automatically increases, which through a high-frequency filter formed by the resistor R4 and capacitor C4, affects the base of the transistor VI and by changing the mode of its operation, it weakens the gain. With relatively weak radio signals, this automatic gain control circuit has virtually no effect on the operation of the receiver.
Briefly about the functions of some other elements of the receiver. Resistor R9 and variable resistor R10 form a divider, due to which, on the basis of the transistor V4 a fixed bias voltage is created. Capacitor C10 creates negative AC feedback between the collector and the base of this transistor, which improves the quality of the stage. Resistors RI1 and R12 in the emitter circuit of the same transistor, the operation of the cascade is thermally stabilized. At the same time, they also play the role of a divider from which to the base of transistors V5 and V6 through the corresponding halves of the secondary winding of the transformer T1 an initial bias voltage is applied. So that between the emitter and the base of the transistor V4 there was no negative AC feedback, reducing the gain of the stage, resistors R11 and R12 shunted by an electrolytic capacitor SP. Resistors R13 and R14, the total resistance of which is 13.5 ohms (there is no such value among small-sized resistors), they are created between the emitters and the bases of the transistors V5 and V6 negative feedback on direct ... and alternating current, which stabilizes and improves the quality of the output stage.
The appearance of the finished receiver is shown in Fig. 77. Its body is a box made of colored polystyrene, into which a second box of slightly smaller dimensions is inserted - the back cover. The position of the cover inside the case depends on. which battery is used to power the receiver, and is fixed in it with a steel bracket-handle. The dynamic head is mounted directly on the front wall of the case. All other parts of the receiver are mounted on a printed circuit board made of foil-clad getinax.
The external view of the board and the diagram of the installation of parts on it are shown in Fig. 78. The battery is connected using the power connector supplied with the receiver parts kit.
Coil L1 The magnetic antenna contour is wound (at the factory) directly on a 400NN ferrite rod with a diameter of 8 and a length of 125 mm. In total, it contains 150 turns of wire PEV-2 0.18, laid in eight sections: seven sections of 20 turns and one section of 10 turns. Communication coil L2,- the number of turns in which (up to 8 turns) is selected when setting up the receiver, wound over the coil L1 the same wire.
High frequency transformer L3 L4 and choke L5 wound (at the factory) with wire PEV-2 0.18 on ferrite rings of 2000NN brand with dimensions of 10X6X5 mm. Coil L3 contains 100 turns, coil L4 - 20 turns, choke L5 - 195 turns.
Low frequency transformers T1 and T2 wound on magnetic cores Ш4Х6. Primary (I) winding of the interstage transformer T1 contains 2500 turns of wire PEL 0.06, secondary (II) - 350 + 350 turns of the same wire. Primary (I) winding of the output transformer T2 has 450 + 450 turns of PEL 0.09 wire, secondary (II) - 102 turns of PEL 0.23 wire.
Other receiver parts: variable capacitor C1 type KPM-1; capacitors C2 and C10- CT (C4 - C6 - MBM, C7 - CD, C13 - CLS; electrolytic capacitors SZ, S8, S9 and C12 - K50-3 or EM; fixed resistors of MLT-0.125, VS-0.125 or ULM types; variable resistor R10, combined with power switch (S1), type SP-3; small dynamic head power IN 1 0.1 W; coefficient h21E transistors not less than 40.
The current-carrying conductors of the printed circuit board, which are thin, and in some places also narrow strips of copper foil, can peel off from the getinax if they are overheated. Therefore, before soldering this or that part to such conductors, make sure that it is in good working order and that its value corresponds to that indicated in the schematic diagram. Pay special attention to the correctness of switching on the transistors and the polarity of the diode, electrolytic capacitors. Excessive soldering can be hazardous to printed conductors.
For the output stage, try to select transistors with as close coefficients as possible h21E and reverse currents of collectors Iko. In the first stage of the RF amplifier, use the one of the high-frequency transistors that has the highest coefficient h21E.
When mounting low-frequency transformers on the board, provide for the possibility of measuring the collector current of the transistor V5 and the total current of the collectors of transistors V6 and V7. For this, the pins of the upper (according to the scheme) output of the primary winding of the TU transformer and the middle (also according to the scheme) output of the primary winding of the transformer T2 wrap. narrow strips of capacitor paper to temporarily isolate them from the board. To measure collector currents, you will connect a milliammeter between these pins and the printed conductors of the negative pole of the batteries going to them.
