The receiver of a novice shortwave observer operates in bands 28; 21; 14.0; 7.0; 3.5 MHz and is intended for receiving radio stations operating by telephone and telegraph.
The main components of the receiver are: a converter on a lamp L1 (6A10S), a grid detector L2 (6K3) with feedback and a two-stage low-frequency amplifier L3 (6N7S).
Fig.1. Schematic diagram of the receiver
To facilitate the manufacture of a receiver by beginning shortwave operators, the input circuits are not rebuilt during the process of receiving the radio station. There is no noticeable decrease in sensitivity at the edges of the range. The converter uses a single IF circuit, to which positive feedback is applied to increase the sensitivity and selectivity of the receiver. In order to eliminate interference on the mirror channel, the IF was selected at a high frequency of 1600 kHz.
The required mode of operation of lamp L1 along the shielding grid, in which stable operation of the local oscillator is obtained, is selected by resistance R2. R3 and C8 perform gridlick functions.
The amount of feedback is regulated by potentiometer R9, connected to the screening grid circuit of the detector cascade lamp. When receiving distant stations operating by telephone, the amount of feedback should be set close to critical; when receiving telegraph stations - above critical.
Details and design
The inductors are wound on cardboard frames with a diameter of 10 mm and a length of 40 mm.
Fig.2. Drawing of inductors L1-L5
Fig.3. Drawing of inductors L6-L10
Coil L12 must be able to move relative to coil L11. The distance between them is selected experimentally. Coils L11 and L12 are enclosed in a copper or aluminum screen. At the top of the screen there is a nut (not shown in the figure) in which the ferrite core screw rotates. Using this core you can configure the L11, L12 circuit.
Fig.4. Drawing of inductors L11-L12
Transformer Tr1 is wound on a Sh15 core, the thickness of the set is 20 mm. Winding 1 contains 3000 turns of PEL 0.12 wire; winding 2 - 70 turns of PEL 0.4 wire. You can use a ready-made one - from an industrial receiver "Voronezh". The power transformer is also ready with suitable supply voltages. The rectifier must provide a current of at least 25 mA at a voltage of 230...250 V.
Setting up the receiver
Setting up the receiver is easy. The low-frequency part and the grid detector usually start working immediately. If generation does not occur when the voltage on the shielding grid of lamp L2 increases, the distance between the coils L11 and L12 should be reduced. If there is no generation in this case, it is necessary to switch the ends of the feedback winding L12 or turn it over. If generation occurs when the potentiometer R9 is in the middle position, the adjustment of the detector cascade can be considered complete.
When setting up the conversion stage, you first need to check whether the local oscillator is working. If the local oscillator is working, then when petal 8 of lamp L2 is short-circuited to the cathode, the voltage drop across R1 increases. In the absence of generation, the voltage on the screening grid L1 should be more carefully selected by changing the value of R2.
Changing the boundaries of the ranges is carried out by changing the capacitance C12-C16 and more carefully selecting the number of turns of coils L6-L10.
By turning on the 40 m range and attaching an antenna to the receiver, they try to receive some radio station. Then, by rotating the core screw L11 and adjusting the capacitor C5, the maximum reception volume is achieved.
V. Polyakov (RA3AAE)
Continuing the series of articles on the basics of amateur radio communications, which began in the August issue of the magazine last year with a description of a simple transmitter with quartz stabilization for the amateur band of 160 meters, we propose the design of a simple heterodyne radio receiver for the same range. The receiver may be of interest to both novice shortwave observers and more experienced radio athletes. Thanks to its cost-effectiveness and small dimensions, the receiver is especially suitable for use in the field.
Conventional mass broadcast receivers are unsuitable for receiving signals from amateur radio stations without such significant modernization that it is easier to build the receiver anew. The point is not even their low sensitivity and excessively wide bandwidth, but the fact that they are designed to receive amplitude-modulated (AM) signals. Amateurs have long abandoned AM due to its low efficiency and use exclusively telegraph (CW) or single-sideband (SSB) speech signal on short waves (KB). For this reason, the receiver must be designed on completely different principles. In particular, it does not require an amplitude detector, and it is advisable to do the main amplification at low audio frequencies, where it is much easier and cheaper.
The CW signal consists of short and long bursts of an unmodulated carrier frequency lying in one of the amateur radio bands, in our case 1.8...2 MHz (160 meters). In order for the signal to sound like the usual Morse code melody, its high frequency must be converted down to the 3H range. This is done by a frequency converter installed at the receiver input (Fig. 1), immediately after the input filter Z1, containing a mixer U1 and a low-power auxiliary oscillator - local oscillator G1.
Let's say we want to receive a CW signal at 1900 kHz. By tuning the local oscillator to a frequency of 1901 kHz, we obtain sum (3801 kHz) and difference (1 kHz) frequency signals at the mixer output. We don’t need the total frequency, but we will filter the difference audio frequency signal (Z2), amplify it in ultrasonic sounder A1 and send it to BF1 phones. As you can see, the receiver is really very simple.
An SSB signal is the same audio signal, but with a spectrum shifted to radio frequencies. On low-frequency amateur bands (160, 80 and 40 meters), the spectrum of the SSB signal is also inverted (the lower sideband, LSB, is emitted). This means that with a SSB signal carrier frequency of 1900 kHz, its spectrum extends from 1897 to 1899.7 kHz, i.e. 1900 kHz - (0.3....3 kHz). The suppressed upper side (USB) occupies the frequency band 1900.3...1903 kHz, as can be seen in the spectrogram (Fig. 2). The emitted LSB is highlighted by thick lines. To receive this signal, it is enough to tune the local oscillator exactly to the frequency of 1900 kHz.
The heterodyne receiver was invented at the dawn of radio engineering, approximately in 1903, when there were no lamps or other amplifying devices, but there were already antennas, telephones and continuous oscillation generators (arc, electric machine). For the next decade, exclusively heterodyne receivers were used for auditory reception of telegraph signals. Then the tube regenerator, or audion (1913), the superheterodyne (1917), which, by the way, got its name from the heterodyne receiver, were invented; AM began to be widely used, and heterodyne receivers were firmly and for a long time forgotten.
Radio amateurs revived this technique in the 60-70s of the last century, proving in practice that a receiver with three or four transistors can receive radio stations from all continents, working no worse than large multi-tube devices. But the name became different - Direct Conversion Receiver (DCR), which emphasized the fact of direct conversion (conversion, not detection) of the radio signal frequency into a low audio frequency.
