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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio receiver Contest-RX. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception This receiver has better parameters than the "Super-Test" receiver developed earlier by the author of the article and published in the March 2002 issue of the magazine. It is more sensitive, it has better dynamic range. In this receiver, the emphasis is on transferring the gain of the receiver to a greater extent on the low-frequency stages. This was done intentionally, since at low frequencies it is easier to obtain a greater signal-to-noise ratio with the same element base than at high frequencies. In addition, the applied scheme for separate gain control for URF and IF made it possible to significantly increase the quality of reception in the low-frequency ranges without deteriorating dynamic performance. Much attention in the receiver is given to the GPA. It uses the Wakar circuit, which has increased frequency stability. Mounting the generator on ceramic racks (including the use of ceramics in coils and capacitors) and the use of a transistor with small throughput capacitances led to an increase in the frequency stability of the GPA. In addition, it became possible to perform thermal compensation only on one range - 18 MHz when using the same type of capacitors with TKE close to zero. The use of the DAC system in this receiver completely banishes the idea of using a multi-detailed and multi-noise frequency synthesizer. It should be said about the AGC system. It has been brought, if not to perfection, then to the desired result (with a limited elemental base). The ability to set the threshold for the AGC system, the autonomy of operation and the ability to read the S-meter readings regardless of the positions of the resistor sliders that control the gain, the prevention of clicks when powerful pulse signals appear at the receiver input - these are not all the useful qualities of this circuit. There are no heat sinks in the receiver (with the exception of a small one in the DA1 chip). It is possible to install two-section filters at the inlet. The use of a full-fledged speaker, the remoteness of the GPA from the speaker and the mains transformer (to prevent unwanted electromagnetic and mechanical feedback), the ability to install large-sized controls on the front panel, free access to radio elements (the digital scale can be easily removed - three screws) are very useful in this design. In a word, this design is the most perfect in comparison with my other designs (with a slightly increased element base).
The circuit diagram of the receiver is shown in fig. 1. It is a superheterodyne with one frequency conversion.
The radio frequency signal through the antenna socket XW1, the capacitor C1 and the switch SA1.1 enters a part of the coil L1, which together with the variable capacitor C4 forms the input circuit. Switching the receiver from range to range is carried out by closing the corresponding part of the turns of the coil with the SA1.2 range switch section. The SA1.1 switch section on any of the ranges connects only a part of the turns (about half) of the input circuit coil to the antenna, thereby providing an acceptable match with the antenna. In the range of 1,8 MHz, capacitor C4 is connected in parallel to KPI C2, which makes it possible to tune in this frequency range while reducing the frequency overlap ratio. From the input circuit, the RF signal through the capacitor C3 is fed to the first gate of the transistor VT1, which operates in the cascade of the RF. The AGC control voltage is applied to the second gate of this transistor. It is fed through the resistor R4, which is used to manually adjust the gain of this stage. From the URF, the signal is fed to a double bridge balanced mixer. This mixer includes two diode bridges VD1-VD4, VD5-VD8, two transformers T1, T2 and two resistors R7, R8. The presence of resistors makes it possible to carry out the switching mode of diodes at a relatively high local oscillator voltage and limit their current at the opening half-wave of voltage to the maximum allowable value. This mixer is one of the high-level mixers that can provide a large dynamic range due to the high local oscillator voltage. The positive qualities of this mixer include a good decoupling of the input and heterodyne circuits. The GPA signal is fed to one of the windings of the transformer T2, and the radio frequency signal is fed to the connection point of the two windings of the transformer T1. The intermediate frequency signal of 5,5 MHz is taken from the fourth winding T1, which is connected in series with the third winding, which ensures good matching with the high-resistance input of the subsequent stage. Further, the IF signal is amplified by a cascade made on VT2VT3 transistors according to a cascode circuit, where VT2 is connected to a common source, and VT3 is connected to a common base. The IF signal isolated on the L3C13 circuit is fed to the main selection filter, which is used as an eight-crystal quartz filter, made according to the ladder circuit. When the contacts of the relay K1.1, K2,1, short circuit are closed. 