The receiver, assembled from known serviceable parts and exactly according to the principle diagram, starts working immediately after turning on the power. But for transistors it is necessary to select the most favorable operating modes.
Approximate quiescent currents of collector circuits and voltages at transistor electrodes are given in the table.
|
Transistors |
Collector current, Ik, mA |
Collector voltage, Uк, v |
Base voltage, Ub, V |
Emitter voltage, Ue. in |
For transistors V5 and V6 the total current of their collectors is indicated. The voltages at the electrodes of the transistors were measured with a high-resistance voltmeter relative to the positive conductor at a power supply voltage of 9 V.
Transistor operating mode V5 and V6 determined by the voltage drop across the resistor R12, resistance, which depends on the mode of the transistor V4. In this regard, first select a resistor R9, to set the recommended collector current of the transistor V4, and then by selecting a resistor R12 - total collector current of transistors V5 and V6. With an increase in the resistance of the resistor R12 negative voltages at the bases and the output stage increase.
When the modes of transistors V4... V6 installed, the output pins of the transformer windings are soldered to the printed conductors of the board.
Collector currents of transistors V1 and V2 set by selection of resistor R5 voltage divider R5 R3. To increase the currents, the resistance of this resistor must be reduced, and in order to reduce the currents, the resistance of the resistor must be increased. If you need to adjust the collector current of the transistor only V1, this can be done by selecting the resistor R1. Thus, the resistors R5 and R1 It is necessary to solder completely only when they are selected.
Resistor R2 is not a necessary element of the high-frequency stage, therefore, when the receiver is first tested, it may not be there. In case of self-excitation of the cascade, try to swap the leads of the coils L3 or L2. If that doesn't help, then plug in a resistor. R2 parallel to the emitter-collector section of the transistor or parallel to the coil L3.
Is it possible to assemble such or a similar receiver only from a set of ready-made parts? Of course not. The coils of the magnetic antenna and the high-frequency transformer can be wound on your own, low-frequency transformers can be purchased (suitable from any transistor receivers with a push-pull transformer output) or you can also wind up yourself, the receiver body can be glued from colored plexiglass, and the circuit board does not have to be printed - the installation is “hinged” ...
Transformerless 2-V-3. You can see the schematic diagram of the second version of the direct amplification receiver in Fig. 79. This receiver, like the receiver of the first option, also 2- V-3 and also with a push-pull power amplifier. But he is not reflexive and transformerless.
Look closely at the diagram. Almost everything in it is already familiar to you. Two-stage RF amplifier on transistors VI and V2 familiar from the ninth workshop, three-stage bass amplifier on transistors V5 — V8 - on the eleventh, diode detector UZ and V4 - on the seventh, and the method of thermal stabilization of operating modes of transistors - on the twelfth workshop.
You are not familiar with the way to turn on the resistor Rl5 t This resistor together with the resistor R16 . forms a divider from which to the base of the transistor V6 bias voltage is applied. But its right (according to the diagram) output is connected not to the negative conductor of the power source, as it was in a similar amplifier of the eleventh workshop, but to the emitters of transistors V7 and V8 output stage, that is, with the point to which the dynamic head is connected IN 1(via electrolytic capacitor C13). What does it do? With this inclusion of the resistor R15 between the output of the amplifier and the base of the transistor V6 negative AC feedback is created, which stabilizes and improves the quality of the amplifier.
Try to pre-assemble and adjust the receiver on a breadboard, and only then mount the parts clean on a permanent board made of durable insulating material. As for the very designs of the finished receiver, you, apparently, can successfully solve this issue on your own. Much of it can be borrowed from industrial receiver designs.
All transistors, capacitors, resistors and a magnetic antenna can be mounted on one common board with dimensions of about 175X70 mm (Fig. 80), and the variable resistor R9, combined with power switch (S1), and mount the dynamic head on the front panel of a suitable ready-made or homemade case. Make the tuning scale of the receiver in the form of marks or numbers on a disk mounted on the axis of the variable capacitor of the magnetic antenna circuit.