Referring again to Fig. 1, let us explain the purpose of filters. The Z1 input bandpass filter attenuates strong out-of-band signals from service and broadcast stations that may cause interference. Its bandwidth can be equal to the width of the amateur band, and if it is narrower, the filter is made tunable. It also weakens side reception channels that are possible at local oscillator harmonics. The Z2 filter is a low-pass filter that passes only the “telephone” band of audio frequencies below about 3 kHz. The lowest frequencies, below 300 Hz, are sufficiently attenuated by separating capacitors in the ultrasonic sounder.
Filter Z2 determines the selectivity of the receiver: signals from radio stations located further than 3 kHz from the local oscillator frequency will create frequencies above 3 kHz at the output of the mixer, and therefore will be effectively filtered in the low-pass filter. Added to the selectivity of the receiver is the selectivity of telephones, which poorly reproduce frequencies above 2.5...3 kHz, and the natural selectivity of human hearing, which perfectly distinguishes the tone of signals and highlights the useful signal against the background of interference - after all, if the frequencies differ in the radio range, after conversion they will vary in the audio range. There is no trace of this in AM receivers with a detector - it doesn’t care what signals to detect (it does not respond to frequency), as a result, all signals passing through the radio path create interference.
The disadvantages of a heterodyne receiver include dual-sideband reception: in our example of CW reception, an interfering signal with a frequency of 1902 kHz will also give a difference frequency of 1 kHz and will be received. Sometimes such interference can be eliminated. The fact is that for a signal with a frequency of 1900 kHz, two settings are possible - upper (the local oscillator frequency is 1901 kHz) and lower (1899 kHz). If interference is audible with one setting, it may not be with another.
On an SSB signal, only one setting is possible - 1900 kHz, but all signals with frequencies of 1900 ... 1903 kHz will create interference (see Fig. 2) and cannot be eliminated. This drawback is significant only during “pile-up” reception, when many stations “huddled together” at close frequencies, hearing, for example, the rare “DX”. During normal reception, when there are few stations and there are significant gaps between their frequencies, this drawback is completely unnoticeable.
The schematic diagram of the receiver is shown in Fig. 3. The input signal from the antenna is fed through a small capacitance coupling capacitor C1 to a double-circuit bandpass filter. The first circuit of the L1C2C3C4.1 filter has a relatively high quality factor and, therefore, a narrow bandwidth, so it is frequency tuned using one section of the dual C4.1 KPI. There is no need to rebuild the second L2C7 circuit, since it is heavily loaded by the mixer, its quality factor is lower, and its bandwidth is wider, so it does not tune and passes the entire frequency band of 1.8...2 MHz.
The receiver mixer is assembled on two diodes VD1 and VD2, connected back-to-back. Through capacitor C8 (it is also included in the low-pass filter), the local oscillator voltage from the tap of coil L3 is supplied to the mixer. The local oscillator is tuned in the frequency band 0.9...1 MHz by another section of the KPI - S4.2. As you can see, the local oscillator frequency is half the signal frequency, which is necessary by the very principle of operation of the mixer. It works as follows. To open silicon diodes, a voltage of about 0.5 V is required, and the amplitude of the heterodyne voltage supplied to the diodes barely reaches 0.55...0.6 V. As a result, the diodes alternately open only at the peaks of the positive and negative half-waves of the heterodyne voltage, i.e. i.e. twice per period.
This is how the signal circuit is switched with double the local oscillator frequency. The mixer is especially convenient for heterodyne receivers, since the local oscillator signal is practically not emitted by the antenna, being greatly attenuated by the input filter, and does not create interference either to others (the first heterodyne receivers sinned with this, in which the local oscillator operated at the signal frequency and it was not easy to suppress its radiation) or to its own reception.
The local oscillator is made according to the “inductive three-point” circuit on transistor VT1. Its circuit L3C6C5C4.2 is connected to the collector circuit of the transistor, and the feedback signal is supplied through capacitor C9 to the emitter circuit. The required base bias current is set by resistor R1, shunted for high frequency currents by capacitor C10.
The converter is designed in such a way that it does not require painstaking work to select the optimal local oscillator voltage on the mixer diodes. This is facilitated by the easy operating mode of the local oscillator at a low collector-emitter voltage of the transistor (about 1.5 V) and a low collector current - less than 0.1 mA (note the high resistance of resistor R2). Under these conditions, the local oscillator is excited easily, but as soon as the oscillation amplitude increases to approximately 0.55 V at the coil tap, the mixer diodes open at the peaks of the oscillations and bypass the local oscillator circuit, limiting further growth in amplitude.
The low-pass filter of the C8L4C11 receiver is the simplest U-shaped filter of the third order, providing a slope of 18 dB per octave (double the frequency) above the cutoff frequency of 3 kHz.
The receiver's ultrasonic frequency is two-stage, it is assembled on low-noise transistors VT2 and VT3 of the KT3102 series with a high current transfer coefficient. To simplify the amplifier, direct communication between the stages is used. The resistances of the resistors are chosen so that the DC mode of the transistors is set automatically and depends little on fluctuations in temperature and supply voltage. The current of transistor VT3, passing through resistor R5, connected to the emitter circuit, causes a voltage drop across it of about 0.5 V, sufficient to open transistor VT2, the base of which is connected through resistor R4 to the emitter VT3. As a result, when opening, transistor VT2 lowers the voltage at the base of VT3, preventing a further increase in its current.
In other words, the ultrasonic sounder is covered by 100% negative feedback (NFE) for direct current, which strictly stabilizes its mode. This is facilitated by the relatively large (compared to generally accepted) resistance of the collector load VT1 - resistor R3 and the small one - resistor R4. On alternating current of audio frequencies, the OOS does not work, since they are closed through a large-capacity blocking capacitor C15. A variable resistor R6 is connected in series with it - the volume control. By introducing some resistance, we thereby create some OOS, which reduces the gain. This method of volume control is good because the regulator is installed in the circuit of an already amplified signal and does not require shielding. In addition, the introduced OOS reduces the already small signal distortion in the amplifier. The disadvantage is that the volume is not adjusted to zero, but usually this is not necessary. The phones are connected to the collector circuit of the VT3 transistor (via connector XS3), and both the alternating signal current and the direct current of the transistor flow through their coils, which additionally magnetizes the phones and improves their performance. It does not require setting up an ultrasonic sounder.
About the details. Start selecting them with headphones. You need ordinary telephones of an electromagnetic system with tin membranes, necessarily high-resistance, with a total direct current resistance of 3.2...4.4 kOhm (they are not suitable for telephone sets - they are low-resistance). The author used TA-56m phones with a resistance of each 1600 Ohms (indicated on the case). TA-4, TON-2, TON-2m, still produced by the Oktava plant, are also suitable. Miniature headphones from players with low sensitivity cannot be used with this receiver.
