Encyclopedia of radio electronics and electrical engineering / radio reception

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The receiver is a double-conversion superheterodyne designed to receive amplitude (AM) and single-sideband (SSB) modulation signals in the range of 20 m. work in the ranges from 15 to 25 m. In the range of 20 m, amateur radio stations were well received in a city apartment on the ground floor on a telescopic antenna about 1 m long. Reception was difficult only in conditions of very poor transmission.

The receiver circuit is a superheterodyne with double frequency conversion. It consists of a UHF on a KT368AM transistor, the first mixer on a K174PS1 chip with a frequency-tunable local oscillator (Fig. 1), a second mixer (Fig. 2) and an AM / SSB detector (Fig. 3 ).

Fig. 1

Fig. 2

Fig. 3

Consider the operation of the receiver. The RF signal from the antenna (Fig. 1) enters the input circuit, tuned to the middle frequency of the range, and then to the resonant UHF. The amplified signal is then applied to the first mixer and transferred to the first intermediate frequency of 6,465 MHz. A parallel circuit is configured on it, consisting of L5 and a 300 pF capacitor. The frequency of the first local oscillator, which is part of the K174PS1 microcircuit, is tuned within a small range by the KV109 varicap using two variable resistors ("Coarse tuning" and "Fine tuning"). From the output of the first mixer, the signal goes to a three-circuit bandpass filter (Fig. 2), and then to the second mixer (K174PS1 microcircuit), at the output of which the second intermediate frequency (465 kHz) is allocated. The frequency of the second local oscillator, which is part of the K174PS1, is stabilized by a quartz resonator at a frequency of 6 MHz.

The first intermediate frequency of the receiver can be selected from 6 to 10 MHz. If the radio amateur has an appropriate quartz resonator at his disposal, it becomes possible to replace the three-loop bandpass filter with a piezoceramic one (for example, a television one, at a frequency of 6,5 MHz).

Next, the signal of the second intermediate frequency is fed to the detector, made on the K157XA2 chip (Fig. 3), which is designed to detect signals with amplitude modulation. To detect SSB signals using a toggle switch, an additional circuit is connected to pin 10 of the microcircuit, consisting of an L12 coil and capacitors of 0,01 μF and 3300 pF.

A variable resistor with a resistance of 22 kOhm, installed at the K157XA2 input, regulates the amplitude of the signal coming from the output of the second mixer. Keep in mind that the SSB detector provides a satisfactory quality of the low-frequency signal only at a certain level of the input signal. Of course, this somewhat complicates tuning to amateur radio stations.

Construction and details

The receiver is powered from a stabilized source with a voltage of 9 V. The supply voltage of the K157XA2 microcircuit is 5 V, therefore, a quenching resistor with a resistance of 1,1 kOhm is connected to the power output of the microcircuit. It should be noted that even small ripples of the supply voltage can lead to distortion of the received SSB signal, so it is advisable to use a battery or batteries as a power source.

It is desirable to install microcircuits in sockets, which will facilitate replacement if there is any doubt about their serviceability. In addition, in the process of setting up the receiver for maximum sensitivity, it is desirable to select the KT368AM transistor and a copy of the K157XA2 microcircuit.

All elements of the detector, with the exception of capacitors and the coil of the additional SSB circuit, must be protected by a screen to prevent interference.

In the author's version, the installation was carried out according to the method proposed in [3]. The side of the square is 3 mm, and all points connected to the common wire and the screen are connected by wire jumpers with foil on the reverse side of the textolite plate, which eliminates parasitic pickups. The width of the wafer is slightly greater than the length of the microchip panels that are mounted across the wafer.

The receiver is mounted on two plates 12 cm long each. UHF, the first mixer and a three-circuit band-pass filter are located on one of them, the second mixer and a detector are located on the second. The latter is shielded around the perimeter with strips of double-sided fiberglass.

The capacitors and coil of the SSB detector (L12) are located behind the screen. The L12 coil is wound on a four-section small-sized frame with a trimming ferrite core, it does not have a screen and contains 60 turns of wire with a diameter of 0,15 mm. The position of the coil is important. It must be located vertically, and the distance to other elements of the circuit and to the walls of the housing or screen must be at least 1,5 cm. If the coil is placed close to the housing or covered with a screen, the detection quality deteriorates. The remaining coils used in the receiver are wound on frames with a diameter of 6 ... 7 mm with ferrite tuning cores and have the following winding data:

• L2, L4, L5, L6, L7, L8, L9 - 18 turns of wire with a diameter of 0,3 ... 0,4 mm each (winding - turn to turn),
• L1, L3, L10 - 6 turns of wire with a diameter of 0,3 ... 0,4 mm over the corresponding windings,
• L11 - 80 turns of wire with a diameter of 0,15 mm (in bulk).