The circuit board was drunk from a sheet of getinax. Or a PCB 1.5 ... 2 mm thick. As reference points of the parts, use pieces of bare, pre-straightened and tinned copper wire 1 ... 1.5 mm thick and 8 ... 10 mm long, driven into the holes in the board or hollow rivets (rivets) pressed into it. Place the parts on one side of the board, and make the connections between them with wiring conductors. on the other side of the board (shown by dashed lines in Fig. 80). The dynamic receiver head can have a power of 0.5 ... 1 W, for example 1GD-18. With such a head, the sound quality will be much higher than with a small-sized one.
For the magnetic antenna (Fig. 80 above), use a 400NN or 600NN ferrite rod with a diameter of 8 and a length of 140 mm. Coils L1 and L2 wrap with wire PEV-1 or PEL 0.12 ... 0.15 on separate paper cylindrical sleeves-frames, which could be moved along a ferrite rod with little friction. For receiving medium-wave radio stations, coil L1 should contain 65 ... 75 turns, L2 - 5 ... 6 turns, laid on frames in one layer, turn to turn, and for receiving long-wave radio stations - 180 ... 200 and 10 ... 12 turns, respectively. It is advisable to wind a loop coil of a long-wave range in four to five sections of 35 ... 40 turns in each section (like a coil L1 radio receiver "Sverchok"). The sectioned winding reduces the turn-to-turn capacitance of the coil, which, with the same tuning capacitor, somewhat expands the range of waves covered by the magnetic antenna circuit.
In the RF amplifier, instead of P422 transistors, you can use any other high-frequency transistors (P401 ... P403, P416, GT308) with a static current transfer coefficient of at least 60 ... 80; in the LF amplifier instead of transistors MP39 - similar low-frequency transistors MP40 ... MP42, instead of MP35 - transistors MP36 ... MP38 with h21e at least 50. For the output stage, select transistors with close coefficients h21E and reverse currents Iko.
As always, before turning on the power, carefully check the installation with the schematic diagram of the receiver - whether the transistors, diodes, electrolytic capacitors are turned on correctly, whether the dynamic head is connected securely. After turning on the power, immediately measure and, if necessary, set the recommended operating modes of the transistors. The total quiescent current consumed by the receiver should not exceed 10 ... 12 mA.
Symmetry voltage across the emitters of transistors V7 and V8, which should be equal to 4.5 V (at a power supply voltage of 9 V), set by selecting a resistor R15, and their collector current is within 2 ... ... 4 mA - by selecting a resistor R18. Do not forget: when replacing these resistors, the amplifier must be de-energized, otherwise the output transistors may have a thermal breakdown due to large collector currents.
Collector currents of transistors VI, V2 and V5, which can be in the range of 1 ... 1.2 mA, set by selecting the resistors related to them Rl, R5 and RW voltage dividers in their base circuits. The normal operating mode of these transistors can also be considered if about half of the voltage of the power supply is on their collectors relative to the positive conductor, and about 0.1 V on the base relative to the emitters.
You can check the quality of the LF path by feeding a signal from a radio broadcasting network to its input - just as you did when testing a similar amplifier at the eleventh workshop.
Set the wavelength range covered by the contour of the magnetic antenna on the scale of the control (industrial) transistor or tube receiver, tuning both receivers to the same radio stations and comparing the readings of their scales. The radio stations of the longest wavelength part of the range should be listened to when highest capacity capacitor C1. To move this section towards longer waves, the coil L1 it is necessary to move closer to the middle of the ferrite rod or increase the number of turns, and in order to shift towards shorter waves, move it closer to the end of the rod or reduce the number of turns.
Here, perhaps, the main thing that, together with the information already familiar to you, must be said about the installation and adjustment of this version of the direct amplification receiver.
Literature: Borisov V.G. Practical work of a beginner radio amateur. 2nd ed., Revised. and add. - M .: DOSAAF, 1984.144 p., Ill. 55k.
Below are single-stage high-frequency amplifiers (UHF) with detectors, which, together with any ultrasonic amplifier circuit, form a direct amplification radio receiver. Single-stage UHF have active detector circuits, and two-stage UHF passive detectors based on a diode full-wave circuit. Receivers can operate in the long or medium wave range, but you can introduce a switching scheme and get a dual-band radio receiver.