The phone power plug is replaced with a standard round three- or five-pin connector from sound-reproducing equipment. A jumper is installed between pins 2 and 3 of the pin part of the connector, which is used to connect the power battery GB1. When the phones are disconnected, the battery will turn off automatically. The former positive terminal of the telephone cord is connected to pin 2, this will ensure the addition of magnetic fluxes created by the bias current and the permanent magnets of the telephones.
The next important detail is the KPI. The author was lucky - he managed to find a small-sized dual KPI from a portable transistor receiver with a built-in ball vernier. It is possible to use a KPI without a vernier; receiving CW stations will not cause problems, but precise tuning on an SSB station will be difficult, since the tuning density of 400 kHz per revolution is too high. Select the maximum diameter adjustment knob or construct your own vernier using a suitable pulley and cable. KPI with an air dielectric is better, but small-sized KPI with a solid dielectric from transistor receivers are also suitable. Often they are already equipped with vernier pulleys. The capacitance of the capacitor is not critical; the required range overlap can be selected using “stretching” capacitors SZ, C5 (their capacitances must be the same) and C2, C6 (the capacitances are also the same).
The receiver coils are wound on standard three-section frames used in transistor receivers. If frames have four sections, the section closest to the base is not used. The turns are evenly distributed in all three sections of the frame, winding is carried out in bulk. The frames are equipped with ferrite cores with a diameter of 2.7 mm. A PEL wire with a diameter of 0.12-0.15 mm is suitable, but it is advisable to use PELSHO, or even better - Litz wire twisted from several (5-7) PEL conductors 0.07-0.1 or ready-made Litz wire in a silk braid, for example, LESHO 7x0.07.
Coils L1 and L2 contain 70 turns each, L3 - 140 turns with a tap from the 40th turn, counting from the terminal connected to the common wire. The low-pass filter coil L4 is wound on a ring K10x7x4 made of ferrite with a magnetic permeability of 2000 and contains 240 turns of PEL or PELSHO wire 0.07-0.1. Winding it in the absence of experience can result in a problem (the author wound it in less than an hour). Use a shuttle soldered from two pieces of copper wire about 10 cm long. At the ends, the wires are slightly separated, forming “forks” into which a thin winding wire is placed. It is better to fold it in half and wind 120 turns, then connect the beginning of one wire to the end of the other (an ohmmeter is needed to identify the terminals). The resulting middle output is not used.
Coil L4 can be replaced with the primary winding of the output or transition transformer from pocket receivers. If its inductance turns out to be too high and the cutoff frequency of the low-pass filter decreases, which will be noticeable by ear by weakening the higher frequencies of the audio spectrum, the capacitance of capacitors C8 and C11 should be slightly reduced. In extreme cases, the coil can even be replaced with a resistor with a resistance of 2.7...3.6 kOhm. In this case, the capacitance of capacitors C8 and C11 must be reduced by 2...3 times, the selectivity and sensitivity of the receiver will decrease somewhat.
The capacitors included in the circuits must be ceramic, mica or film, with good capacitance stability. Miniature capacitors with non-standardized TKE (temperature coefficient of capacitance) are not suitable here; they are usually orange. Don’t be afraid to use vintage capacitors of the KT, KD (ceramic tubular or disk) or KSO (pressed mica) types. The requirements for capacitors C8-C11 are less stringent; any ceramic or metal-paper (MBM) are suitable here, except for capacitors made of low-frequency ceramics of the TKE H70 and H90 groups (the capacity of the latter can change almost 3 times with temperature fluctuations). There are no special requirements for other capacitors and resistors. The capacitance of capacitor C12 can range from 0.1 to 1 µF, C13 - from 50 µF and above, C15 - from 20 to 100 µF. Variable volume control resistor - any small-sized one, for example, type SPZ-4.
It is permissible to use almost any silicon high-frequency diodes in the mixer, for example, the KD503, KD512, KD520-KD522 series. In addition to the KT361B (VT1) transistor indicated in the diagram, any of the KT361, KT3107 series will be suitable. Transistors VT2, VT3 - any silicon with a current transfer coefficient of 150...200 or more.
The flat six-volt battery was taken from a used Polaroid camera cassette. Other options are also possible: four galvanic cells in series connection, a Krona battery. The current consumed by the receiver does not exceed 0.8 mA, so any power source will last for a long time, even with daily long-term listening to the air.
The design of the receiver depends on the housing you choose. The author used a thread box made of thick plastic (see photo of the receiver in Radio, 2003, No. 1) with dimensions of 160x80x40 mm. Actually, the entire receiver is mounted on the front panel, which also serves as a cover for the box. The panel must be cut from one-sided foil-coated getinax or fiberglass. It is advisable to choose a material with a beautiful non-foil surface (the author uses black getinaks). Holes are drilled in the panel for the antenna and grounding sockets, KPI, volume control, then the foil is sanded to a shine with fine sandpaper and washed with soap and water.
The telephone connector is installed on the lower side wall of the box (Fig. 4). The power battery is placed at the bottom of the box and pressed through a cardboard spacer with a bracket made of thin elastic brass or tin, resting against the side walls of the box. The battery terminals are made from ordinary wiring wires. Their stripped ends are inserted into the windows provided in the cardboard battery case before installing the battery in the receiver. The negative terminal is soldered to the body of the telephone connector, the positive terminal to socket 2. The connector is connected to the receiver board with four twisted conductors of sufficient length.
Mounting the receiver mounted. Those parts, one terminal of which is connected to a common wire, are soldered with this terminal (shortened to the minimum length) directly to the foil. Then the remaining terminal also serves as a mounting stand, to which the terminals of other parts are soldered, in accordance with the diagram. It is even recommended to bend one of the connected terminals in the form of a ring or mounting tab. If the design of the part allows it (KSO type capacitors, oxide capacitors), it is useful to secure its body to the board with a drop of glue. Other mounting tabs are the terminals of the control unit and the volume control. The spring output from the rotor plates of the KPI must be connected to the foil of the board with a separate conductor - this will eliminate possible frequency jumps when rebuilding the receiver, since the electrical contact through the bearings is by no means the best.
When installing the low-pass filter coil, solder a short piece of single-core mounting wire to the board and bend it perpendicular to the board. A thick cardboard or plastic washer, a coil, and another similar washer are put on it in succession, and everything is secured with a drop of solder. The top end of the support wire must be insulated to prevent short-circuited turns. If the top washer is made wider, then it is convenient to attach the terminals of capacitors C8 and C11 to it. Even without drilling holes, the lead can be “melted” through the plastic with a soldering iron.