In the author's version, the coils do not have screens. If they are screened, then the number of turns should be increased by about 1,3 ... 1,4 times.

The remaining parts in the receiver are small-sized. Variable resistors for coarse and fine frequency tuning and gain control should preferably be used with a linear dependence of the resistance change on the rotation angle.

When setting up a receiver to stabilize the frequency of the first local oscillator, you will have to select the TKE of the capacitors included in the local oscillator circuit. An approximate TKE of capacitors can be as follows: 200 pF - M1500, 10 pF - M750, 5 pF - M75. For a more accurate fit, small capacitors with different TKE can be soldered parallel to the L6 coil.

Setting

The receiver tuning was carried out without the use of special instruments, and its description may be useful to many novice radio amateurs. It is only necessary to have an avometer to control the supply voltage and current consumption.

For the initial check of the circuit and its adjustment, the "chessboard" should be taken larger, with a "squares" side of about 4 ... 5 mm. Parts will be located quite freely, and it will be easy to change them if necessary. After the final configuration of the circuit, all radio elements can be mounted on smaller boards.

It is recommended to start mounting the receiver with the detector circuit (Fig. 3). A variable resistor with a resistance of 22 kOhm and an L12 coil can be omitted at this stage. When a supply voltage is applied to the microcircuit, noise should appear at the output of the ULF connected to the detector, which will increase if you touch pin 1 with a metal object through the capacitor or connect a piece of wire. The voltage at pin 11 should be 5 V.

Next, the first mixer with a tunable local oscillator and UHF is assembled (Fig. 1). The supply voltage to the UHF can not be applied. Instead of the L5 coil and a 300 pF capacitor, a 2 kΩ resistor is soldered (between pins 2 and 3), and pin 2 is connected to the detector input, i.e. connected to a piezo filter at 465 kHz (Fig. 3). Then, to pin 7 of the K174PS1 microcircuit (Fig. 1), through a capacitor with a capacity of 100 pF, an antenna is connected in the form of a piece of wire about 1,5 m long, and the capacitor connected to pin 8 is connected to a common wire. Thus, at this stage, a receiver with one frequency conversion and an intermediate frequency of 465 kHz is obtained, which can receive AM signals. A voltage of 9 V is applied to the mixer. An air noise and, possibly, a signal from some radio station should appear at the ULF output. If by moving the L6 core it is possible to "catch" the signals of AM radio stations, it can be argued that the first mixer and detector are operational. Otherwise, the K174PS1 chip may be faulty and should be replaced. Usually, with proper assembly and serviceable parts, the circuit starts working immediately.

At this stage, you can pick up a copy of the K157XA2 chip with the highest sensitivity. To do this, you should tune in to a weak signal, and from several microcircuits, select an instance that provides the most efficient and high-quality reception.

Then the second mixer is made (Fig. 2). Its performance is checked separately by applying a unipolar pulse voltage with an amplitude of 9 V and a frequency of approximately 1000 Hz, which can be obtained from a multivibrator (Fig. 4.).

Fig. 4

As an antenna, a piece of wire 13..174 cm long is soldered to pin 1 of the K2PS5 chip (Fig. 6). The modulated signal of a working quartz local oscillator at a frequency of 6 MHz is easy to detect with any broadcast AM receiver if the antenna of the latter is brought closer to the mixer board. By switching the ranges and turning the tuning knob of the broadcast receiver, you can "catch" the signal of a working local oscillator (most likely, its harmonics), which will indicate that the circuit is working. install KPI with a maximum capacitance of up to 200 pF. By rebuilding the KPI, they are trying to find the local oscillator signal. After the successful completion of this procedure, the KPI is replaced by a fixed capacitor. If the local oscillator signal could not be detected, the quartz resonator or microcircuit should be replaced. Usually, with serviceable parts and proper installation, the mixer works immediately.