The radio receiver according to the diagram in Fig. 5.3 contains one stage of high frequency amplification on two transistors VT1 and VT2. Transistor VT2 is switched on according to the scheme with a common collector, VT1 - with common base... One of the main advantages of such a cascade is that the output circuit of the circuit is weakly connected to the input circuit and it is possible to obtain a higher gain in comparison with a circuit based on a single transistor. The base of the transistor VT2 is grounded at high frequency using the capacitor C3. The load of the stage is a high-frequency choke L3. From the collector of the transistor VT1, the modulated high-frequency signal through the coupling capacitor C4 is fed to the detector, made according to the scheme with a common collector on the transistor VT3. Although the detector has a voltage gain of less than unity, its gain is still higher than that of a diode, and the distortion of the low-frequency signal is lower. The chain C6, R5, C7 filters the low-frequency signal, from the resistor R6 through the blocking capacitor CU it is supplied
Fig. 5.3. Single-stage UHF OK-OB with a detector on a transistor according to the scheme with OK
Fig. 5.4. UHF circuit board (a) and techniques for mounting parts on it (b, c)
resistor R7, which serves as a volume control, and then from the variable resistor slider to the UZCH input. The power supply of the circuit is well filtered by the R8, C8, C9 circuit.
The layout of the parts on the circuit board is shown in Fig. 5.4. The supporting mounting points of resistors, capacitors, connecting conductors and other parts can be hollow rivets (rivets) or studs - pieces of tinned copper wire with a diameter of 0.9 ... 1.3 mm, pressed into the holes of the board (Figure 5.4, b and Figure 5.4 , c, respectively. Fig. 5.4, b shows devices for flaring the caps and an example of installing a part in them. Dowels sharpened on emery used for construction work are well suited as devices. One of them is clamped in a vice, and the other with the help of light blows hammer flare the piston. Pistons can be pre-cut pieces of copper tubes, the length of which is 0.6 ... 1.5 mm longer than the thickness of the board. You can make similar pistons from a copper plate or tin-plated sheet with a thickness of 0.5 ... 0.8 mm. Hole diameter in the board, it is desirable to choose in the range of 2 ... 3 mm.
To press studs into the holes of the boards, a device is also used - a steel bar with a guide hole in the end (Figure 5.4, c). Using this device, the pin is directed into the hole in the board, the diameter of which is about 0.1 mm less than the diameter of the pin, and pressed in with a hammer blow. In fig. 5.4, the dimensions of the device for pressing the studs with a diameter of 1 mm and a length of 10 mm into a board with a thickness of 1.5 ... 2 mm are given.
The circuit of the radio receiving device (Fig.5.5) consists of a single-stage high-frequency amplifier on transistors VT1, VT2, forming the so-called cascode circuit. The first transistor of the amplifier VT2 is connected according to the scheme with a common emitter, and the second VT1 - with a common base. As a result, the input and output of the stage are well decoupled from each other and it is possible to obtain sufficient voltage gain even when using a single high-frequency amplification stage. The load of the transistor VT1 is the transformer L3, L4. A high-frequency transformer is used to obtain two antiphase high-frequency voltages required for the operation of an active full-wave detector on transistors VT3, VT4. The harmonic coefficient of the detector is much less than that of the diode one, and the transmission coefficient is higher. After filtering by the C7, R9, C8 circuit, the audio frequency voltage is fed through the SI blocking capacitor to the R11 volume control. The circuit is powered through the filter R10, C9, CU.
The connections of the parts of this UHF are shown in Fig. 5.6. Capacities of capacitors SZ-C6 can be in the range from 6800 pF to 0.068 μF. KT315 transistors can be with any letter indices. They can be replaced with similar transistors of the KT312, KT316, KT342, KT358 series with a coefficient
Fig. 5.5. Single-stage UHF OE-OB with a full-wave transistor detector
transmission ratio of at least 50. It is desirable that the transmission ratios of transistors VT1, VT2 differ by no more than 20%, and VT3 and VT4 were as close as possible.