Loop coil frames typically have four pins for mounting on a printed circuit board. Three of them are soldered to the foil of the receiver board, the remaining one is used to secure the “hot” output of the coil and as a mounting tab. The distance between the axes of the coils L1 and L2 should be about 15 mm to obtain optimal connection. If you plan to take the receiver with you on hikes, when wet weather often occurs, it is better to fill the turns of all coils with paraffin. All you need is a soldering iron and a candle stub. The same applies to all cardboard insulating parts.
The approximate location of parts on the receiver board is shown in Fig. 5. An “instrument” version of the receiver design (for home use) is also possible, when the front panel is located vertically, the antenna jack is on the right, and the volume control is on the left. In this case, it is advisable to install the telephone connector on the front panel on the left, next to the volume control, and make the case out of metal to protect it from interference created by other equipment standing on the table.
For other receiver design options, general rules should be followed: input circuits and circuits should not be placed close to the local oscillator; it is better to place them on opposite sides of the control unit, the housing of which will serve as a natural screen; the local oscillator coil should not be placed close to the edge of the board to prevent the influence of hands on the frequency; The input and output circuits of the ultrasonic sounder should be spaced further apart to reduce the likelihood of its self-excitation. At the same time, the connecting conductors should be short and laid close to the metallized surface of the board. It is better to do without connecting conductors altogether, using only the leads of the parts. The more metal connected to the common wire in the structure, the better. It is easy to see from the illustrations that these rules are observed in the proposed design.
Setting up the receiver is simple and comes down to setting the required local oscillator frequency and adjusting the input circuits to maximize the signal. But before turning on the receiver, carefully check the installation and eliminate any errors found. The functionality of the ultrasonic filter is verified by touching one of the terminals of the low-pass filter coil. A loud "growling" sound should be heard in the phones. In operating mode, the noise from the first stage will be faintly audible.
The easiest way to check the operation of the local oscillator and set its tuning range is 0.9...1 MHz using any broadcast receiver with a mid-wave range. In this receiver, the local oscillator signal will be heard as a powerful radio station during transmission pauses. The receiver with a magnetic antenna must be placed nearby, and if the receiver only has a socket for connecting an external antenna (such receivers are now a rarity), then a piece of wire must be inserted into it, connected to the local oscillator coil. In the absence of generation, it is necessary to install transistor VT1 with a high current transfer coefficient and/or solder resistor R2 of lower resistance. You can clarify the scale calibration of the auxiliary receiver using signals from local radio stations whose frequencies are known. In the center of Russia - "Radio Russia" (873 kHz), "Free Russia" (918 kHz), "Radio Church" (963 kHz), "Slavyanka" (990 kHz), "Resonance" or "People's Wave" (1017 kHz) .
These same signals can be used to calibrate the scale of our receiver. The technique is as follows: tune the auxiliary receiver to the frequency of the radio station, turn on the tuned receiver and change the frequency of its local oscillator using the tuning knob and the L3 coil trimmer until the local oscillator signal is superimposed on the station signal. A whistle will be heard in the loudspeaker of the auxiliary receiver - the beating of two signals. Continuing the adjustment, lower its tone to zero beats and mark a point on the scale - here the tuning frequency of our receiver is exactly equal to twice the frequency of the radio station. If the station signal in the auxiliary receiver is completely clogged with the signal of our local oscillator, slightly increase the distance between the receivers.
The last operation is to configure the input circuits. Connect an antenna at least 5 m long, or even an indoor one. Surely you will already receive some signals. By alternately rotating the trimmers of coils L1 and L2, achieve maximum reception volume. It is more convenient to finally adjust the input circuits in a part of the range free from radio stations, simply to the maximum noise level. It should be noted that adjusting the L2C7 circuit slightly affects the local oscillator frequency, but when tuning for noise this does not make any difference. You can verify that the settings are correct by connecting and disconnecting the antenna: the noise on the air should be many times greater than the internal noise of the receiver.
Receiver operation test results. Its sensitivity, measured using a standard signal generator (SSG), turned out to be about 3 μV. This is not surprising, given the high gain of ultrasonic frequencies (more than 10,000) and the presence of sensitive phones. The receiver mixer introduces virtually no noise of its own, and there is no amplifier in it.
It is preferable to listen to the broadcast in the evening and at night, when the range of 160 meters is “open” (there is a long range of radio waves). During the daytime, you can only hear local stations if they are working (and amateurs, knowing the conditions for the passage of radio waves, usually do not go on the air in this range during the day).
At this time, not having an antenna for the 160-meter range, the author tested the receiver with a temporary wire antenna no more than 10 m long, including descent. It was stretched from the balcony to the roof railing and there fixed on a pole no more than 1.5 m high. Nevertheless, SSB stations in the European part of Russia from Karelia to the Volga region and Krasnodar Territory, as well as Ukraine and Belarus were confidently received. Telegraphs could be heard from stations in Spain and Siberia (I’m only mentioning the most distant ones). “Grounding” to a heating radiator or water pipe significantly increased the reception volume. Thus, almost everything that could be heard on any other, much more complex receiver was accepted.
At the same time, they are well protected from breakdown by static electricity. Using such transistors, simple and effective mixers for radio receivers can be constructed. In Fig. Figure 1 shows a typical diagram of such a mixer.
The signal voltage is applied to the first gate of the transistor, and the voltage of the local oscillator (smooth range generator, VFO) is applied to the second. The dynamic range of the mixer (for intermodulation - about 70 dB, for blocking - more than 90 dB) reaches its maximum value at a bias voltage at the transistor gates close to to zero. The high output resistance of the transistor (10...20k0m) is in good agreement with widely used magnetostrictive electromechanical filters at a frequency of 500 kHz, and the low drain current (approximately 1...1.5 mA) allows the use of direct connection of the EMF excitation winding. At the same time, a significant conversion slope (approximately 1.5...2 mA/V) ensures acceptable receiver sensitivity even without an amplifier. The high input impedance for both inputs greatly simplifies the matching of the mixer with the preselector and GPA.
Based on these mixers, using a disk EMF at a frequency of 500 kHz with an average bandwidth, in a couple of hours of leisurely, enjoyable work, a fairly sensitive and noise-resistant observer receiver for the range of 80 meters was made, both in design and in setting up. Its diagram is shown in Fig. 2. An input signal with a level of 1 μV is supplied to an adjustable attenuator made on a dual variable resistor R27. Compared to a single resistor, this solution provides an attenuation control depth of more than 60 dB throughout the entire HF range, which allows for optimal receiver operation with almost any antenna.