1, K4.1 the filter bandwidth is narrowed from 2,4 to 0,8 kHz. From the output of the quartz filter, the IF signal through the matching transformer TZ is fed to the second IF, made on the transistor VT4 according to the common-source circuit. The control voltage of the AGC is supplied to the second gates of the field-effect transistors of both IF amplifiers. Resistor R69 perform manual gain adjustment of the above stages. From the L5C35 circuit, the IF signal enters the SSB signal detector, made on VD9-VD12 diodes according to a ring balanced circuit. Through the balancing resistor R23, it also receives a signal of an exemplary quartz local oscillator with a frequency of 5,5 MHz, which is assembled on a VT13 transistor. From the SSB output of the detector, signal 34 through a low-pass filter (C37R24C42) and an artificially created non-polar capacitor C40C41, necessary to prevent imbalance of the ring mixer with a constant voltage that can come from the VT5 base when the parameters of the electrolytic capacitor C44 change over time, is fed to a low preamplifier frequency, made on low-noise transistors VT5 and VT6 according to the cascode scheme. The first transistor is connected according to the scheme with a common emitter, the second - with a common base. From the VT6 collector, the 3H signal goes through the LF gain control resistor R32 to the final ULF (DA1), and from its output either to the BA1 speaker or to the phones, depending on the position of the SA3 switch. From the VT6 collector, the 3H signal also enters through the cascade on the VT7 transistor and the SA2 switch to the automatic gain control (AGC) circuit made on the VT14 transistor. An AGC rectifier is made on diodes VD17 and VD18. The resistance value R74 determines the threshold for the operation of the AGC system, and the value of capacitance C120 determines the response time. Diodes VD5, VD6 prevent VT14 from completely closing when a powerful pulse signal appears at the receiver input, which prevents clicks in the speaker The presence of the resistor R68 allows you to limit the control voltage of the AGC from above, and the resistor R70 - to remove the non-working area from the bottom. The VT14 emitter includes a PA1 measuring device as an S-meter. R71 limits the signal applied to PA1 from above, and VD25 creates a non-linearity for signals with high levels, which is convenient when reading them. Capacitor C119 blocks RF interference. The input "B" is supplied with a control voltage of + 12 V to lock the receiver when the transmitter is operating at the rate of transmission. The smooth range generator (GPA) is made on a VT8 transistor. The advantages of the GPA include the use of a doubler amplifier stage and an intermediate frequency of 5,5 MHz. This IF has fewer affected points in the conversion compared to other IF values. Parametric voltage stabilizer VD14R50 and capacitor C86 prevent leakage of high-frequency voltage in the power circuit and provide increased stability of the output signal parameters. Switch section SA1.3 connects GPA capacitors at different frequency ranges, and SA1.4 section connects capacitors C90 and C91, used to obtain the necessary stretch on different ranges. Resistor R44 improves the decoupling between the generator and the subsequent stage. The frequencies generated by the GPA are shown in Table. 1.
A broadband GPA amplifier is made on the VT9 transistor. The low capacitance of the gate circuit and the high input impedance of the cascade contribute to a good decoupling of the generator from other cascades. The output of the GPA amplifier is loaded on an elliptical low-pass filter of the seventh order with a bandwidth of 7,33 ... 12,668 MHz. The cutoff frequency of the filter is 12,72 MHz. For all parasitic components of the spectrum of the generated signal, suppression of more than 35 dB is provided. The low-pass filter output is connected to the input of a cascade made on transistors VT10 and VT11, which is a switchable doubler amplifier. The switching of the modes of this cascade is carried out using the contacts of the relay K5.1. On the ranges 1,9; 3,5; 7; 14; The 18 MHz amplifier-doubler works as an amplifier, and on the rest - as a doubler. When switching from doubling mode to amplification mode, the collector of transistor VT10 is turned off, and transistor VT11 is switched to class A linear mode by supplying an additional positive bias to the base circuit due to the connection of resistor R57. In the doubling mode, the signal from the input transformer T5 is supplied in antiphase to the bases of the transistors. The transistor collectors are connected in parallel and loaded on the input winding of the T4 transformer. From the output winding T4, the GPA signal is fed to the first receiver mixer through the emitter follower (VT12), and from its middle (output "B") - to the digital scale and the transmitting attachment. The output "A" is used when viewing the frequency response of the quartz filter and its tuning according to the method described in [1]. If it is intended to use the receiver in conjunction with the transmitting set-top box, then a detuning system should be introduced into the GPA, and when working with digital modes of communication, the TsAPCh system [8] This system works in conjunction with the V. Krinitsky scale [2], and its operation is described in detail in [ 3]. The receiver can use not only this digital scale, but also others, for example, the authors V. Buravlev, S. Vartazaryan, V. Kolomiytsev [4]. When using the scale of V. Krinitsky, for the correct reading of the frequency in the counters, it is necessary to write the numbers 945000 on the low bands (up to 10 MHz inclusive) and 055000 on the high bands. A fragment of the circuit diagram of the central line with the elements of recording the above-mentioned digits and a switching circuit for recording digits on the scale are shown in [8]. The power supply consists of a T6 network transformer, a VD21-VD24 rectifier bridge and a stabilizer made on DA2, VT15, VT16 and VT17. The collector of the transistor VT17 is "planted" directly on the chassis case. There is a negative voltage at the emitter of this transistor relative to the body, which can be used to additionally block the receiver stages when used in conjunction with a transmitter. The output voltage stabilization coefficient of this stabilizer is at least 4000. The receiver is made in a housing measuring 290x178x133 mm from duralumin 1,5 mm thick. The chassis is made of duralumin 4 mm thick. The view of the chassis from two sides is given in [8]. Depth of the chassis from below - 53 mm. The compartments of the GPA, as well as the C76 capacitor, are made of duralumin plates 5 and 1,5 mm thick. GPA parts are mounted on racks made of failed ceramic fuses (remains of conductive wires should be removed from the fuses). The uprights are inserted into recesses drilled (not through) into the chassis and secured with Moment glue. This arrangement improves frequency stability. From below, the GPA compartment is covered with a 1,5 mm thick duralumin cover. A similar cover is covered from above and capacitor C76. Figured holes for mounting printed circuit boards are sawn into the chassis, and MZ threaded holes are made for their fastening. Capacitors C124 and C126 pass through round holes in the chassis. Chip DA1 is equipped with a small heat sink. In the input circuits of the receiver, it is possible to use two-section filters. For this, it is possible to shift the capacitor C4 forward to the tuning capacitors C55-C65. A hole is cut in the vacated place for installing a board with filters. The digital scale is fastened with three screws to the threaded bushings. A view of the front panel of the receiver is shown in [8]. It is made of duralumin 2 mm thick and painted with black nitro paint. Rectangular pieces of paper with explanatory inscriptions are glued to the paint. From above, the front panel is covered with a false panel made of transparent, colorless organic glass 2 mm thick, which acts as a glazing for the digital scale and, at the same time, protects the inscriptions from damage. A decorative overlay made of white polystyrene 2 mm thick is superimposed on the false panel. Inserts of colored plastic in blue and red colors are glued into the white overlay to frame the digital scale and S-meter. Inside the digital scale, a green color filter made of plexiglass (2 mm) is installed. The loudspeaker is covered with a decorative red grille. The main part of the radio components is installed on four printed circuit boards. Printed circuit boards are made of double-sided fiberglass 1,5 mm thick. The copper foil on the side of the radio components has not been completely removed. At the edges of the boards, as well as under the screen partitions, tracks 3 mm wide are left, to which the screens are soldered (brass 0,5 mm thick). The box screens of the crystal filter and reference crystal oscillator are removable. The topology of printed circuit boards is given in [8]. The receiver uses widely used radio components. Resistors of types MLT-0,125, MLT-0,5, MLT-1. Variable resistors - SPZ-9a Transistors KP350B can be replaced with KP306, KT339B - with 2T3124A-2, KT342 - with KT306, KT660B - with KT603B, KT608B, KT646B, KT606B - with KT904A, KT312B - with KT306, KT342, MP25B - on KT501M . Loudspeaker - dynamic head type 1GD50. The HL1 incandescent lamp is used for a voltage of 28 V (CAM-28). It can be replaced with several yellow LEDs connected in series with 300-500 Ohm resistors and placed around the perimeter of the RA1 device. In this case, the illumination of the S-meter will slightly decrease, but the thermal regime of the GPA will be facilitated, which will positively affect the stability of its frequency. Relay K1-K5 - RES49 passport RS4.569.423 or RS4.569.421-00. The receiver uses capacitors of types KT-1, KD-1, KM, KLS, K50-6. Capacitor C80 - PZZ groups, and C81 - M47. To tune the receiver in frequency and tune its input circuit, the so-called differential KPI ("butterfly") passport YaD4.652.007 from the radio station R-821 (822) was used. To increase the maximum