The coils of the high-frequency transformer L3 and L4 are wound with a PEV-1 wire 0.08 ... 0.1 mm on a ferrite ring of standard size K7 X 4 X 2 (outer diameter 7 mm, inner diameter 4 mm, and height 2 mm). Coil L3 contains 250 turns, coil L4 is wound in two wires and contains 100 turns. Then the beginning of one winding is connected to the end of the other, thus obtaining the middle lead of the coil L4. For the convenience of winding the wire on the ferrite ring, make a special device - a shuttle. Wrap the wire on the shuttle so long that there is enough for the entire spool with a small margin. Try to lay the coils tightly to each other and make sure that the wire does not twist into loops during winding.
The high-frequency transformer is last mounted on the printed circuit board, attached with a small amount of glue, for example, Moment glue.
After checking the installation, connect the magnetic antenna, audio amplifier and turn on the power of the radio receiver. Check the modes of operation of the cascades for direct current and, if necessary, select resistors R1, R5. If the receiver is functional, you will be able to tune into one of the powerful radio stations. If the receiver is self-excited (accompanied by whistles and strong transmission distortions), try removing the magnetic antenna from the coils L3, L4 of the high-frequency transformer, or swap the leads of the coil L3.
Laying the ranges should be done using a factory radio receiver that has the required range (LW or MW).
A feature of the radio receiver (Fig. 5.7) is the use of an amplifier stage on a field-effect transistor VT1. The high input impedance of the field effect transistor allows you to completely include the oscillatory circuit in the input circuit and thereby increase the signal at the input of the high frequency amplifier. The amplified signal from the load of the amplifier VT1 - the resistor R1 is fed to the input of the precision detector on the operational amplifier and diodes VD1, VD2. Diodes VD1, VD2 are included in the feedback circuit of the operational amplifier. This circuit allows you to vary the detector transmission coefficient within a wide range using a variable resistor R4. In the lower (according to the schematic diagram) position of the engine
Fig. 5.7. Single-stage UHF on a field-effect transistor with a detector on an operational amplifier
resistor, the transfer coefficient is maximum, and in the upper one - minimum. Resistor R4 is the volume control. After filtering by the KB, C7 chain, the low-frequency signal is fed to the input of the audio amplifier. The power supply of the high-frequency stage and the detector is supplied through the decoupling filter K7, C4, C5.
The connection diagram of the parts on the circuit board is shown in Fig. 5.8. The field-effect transistor VT1 is mounted with the leads up, and the required leads of the op-amp DA1 are lengthened with a bare mounting wire.
Establishment begins with the installation of DC UHF modes. They will be installed automatically if there is a floor on the drain; the second transistor VT1 will have a voltage of +4.3 V. Set the recommended operating mode of the transistor by selecting the resistor K2.
When connecting an audio frequency amplifier, please note that there is a constant voltage at the UHF output. Connect it through an adapter capacitor with a capacity of 2.2 ... 4.7 μF. If the capacitor is oxide, its positive terminal is connected to the UHF output.
Fig. 5.8. Circuit board
Two-stage high-frequency amplifiers (circuits shown in Fig. 5.9, 5.11, 5.13) consist of a magnetic antenna W1, amplifier stages and a diode detector VD1, VD2, connected in a voltage doubling circuit. The voltage of the low-frequency signal from the output of the detector is filtered by an additional RC-circuit and isolated at the load - a variable resistor that is a volume control. Any audio amplifier described earlier can be used with these circuits.
Fig. 5.9. Two-stage UHF from identical stages according to the scheme with OE
The diagrams shown in Fig. 5.9, 5.13, have a sensitivity of 10 ... 20 mV / m and allow you to receive powerful radio stations in the long ranges 750 ... 2000 m (400 ... 150 kHz) or (and) medium waves 187 ... 570 m (1600 ... 525 kHz), remote at a distance 100 ... 250 km. In the diagram in Fig. 5.11 due to resonant circuits in all stages, the sensitivity is raised to 5 ... 7 mV / m. As a result, the range of the receiver is 300 ... 500 km.
It should be noted that the sensitivity of the circuits shown in Fig. 5.9, 5.13, can also be improved up to 7 ... 8 mV / m due to the inclusion of a resonant circuit in the second stage of the amplifier. Such a circuit can be a high-frequency broadband choke L5, used in the circuit shown in Fig. 5.11.
The range of all receivers can be increased by connecting an external antenna.