Next, the signal is fed to the input bandpass filter formed by elements L1, L2, C2, SZ, C5 and C6 with external capacitive coupling through capacitor C4. The connection of the attenuator to the primary circuit through the capacitive divider C2SZ shown in the diagram is recommended for low-impedance antennas (quarter-wave “beam” about 20 m long, dipole or “delta” with a coaxial cable feeder). For a high-impedance antenna in the form of a piece of wire with a length significantly less than a quarter of the wavelength, the output of the attenuator (the upper terminal of resistor R27.2 in the diagram) should be connected to terminal X1 of the board, connected to the first circuit of the input filter through capacitor C1. The connection method for a specific antenna is selected experimentally based on maximum volume and reception quality.
The two-circuit DFT is optimized for an antenna resistance of 50 Ohms and a load resistance of 200 Ohms (R4). The DFT transmission coefficient due to the transformation of the resistances is approximately +3 dB. Since an antenna of any random length can be used with the receiver, and when adjusted by an attenuator, the resistance of the signal source at the DFT input can vary over a wide range, a matching resistor R1 is installed at the filter input, which provides a fairly stable frequency response under such conditions. The selected DFT signal with a level of at least 1.4 μV is supplied to the input of the mixer - the first gate of transistor VT1. Its second gate receives a local oscillator signal voltage with a level of 1...3 Veff through capacitor C7.
An intermediate frequency signal (500 kHz), which is the difference between the frequencies of the local oscillator and the input signal, with a level of the order of 25...35 µV is allocated in the drain circuit of transistor VT1 by a circuit formed by the inductance of the filter winding Z1 and capacitors C12 and C15. Circuits R11C11 and R21C21 protect the general power supply circuit of the mixers from local oscillator, intermediate and audio frequency signals entering it.
The first local oscillator of the receiver is made according to a capacitive three-point circuit on transistor VT2. The local oscillator circuit is formed by elements L3C8-C10. The local oscillator frequency can be adjusted using a variable capacitor C38 in the range of 4000...4300 kHz (with some margin at the edges). On the 80 meter band, amateur radio stations use the lower sideband, and the receiver IF path (see below) is focused on highlighting the upper sideband. To ensure sideband inversion of the received signal, the VFO frequency must lie above the amateur band of 80 meters. Resistors R2, R5 and R7 determine and rigidly set (due to deep OOS) the direct current operating mode of the transistor. Resistor R6 improves the spectral purity (shape) of the signal. The power supply of both local oscillators (+6 V) is stabilized by the integrated stabilizer DA1. Circuits R10C14C16 and R12C17 protect the common power supply circuit of both local oscillators and decouple them from each other.
The main selection of signals in the receiver is performed by the EMF Z1 with an average passband width of 2.75 kHz. Depending on the type of EMF used, the selectivity in the adjacent channel (with a detuning of 3 kHz above or below the passband) reaches 60...70 dB. From its output winding, tuned to resonance by capacitors C19, C22, the signal is supplied to a mixing detector made on transistor VT4, according to a circuit similar to the first mixer. Its high input resistance made it possible to obtain the minimum possible signal attenuation in the EMF (about 10... 12 dB), and therefore at the first gate of transistor VT4 the signal level is at least 8...10 µV.
The second local oscillator of the receiver is made on transistor VT3 in almost the same circuit as the first, only instead of an inductor, a ceramic resonator ZQ1 is used. In this circuit, generation of oscillations is possible only with inductive reactance of the resonator circuit (when the oscillation frequency is between the frequencies of series and parallel resonances). Often in such receivers, a rather scarce set is used in the second local oscillator - a quartz resonator at 500 kHz and an EMF with an upper passband. This is convenient, but it significantly increases the cost of the receiver. In our receiver, a widely used ceramic resonator at 500 kHz from remote controls, which has a wide inter-resonant interval (at least 12... 15 kHz), is used as a frequency-setting element. With capacitors C23 and C24, the second local oscillator is easily tunable in frequency within the range of at least 493...503 kHz and, as experience has shown, with the exception of direct temperature effects, it has sufficient frequency stability for practice.
Thanks to this property, almost any EMF with an average frequency of about 500 kHz and a bandwidth of 2.1...3.1 kHz is suitable for the receiver. This could be EMF-11D-500-3.0V or EMFDP-500N-3.1 or FEM-036-500-2.75S, used by the author. The letter index indicates which sideband relative to the carrier is allocated by this filter - upper (B) or lower (H), or whether the frequency of 500 kHz falls in the middle (C) of the filter passband. In our receiver this does not matter, since during setup the frequency of the second local oscillator is set 300 Hz below the filter passband, and in any case the upper sideband will be highlighted.
The second local oscillator signal with a frequency of about 500 kHz (498.33 kHz in the author’s copy) and a voltage of approximately 1.5...3 Veff is supplied to the second gate of transistor VT4. As a result of the conversion, the signal spectrum is transferred to the audio frequency region. The conversion factor (gain) of the detector is about 4.
The signal from the ultrasonic sound output is detected by diodes VD1. VD2, and the AGC control voltage is supplied to the gate circuit of the control transistor VT5. As soon as the voltage level exceeds the threshold (about 1 V), the transistor opens and the voltage divider formed by it and resistor R20 stabilizes the audio frequency output signal at a level of approximately 0.65 ... 0.7 VEff, which corresponds to a maximum output power of approximately 60 mW. With such power, modern imported speakers with high efficiency are capable of sounding a three-room apartment, but for some types of domestic speakers this may not be enough. In this situation, you can double the AGC threshold voltage. installing red LEDs as VD1, VD2 and increasing the supply voltage of the ultrasonic unit to 12 V.
In rest mode or when working with high-impedance headphones, the receiver is quite economical - the current consumption does not exceed 12 mA. With a dynamic head with a resistance of 8 Ohms at maximum sound volume, the current consumption can reach 45 mA. To power the receiver, any industrial or homemade power supply is suitable, providing a stabilized voltage of +9 V at a current of at least 50 mA. For autonomous power supply, it is convenient to use galvanic cells placed in a special container or batteries.
For example, an HR22 rechargeable battery (Krona size) with a voltage of 8.4 V and a capacity of 200 mAh provides more than three hours of listening to the air on a dynamic head at medium volume and more than ten hours on high-impedance phones. All parts of the receiver, except connectors, variable resistors and KPE, mounted on a board measuring 45×160 mm made of one-sided foil fiberglass. Drawings of the board from the side of printed conductors and the location of parts are shown in Fig.