capacity, their stators are connected to each other, and the rotors are connected to a common wire. Measuring head RA1 - microammeter M476/3 with a total deflection current of the arrow 100 μA (from the tape recorder "Romantic-3"). Switches SA2, SA3, SA4, SA5, "On. Stabilization" and "On. Detuning" applied type VKZZ-B15. In the quartz filter and the quartz oscillator, quartz resonators are used from the set "Quartz resonators for radio amateurs" No. 1 (passport IG2.940.006 PS), manufactured by the Omsk Instrument-Making Plant named after. Kozitsky. Network transformer Т6 type ТН 34-127/220-50. It can be replaced by any incandescent transformer with a power of more than 30 W and having 2-3 filament windings for a voltage of 6,3 V and a current of more than 0,9 A. If all three windings are used, then it is advisable to use five-volt taps. The winding data of the contours are indicated in Table. 2. The design of the coil L1 is shown in fig. 2
Establishing the receiver begins with checking the performance of the power supply and setting the voltage to +12 V with resistor R79. After that, all stages are checked for the absence of a short circuit in the supply circuits and then they are energized. Next, they begin to tune the local oscillators. Tuning the reference quartz local oscillator (VT13) consists in rotating the core of the L12 coil until stable generation and maximum output amplitude are obtained. By adjusting the core of the L14 coil, the generation frequency is set behind the lower slope of the quartz filter characteristic. In the absence of generation, the generator parts should be checked for serviceability. By the way, it is desirable to do this with each part (and with new ones, especially) before installing it on the printed circuit board. The generation at the output is controlled by a high-resistance RF voltmeter or, even better, an oscilloscope, as well as a frequency meter.
The tuning of the smooth range generator (VT8) begins with laying the 18 MHz range by rotating the rotor of the tuned capacitor C60. Switch SA1 is shown in the 14 MHz position. After laying, thermal compensation is performed by replacing capacitors C80, C81 with equal capacitance, but with different temperature coefficients (TKE). Next, the rest of the ranges are laid in the same way as described above by adjusting the capacitors C55-C59, C61-C65, and, if necessary, the selection of capacitors C66-C74. If capacitors with zero TKE are used (the use of capacitors of the KSO type with the letter G also gives good results), then thermal compensation in these ranges can be omitted. By selecting the values of capacitors C90, C91, the necessary stretching is carried out over the ranges (according to the positions of the SA1.4 switch) so that the overlap margin is 10-15%. Laying frequencies by ranges is carried out according to table. 1. Next, set up a cascade made on the transistor VT9 by selecting the value of the resistor R49 according to the maximum signal at the drain of this transistor (the shape is a regular sinusoid). They do it this way: temporarily replace R49 with a variable resistor with a nominal value of 47 kOhm (the connecting conductors should be as short as possible), set up the cascade, and then, having measured the value of the resistance obtained, replace it with a constant resistor close in value. The low-pass filter is adjusted by rotating the cores of the coils L9, L10, L11 in order to obtain a uniform characteristic in the frequency band 7,33-12,668 MHz. The cutoff frequency should be 12,72 MHz. Control the setting with a frequency response meter or an oscilloscope. Next, tune the amplifier / doubler (VT10, VT11). The tuning starts in the doubling mode on the 28 MHz range by selecting the value of the resistor R56 until the maximum amplitude of the signal of the correct sinusoidal shape is obtained at the output ("B"). Then SA1 is switched to a 1,9 MHz range, in which this stage operates in amplification mode. The setting is carried out by selecting the value of the resistor R57 until the maximum signal at the output "B" of the correct sinusoidal shape is obtained. The emitter follower (VT12) is tuned by selecting the value of the resistor R61 until a maximum signal of the correct sinusoidal shape is obtained on its emitter. If there is an uneven amplitude of the GPA output signal, then the rotation of the cores of the coils L9, L10, L11 should eliminate the latter. If at the output of the GPA there are signal distortions in the form of a meander or the signal amplitude is higher than 4 V (effective), then it is necessary to increase the value of the resistor R44. When establishing detuning systems the slider of the resistor R12 is set to the middle position, and by selecting the value of the resistor R11, the frequencies match when the detuning is on and off. By adjusting the resistor R9, the transmit and receive frequencies are matched. By selecting the value of the resistor R3, the frequencies are matched with and without the DAC system turned on. Checking the performance of the low-frequency amplifier comes down to monitoring the voltage at