Coil L1 and variable capacitor C2 form an oscillatory circuit tuned to the signals of broadcasting stations. So that the relatively low-impedance input of the amplifiers (the input impedance is a few kilo-ohms) does not shunt the oscillatory circuit (the loop resistance when tuning to the signal of the received station is hundreds of kilo-ohms), high-frequency voltage is supplied from the L2 communication coil located on the magnetic antenna rod and forming with the L1 coil a step-down transformer. As a result, you can set the most advantageous connection circuit with an amplifier, selecting the number of turns of the coupling coil and the distance between it and the loop coil L1 of the magnetic antenna.
The UHF circuit shown in Fig. The 5.9 RF amplifier consists of two identical common-emitter amplification stages. It uses a highly efficient method of temperature stabilization of the operating mode of the transistor. In addition, the stage is insensitive to changing transistors with specifications within the limits set by the technical specifications.
Capacitors C5, C7 in stages eliminate negative AC feedback between the emitter and the base of the transistor. Their capacitance should be such that the AC resistance at the lowest frequency of the operating range is much less than the resistance of the resistor R4 (R8). In practice, the value of the capacitance can be in the range of 4700 ... 68000 pF.
The operating modes of each of the stages for direct current are independent of each other and can be changed by selecting the resistors R1, R5. The collector current of each of the stages is chosen equal to 1 mA. However, it is more convenient to control the modes of transistors by measuring not the current, but the voltage at their electrodes. The diagrams show the voltages measured relative to the common ("grounded") conductor of the receiver with a voltmeter with a relative resistance of more than 10 kOhm / V.
The connection between the stages, as well as between the coupling coil and the magnetic antenna, is capacitive through the coupling capacitor C4.
Fig. 5.10. Placement of elements and printed circuit board of a two-stage UHF from identical stages
Fig. 5.11. Two-stage UHF with transformer coupling
place the receiver's self-excitation as far as possible from the magnetic antenna WA1 and the variable capacitor C2. With the small dimensions of the printed circuit board, the part of the board on which the detector is located may have to be covered with a brass or aluminum screen connected to the common wire.
In the diagram in Fig. 5.11 applied amplifier stages, similar to the previous UHF. However, the connection between the first and second stage is transformer. The high-frequency transformer (transformer coils L3 and L4) allows much better than in a circuit with resistors in the collector circuit to match the relatively large output impedance of the first stage with the low input impedance of the second stage of the high-frequency oscillation amplifier. The collector load of the transistor VT2 is a high-frequency choke L5. The voltage of the modulated signal of the broadcasting station created on it is fed through the coupling capacitor Cb to the input of the detector stage. As mentioned above, the detector stage is assembled according to the voltage doubling circuit. Compared with a single-diode detector, such a detector can significantly increase the signal level at the receiver output, and hence the volume of radio reception.
The operation mode of the cascades for direct current is set in each cascade independently using dividers R1, R2 and R4, R5 in their base circuits and resistors R3, R5 in the emitter circuits. The operating mode of the first stage is set (at
Fig. 5.12. Circuit board
Fig. 5.13. Two-stage UHF OK-OE
necessary) by changing the resistance of resistor R1, the second - resistor R4.
The use of resonant circuits in the collectors of the amplifier stages makes it possible to obtain good sensitivity and selectivity of the direct amplification receiver, however, they require a lot of effort in setting up.
Since with this UHF it is possible to carry out a number of experiments that require soldering of parts, they are placed on the circuit board shown in Fig. 5.12.
The transformer coils L3 and L4 and the high-frequency choke L5 are wound with a PEV wire 0.08 ... 0.1 on ferrite rings of the 600NN or 1000NN brand with an outer diameter of 7 and a height of 2 mm (standard size K7 x 4 x 2). Coil L3 contains 250, coil L4 - 100, choke L5 - 250 turns. Before winding, round off the sharp edges of the rings with an emery cloth so as not to damage the insulation of the wire.
In the diagram in Fig. 5.13 high-frequency amplifier is aperiodic two-stage. In the first circuit, the transistor VT1 is connected according to the circuit with a common collector, and VT2 - with a common emitter. A possible version of the printed circuit board with the placement of elements is shown in Fig. 5.14.