Transistors VT1, VT4 can be any of the BF961, BF964, BF980, BF981 series or the domestic KP327 series. For some of these types, it may be necessary to select a resistor value in the source circuit to obtain a drain current of 1 ... 2 mA. For local oscillators, imported transistors of the p-p-p structure - 2SC1815, 2N2222 or domestic KT312, KT3102, KT306, KT316 with any letter indices are suitable. The 2N7000 field-effect transistor can be replaced by its analogues BS170, BSN254, ZVN2120A, KP501A. Diodes 1N4148 - any silicon, for example, KD503, KD509, KD521, KD522 with any letter index.
Fixed resistors - any type with a dissipation power of 0.125 or 0.25 W. Parts mounted on the chassis can also be of any type. Double variable resistor R27 can have a resistance of 1...3.3 k0m, and R26 - 47...500 Ohms. Tuning capacitor C38 is a small-sized one with an air dielectric and a maximum capacity of at least 240 pF, for example, a small-sized KPI from a transistor broadcast receiver. The capacitor should be equipped with a simple vernier with a retardation of 1:3...1:10.
Loop capacitors - small-sized ceramic KD, KT, KM, KLG, KLS, K10-7 with small TKE (groups PZZ, M47 or M75) or similar imported ones (orange disk with a black dot or multilayer with zero TKE - MP0). Trimmer capacitors - CVN6 from BARONS or similar small-sized ones. It is advisable to use heat-stable film or metal film capacitors C26 and C29, for example, the MKT, MKR series and similar ones. The rest of the blocking ceramic and oxide ones are of any type, imported, small-sized. Standard small-sized EC24 chokes with an inductance of 22 μH are used as DFT coils L1 and L2. This option allows you to abandon homemade coils, which are so unloved by many beginning radio amateurs.
The local oscillator coil L3 is homemade. For its winding, a ready-made frame with a trimmer with a diameter of 2.8 mm made of 600NN ferrite and a screen from standard 465 kHz IF circuits of domestic transistor radios is used. To obtain an inductance of 8.2 μH, 31 turns of wire with a diameter of 0.17...0.27 mm are required. After winding the coil evenly in three sections, a trimmer is screwed into the frame, and then this structure is enclosed in an aluminum screen. The standard cylindrical magnetic circuit is not used. In general, as a frame for homemade coils, you can use any available to a radio amateur, of course, with appropriate adjustments to the printed conductors. Very convenient and thermally stable are imported 455 kHz IF circuits, the trimmer of which is a ferrite pot that has a thread on the outer surface and a slot for a screwdriver. Wire in all variants with a diameter of 0.17...0.27 mm.
As noted above, the DFT uses standard imported small-sized EC24 type chokes and similar ones as inductors. Of course, if it is problematic to purchase ready-made chokes of the required inductance, you can also use homemade coils in the DFT, calculating the number of turns using the above formulas. Conversely, if difficulties arise with winding homemade coils, you can also use a ready-made imported 8.2 µH inductor as L3. Choke L4 - any ready-made one with an inductance in the range of 70...200 µH. You can make it yourself by winding 20-30 turns with PEV-2 0.15 wire on a magnetic core of standard size K7x4x2 (K10x6x3) made of ferrite with a permeability of 600...2000 (a larger number of turns corresponds to smaller values of diameter and/or permeability).
A correctly mounted receiver with serviceable parts begins to work, as a rule, the first time it is turned on. Nevertheless, it is useful to carry out all the operations to set it up in the sequence outlined below. The volume control is set to the maximum signal position. Using a multimeter connected to the power supply circuit, check that the current consumption does not exceed 12...15 mA and the receiver’s own noise can be heard in the speaker. Then, switch the multimeter to DC voltage measurement mode. measure the voltage at the terminals of the DA2 microcircuit and transistors. They must correspond to the data given in table. 1 and 2.
Next, a simple check of the general performance of the main components is carried out. If the ultrasonic sound system is working properly, touching pin 3 of DA2 with your hand should cause a loud, growling sound to appear in the speaker. Touching the common connection point of elements C27, R19, R20 should lead to the appearance of a sound of the same timbre, but noticeably lower volume - this is where the AGC is activated. We check the drain currents of the field-effect transistors by the voltage drop across the source resistors R9 and R16. If it exceeds 0.44 V (i.e., the transistor drain current exceeds 2 mA), the resistance of the source resistors should be increased and the drain current reduced to 1 ... 1.5 mA.
To set the calculated frequency of the second local oscillator, remove the technological jumper J2 and connect a frequency meter to this connector instead. In this case, transistor VT4 performs the function of a decoupling (buffer) amplifier of the signal of the second local oscillator, which almost completely eliminates the influence of the frequency meter on the frequency setting accuracy. This is convenient not only at the setup stage, but later, during operation, allowing for operational monitoring and, if necessary, adjustment of local oscillator frequencies without completely disassembling the receiver. The required frequency is set by selecting capacitor C24 (roughly) and adjusting capacitor C23 (exactly). Return jumper J2 to its place and similarly, by connecting the frequency meter instead of the process jumper J1, check and, if necessary, adjust (by adjusting the inductance L3) and the GPA tuning range will be too wide, which is quite likely when using a KPI with a larger maximum capacity in series with it You can include an additional stretching capacitor, the required capacity of which will need to be selected independently.
in resonance of the input and output windings of the EMF with the GSS, an unmodulated signal with a frequency corresponding to the middle of the filter passband is supplied to the first gate of transistor VT1 through a capacitor with a capacity of 20 ... 100 pF. By selecting capacitors C12, C22 (roughly) and fine-tuning capacitors C15, C19, the filter is adjusted to the maximum output signal. To avoid AGC operation, the GSS signal level is maintained such that the signal at the ULF output does not exceed 0.4 Veff. As a rule, for an EMF of unknown origin, even the approximate value of the resonant capacitance is unknown, and it, depending on the type of EMF, can range from 62 to 150 pF. For normal operation of the receiver on a range of 80 meters, it is advisable to connect an external antenna with a length of at least 10...15 m. When powering the receiver from batteries, it is useful to connect a ground wire or a counterweight wire of the same length. Good results can be obtained by using metal pipes for water supply, heating or balcony railings in panel reinforced concrete buildings as grounding.
Homemade HF (short wave) receivers are made on the basis of resistor switches. Many modifications include a wired adapter and are equipped with amplifiers. The standard circuit has high frequency stabilizers. To adjust the channels, knobs with pads are used.
It should also be noted that receivers differ from each other in conductivity and frequency of tetrodes. In order to understand this issue in detail, it is necessary to consider the circuits of the most popular receivers.