Next, the second mixer is connected to the detector. By applying a supply voltage to these nodes and changing the position of the L11 core, they achieve the maximum noise signal at the ULF output, which increases when a piece of wire about 1 m long is connected through the capacitor to terminal 7 of the K174PS1 microcircuit of the second mixer. This indicates that in this case the receiver is roughly tuned to 6,465 MHz (or 5,535 MHz). At this stage, you can connect a three-loop bandpass filter to the input of the second mixer. The filter setting is carried out in the following order. First, the right (according to the diagram) circuit is connected (capacitor with a capacity of 300 pF and coils L9 and L10) and, by changing the position of the coil cores, they achieve maximum noise at the ULF output with an antenna connected to the tuning capacitor. Then, the second circuit is connected through the coupling trimmer capacitor (with the L8 coil), and it is again adjusted to the maximum noise (the antenna is connected to the next trimmer capacitor). Note that the capacitance of the coupling capacitor also affects the tuning of the loops. Then the third circuit is connected, and the band-pass filter is tuned in the complex.

The next step is to connect the output of the first mixer to the input of the band pass filter (Fig. 1). Instead of the previously installed 2 kΩ resistor, a circuit is connected (L5 and a 300 pF capacitor). UHF is not connected at this stage. The antenna is connected to terminal 7 through a 100 pF capacitor. The capacitor connected to pin 8 is connected to a common wire.

When the supply voltage is applied, the ether noise should appear at the ULF output, which reaches a maximum when L5 is adjusted. By tuning the inductance of the coil L6, you can tune into a broadcast radio station operating in the range of 19 or 25 m. You may need to increase the length of the antenna for better reception. Further, on the signal of some radio station, the mixers and the band-pass filter are adjusted to achieve the best reception quality. The cores of the coils are fixed with paraffin after tuning is completed.

Now it's time to connect the gain control (a 22 kΩ variable resistor installed at the detector input) and the SSB detector circuit (Fig. 3). When the latter is turned on, whistles should appear in the dynamics accompanying the reception of AM signals. By connecting the antenna longer, they are trying to catch amateur radio stations operating in single-sideband modulation. If this is successful (which depends on the passage and time of day), then by adjusting the L12 core, the best speech intelligibility is achieved.

By adjusting the voltage level of the second intermediate frequency with a 22 kΩ resistor, the detector is tuned to the most efficient mode of operation.

It should be remembered that since the width of the emission spectrum of single-sideband transmitters is less than that of amplitude-modulated transmitters, when receiving SSB signals, tuning must be done carefully, accurately “adjusting” the local oscillator frequency with the “Tuning Fine” potentiometer.

In the switching circuit K157XA2 (pin 4) there is a resistor marked with an asterisk. It serves to set the bass gain, and its resistance is selected during tuning. The expediency of using a capacitor, indicated by a dotted line, is determined based on the quality of the detection of the SSB signal.

The final stage is the connection of the UHF (Fig. 1) and the subsequent adjustment of the circuits installed at its input and output, according to the maximum sensitivity of the receiver. First, connect the antenna through a 56 pF capacitor directly to the base of the KT368AM transistor, and set up the circuit in the collector. Then the input circuit is connected and configured. The setting of the latter depends on the antenna used.

The current consumed by the receiver without ULF is about 30 mA.

Based on the described design, it is possible to manufacture a multi-band receiver for receiving radio stations with amplitude and single-sideband modulation. In practice, it is also possible to listen to FM signals on the CB band (with the AM detector turned on), although intelligibility leaves much to be desired. If, however, a separate FM detector on the K174XA26 chip is included in the receiver, by connecting it to the output of the first IF path (6,465 MHz), full-fledged FM reception will become possible. To do this, using the described technology, the first mixer with a tunable local oscillator and UHF is manufactured separately for each range. The dimensions of such modules are approximately 2,5..3 by 7..8 cm. In this case, an ordinary switch with 4 sections is suitable for switching ranges, which will respectively switch the antenna circuits, supply voltages, settings and output of the first IF.

In conclusion, it should be noted that sometimes, with an unsuccessful combination of the parameters of the coil and capacitors of the first local oscillator (Fig. 1), frequency “vibration” is possible, which sharply reduces the quality of SSB detection. In the event of such an effect, it is necessary to replace the capacitors or remake the L6 coil. In general, setting up the receiver does not cause any particular difficulties, and if the installation is completed without errors, and the parts are in good order, success is guaranteed.

Literature

1. Gorbaty V.I. Amateur VHF radio complexes, 1984
2. Bulychev A. L., Galkin V. I., Prokhorenko V. A. Analog integrated circuits. Reference book - Minsk Belarus, 1985
3. V. Khodyrev. "Chessboard" for layout - Radiomir, 2001, No. 11

Author: V.Khodyrev, Perm

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