pin 12 of the DA1 chip. It should be equal to half the supply voltage. A signal with a frequency of 1 kHz and a voltage of 20 mV is fed to the ULF input. By changing the frequency of the generator in the audio range, they make sure that there are no noticeable signal distortions at the ULF output by controlling it with an oscilloscope. The characteristics in the high-frequency region are corrected by the selection of capacitors C51, C52, C53. The preliminary ULF is adjusted by selecting the resistor R25 until the maximum output signal is obtained in the absence of visually noticeable distortion. After ULF, they start setting up the IF (VT2, VT3. VT4). A signal with a frequency of 5,5 MHz and a voltage of 10 mV (not modulated) is supplied from the GSS to the lower output of the capacitor C9 according to the circuit through a capacitor with a capacity of 5 ... 10 pF. Further, rotating the cores of the coils L3, L5 in turn, they achieve the maximum signal at the ULF output. The crystal filter should be in wide band mode, resistor R69 should be in the maximum gain position. By rotating the core of the L14 coil in the reference quartz local oscillator, an output signal tone of about one kilohertz is achieved. The final installation of the laser and the adjustment of the quartz filter is carried out after the receiver has been fully tuned. As the maximum output readings are approached when setting L3, L5, the generator voltage at the input should be gradually reduced. Next, the GSS signal is fed to the antenna input with a frequency corresponding to the selected range, and by adjusting the capacitor C4, the maximum output signal is achieved. In this case, the slider of the resistor R4 "URCh" must be in the position corresponding to the maximum gain (down the diagram). On the 1,9 MHz band, it may be necessary to select capacitor C2. After that, proceed to setting up the quartz filter. To do this, a signal from the GSS or from the transceiver (the vernier of the transceiver allows you to change the frequency very smoothly) is fed to the antenna input of the WV1 receiver with a frequency of the selected range and a voltage of 0,3 μV. By smoothly changing the receiving frequency of the tuned receiver, the readings of the S-meter and the corresponding readings of the digital scale are taken and recorded in a table. Then, according to this table, draw a graph of the frequency response of the filter. The readings of the S-meter are plotted vertically (in relative units), and horizontally - the frequency every 200 Hz. The shape of the frequency response is used to judge the quality of the filter. If the characteristic has large irregularities (attenuation of more than 6 dB, blockages and humps) or a small bandwidth (less than 2 kHz), or an unsatisfactory squareness factor (worse than 1,4 at -80/-3 dB levels), then the filter must be adjusted by alternately changing the values of its capacitors. The control is carried out by analyzing repeated plotting of the frequency response. If it is not possible to obtain an acceptable frequency response, then the quartz should be replaced. In the narrow band mode (SA4 contacts are closed), the filter is adjusted by selecting capacitors C18, C22, C26, C29, achieving narrowing of the band. A bandwidth of 0,8 kHz is optimal for this filter design. The easiest way to adjust the filter is to use a frequency response (AFC) meter. To view the frequency response of the filter (as well as its settings), you can use the method described in [1]. Finally, the frequency of the reference quartz local oscillator is set after tuning the quartz filter by tuning L14, behind the lower slope of the frequency response. The SSB detector is balanced by adjusting the resistor R23 to the minimum of the OCG signal (5,5 MHz) on the resistor R24, while the capacitor C37 must be disconnected during the balancing procedure (do not forget to reconnect it later). Setting up the AGC system consists in selecting the value of the capacitor C120, on which its response time depends. The selection of this capacitor is carried out in a wide band mode according to the best correspondence between the movement of the pointer of the PA1 device and changes in the signals and the sufficiency of the time to keep the pointer at the signal maxima in order to enable visual reading of the device. In this case, the necessary smoothness of the change in the amplification factor of the IF is achieved. When the RA1 device goes off scale at the peaks of the signals, it is necessary to reduce the value of the resistor R71. By selecting the resistor R74, the required level of the AGC system operation threshold is achieved, and the resistor R68 - the maximum gain in the IF when the R69 knob is set to the maximum gain position. In this case, the constant voltage at the second gates VT1, VT2, VT4 should not exceed +5 V. By selecting the resistor R70, the non-working section of the resistor R69 is removed (when the IF gain does not change when the R69 knob is rotated). Literature
Author: V.Rubtsov (UN7BV), Astana, Kazakhstan
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