The circuit of a homemade HF receiver includes a controlled modulator, as well as a set of capacitors. Resistors for the device are selected at 4 pF. Many models have contact triodes that operate from converters. It should also be noted that the receiver circuit includes only single-pole transceivers.
To adjust the channels, regulators are used, which are installed at the beginning of the chain. Some models are made with only one adapter, and the connector for them is selected as a linear type. If we consider simple models, they use a grid amplifier. It operates at 400 MHz. Insulators are installed behind the modulators.
Homemade tube HF high-frequency receivers include contact transducers and low-conductivity sensors. Some experts speak positively about these devices. First of all, they note the ability to connect transceivers. Triggers for modification are suitable for the controller type. The most common devices are those with semiconductor resistors.
If we consider the standard circuit, then the comparator is of an adjustable type. Output resistors are installed with a capacity of at least 3.4 pF. Conductivity does not fall below 5 microns. The controls are installed on three or four channels. Most receivers use only one phase filter.
A homemade pulse HF receiver for amateur bands is capable of operating at a frequency of 300 MHz. Most models fold with contact stabilizers. In some cases, transceivers are used. The increase in sensitivity depends on the conductivity of the resistors. the output is 3 pF.
The average conductivity of contactors is 6 microns. Most receivers are manufactured with dipole adapters that accept PP connectors. Very often there are capacitor blocks that operate from thyristors. If we consider lamp models, it is important to note that they use single-junction comparators. They turn on only at 300 MHz. It should also be said that there are models with triodes.
Single-pole homemade HF tube receivers are easy to set up. The model is assembled with your own hands with variable comparators. Most modifications are designed with low conductivity stabilizers. The standard one involves the use of dipole resistors with an output capacitance of 4.5 pF. Conductivity can reach up to 50 microns.
If you assemble the modification yourself, then the comparator must be prepared with a transceiver. Resistors are soldered onto the modulator. The resistance of the elements, as a rule, does not exceed 45 Ohms, but there are exceptions. If we talk about relay receivers, they use adjustable triodes. These elements operate from a modulator, and they differ in sensitivity.
What are the advantages of a multi-pole HF detector receiver for the amateur bands? If you believe the reviews of experts, these devices produce a high frequency and at the same time consume little electricity. Most modifications are assembled with dipole contactors, and adapters are used of the wired type. Connectors for devices are suitable for different classes.
Some models contain phase filters that reduce the risk of interference from wave interference. It should also be noted that the standard receiver circuit involves the use of a regulator to adjust the frequency. Some instances have comparators of the channel type. In this case, the triode is used with only one insulator, and its conductivity does not fall below 45 microns. If we consider expander receivers, they are only capable of operating at low frequencies.
HF receivers for amateur bands with two-junction converters are capable of stably maintaining a frequency of 400 MHz. Many models use a pole zener diode. It is powered by a converter and has high conductivity. The standard modification circuit includes a controller with three outputs and a capacitor. The amplifier for the model is suitable with a varicap.
It should also be noted that high-frequency devices with a converter of this type can cope perfectly with impulse noise from the unit. Comparators are used with grid and capacitive resistors. The resistance parameter at the input of the circuit is about 45 Ohms. In this case, the sensitivity of receivers can vary greatly.
A homemade HF receiver for amateur bands with a three-wire converter has one contactor. The connectors can be used with or without a cover. It should also be noted that resistors are used of different conductivities. At the beginning of the circuit there is a 3 micron element. As a rule, it is used as a single-pole type and allows current to flow in only one direction. The capacitor behind it is located with a linear conductor.
It should also be noted that the resistors at the output of the circuit have low conductivity. Many receivers use them as an alternating type and are capable of passing current in both directions. If we consider modifications at 340 MHz, then in them you can find comparators with grid triodes. They operate at high resistance, and the voltage is as much as 24 V.
A homemade HF receiver for the amateur bands with a frequency of 200 MHz is very common. First of all, it should be noted that the models are not able to work on comparators. Linear modifications are common. However, the most common devices are considered to be models with transition decoders. They are installed with a set of adapters. Resistors at the beginning of the circuit are used with high capacitance, and their resistance is at least 55 Ohms.
Amplifiers are available with and without filters. If we consider switched modifications, they use duplex capacitors. In this case, the stabilizer is used with a regulator. A modulator is required to configure channels. Some receivers work with receivers. They have a PP series connector.
A homemade HF receiver for amateur bands with a frequency of 300 MHz includes two pairs of resistors. Comparators in models have a conductivity of 40 microns. Some modifications contain wired extenders. These elements can significantly relieve the load on capacitors.
If you believe the reviews of experts, then models of this type are distinguished by increased sensitivity. Homemade devices are produced without tetrodes. To improve signal conductivity, only transistors are used. It should also be noted that there are devices with channel filters.
The 400 MHz device circuit involves the use of a dipole adapter and a network of resistors. The model's transceiver is used with an open filter. To assemble the device with your own hands, first of all, a tetrode is prepared. Capacitors for it are selected with low conductivity and sensitivity at the level of 5 mV. It should also be noted that receivers with low-frequency type converters are considered common devices. Next, to assemble the device with your own hands, take one modulator. This element is installed in front of the converter.
A tube HF receiver for low-sensitivity amateur bands is capable of operating on different channels. The standard design of the device involves the use of one stabilizer. In this case, the adapter is used as an open type. The conductivity of the resistor must be at least 55 microns. It is also important to note that receivers are manufactured with covers. To assemble the device with your own hands, a set of capacitors is prepared. Their capacitance must be at least 45 pF. It is especially important to note that receivers of this type are distinguished by the presence of duplex adapters.
The high sensitivity device operates at 300 MHz. If we consider a simple model, it is assembled on the basis of a comparator with a conductivity of 4 microns. In this case, filters under it can be used with a lining.
Transistors on the receiver are installed of the unijunction type, and filters are used at 4 pF. Wired transceivers are quite common. They have good conductivity and do not require large energy consumption.
The modulator may only be used with one varicap. Thus, the model is able to work on different channels. To solve problems with negative resistance, an expansion capacitor is used.
The radio receiver is designed for listening to amateur radio stations operating in the 1.8 MHz bands; 3.5 MHz; 7 MHz; 10 MHz; 14 MHz; 18 MHz; 21 MHz; 24 MHz; 28 MHz; 28.5 MHz; 29 MHz. The receiver has a switch for the bandwidth of the IF path, in the mode of receiving telephone exchanges operating with one sideband (SSB) the bandwidth is 2.4 kHz, when receiving telegraph signals (CW) 0.8 kHz. The receiver is a superheterodyne with one frequency conversion.The main selection element is a four-section quartz filter on identical resonators at a frequency of 9050 kHz, this frequency is intermediate.
The schematic diagram of the high-frequency unit is shown in Figure 1. The signal from the antenna through capacitor C1 enters the input circuit, which consists of one universal coil with taps, common to all ranges and loop capacitors C2 and C3.1. The receiver uses a variable air dielectric capacitor from a broadcast receiver, and its capacitance overlap is greater than necessary.
To reduce overlap and, as a result, increase tuning accuracy, a constant C2 is connected in series with the variable capacitor. In either case, the input circuit consists of part of the loop coil L1 and these two capacitors. In the range of 160 m (1.8 MHz), as the lowest frequency, to reduce the tuning frequency of the circuit, capacitor C4 is used, which is connected in parallel with circuit C3.1 C2.
Smooth change in tuning frequency using a variable capacitor, stepwise, when switching ranges - using switch S1 (its section S1.1).
The receiver does not have an input RF amplifier, and uses a passive mixer based on field-effect transistors VT1 VT2, to which the input circuit is connected directly, without transition capacitors or coupling coils. A significant advantage of such a mixer over diode ones is that it provides a sufficiently high transmission coefficient, so much so that there is no need for an input amplifier.
In addition, the use of field-effect transistors, characterized by good linearity, has made it possible to reduce the noise level and significantly expand the dynamic range, which is most important in communications technology.
To further reduce the noise level and increase the transmission coefficient, a bias voltage is created at the gates of the field-effect transistors, the value of which, during the setup process, can be set by trimming resistor R1. Thanks to the use of a parametric stabilizer on R9 VD1, the potential of the common wire point of the converter increases, and the bias voltage turns out to be negative relative to the common wire and the input and output circuits.
Winding 3 of phase transformer T1 receives local oscillator voltage from the GPA, consisting of a master oscillator on transistors VT3 VT4 and a buffer stage on transistor VT5, which matches the high output resistance of the local oscillator circuit and the low input resistance of the transformer.
The local oscillator frequency is determined by a circuit that consists of a universal coil L2 with taps switched by the range switch section and a set of pairs of capacitors switched by section S1.3. Smooth adjustment is made using the second section of the variable capacitor C3.2, stepwise using two sections of the switch S1.2 and S1.3.
Figure 2
The schematic diagram of the IFF circuit is shown in Figure 2. It is built on bipolar transistors. There are two stages of the amplifier in total, both are made according to a cascade scheme.
The IF signal from the output circuit of the mixer is supplied to the input of the first stage of the IF at VT1 and VT2. Its collector circuit includes circuit L1C3, tuned to an IF frequency of 9050 kHz.
Through the coupling coil, the IF signal is fed to a four-section quartz filter on resonators Q1-Q4. The filter passband is adjusted using a small-sized electromagnetic relay, when the SP1 contacts are closed, the passband is reduced from 2.4 kHz to 0.8 kHz. From the output of the filter, the signal goes to the second stage of the amplifier using transistors VT3 VT4, which is made according to the same circuit.
The AGC system regulates the supply voltage of the entire amplifier, and accordingly controls its gain. The IF signal from the output of the second stage is supplied to the rectifier at VD1 VD2. As a result, a voltage appears at the base of VT8, which increases the higher the signal level. And as this voltage increases, VT8 begins to open. Which leads to a decrease in the DC voltage based on the regulating transistor VT7.
As a result, it begins to close, and the supply voltage of the entire amplifier decreases accordingly (both stages of the amplifier are powered by the emitter voltage VT7). The signal level can be judged by the IP1 indicator, which shows the actual supply voltage of the amplifier.
The demodulator is made using a field-effect transistor VT6. It is a switch that periodically interrupts the IF signal at the frequency of the reference oscillator. The input and output impedances of the demodulator are equal, however, there is no difference between its input and output.
The demodulated signal is supplied through the volume control R17 to a two-stage ultrasonic sounder using transistors VT9-VT11. The amplifier can work with any phones, but dynamic 8-40 ohms are preferable.
The reference oscillator is made using a VT5 transistor. Its frequency is stabilized by the same quartz resonator as used in the quartz filter, but its resonant frequency is shifted using capacitors C15 and C16.
Structurally, the receiver is mounted on two printed circuit boards made of single-sided fiberglass. To switch the ranges, a ceramic biscuit switch is used; it is located in close proximity to the high-frequency block board, near the heterodyne and input coils, which in turn are located mutually perpendicular. Capacitors C9-C31 are mounted directly on the contacts of this switch.
The coils of the heterodyne and input circuits are wound on cylindrical ceramic frames with a diameter of 8 mm. Winding is carried out in accordance with Figure 6.
The inverter coils are wound on frames with a diameter of 5 mm with tuning cores with a diameter of 2.0 mm made of 100 NN ferrite. After winding and installation on the board, the frames are covered with aluminum screens, which are connected to a common wire. Coils L3 and L4 of the high-frequency unit are wound on one frame; they contain 30 and 10 turns, respectively, PEV wires 0.12.
Coils L1 L3 and L5 of the IF amplifier contain 25 turns, and L2 and L4 10 turns of the same wire. The setting indicator is any microammeter for 100-150 µA. The operating modes of the high-frequency unit are shown in the diagram; for the IF path - in the absence of an input signal, the voltage on the collector VT2 and VT3 should be 1.5 V each (set by selecting R2 and R5).
Figure 4 and 5
The voltage at the emitter VT7 is 6.5V - by selecting R16. The IF path is tuned in the traditional way using a 9.05 MHz generator. Coil L5 is adjusted in such a way as to provide the highest quality sound (the frequency should be on the left slope of the frequency response of the quartz filter).
When setting up the GPA, you need to adjust the capacitors in such a way as to ensure the following frequency overlap at the GPA output:
for range 29 MHz - 19.95-20.45 MHz,
for range 28.5 MHz - 19.45-19.95 MHz,
for range 28 MHz - 18.95-19.45 MHz,
for range 24 MHz - 15.84-15.94 MHz,
for range 21 MHz - 11 95-12.4 MHz
for range 18 MHz - 9.02-9.12 MHz,
for range 14 MHz - 4.95-5.3 MP4,
for range 10 MHz - 19.15-19.2 MHz,
for range 7 MHz - 16.05-16.15 MHz,
for range 3.5 MHz - 12.55-10.1 MHz,
for range 1.8 MHz - 10.88-10.1 MHz.
Figure 6
