Encyclopedia of radio electronics and electrical engineering / radio reception

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The use of a synchronous detector in an AM receiver can significantly improve the quality of signal demodulation by eliminating distortion caused by the non-linearity of a conventional envelope detector. At the same time, the noise level is reduced, interference from neighboring stations is reduced. The latter are not detected by a synchronous detector, but are only converted in frequency, therefore, with a detuning of more than 10 ... 20 kHz, interfering signals are in a hard to hear and easily filtered ultrasonic region of the spectrum. The synchronous detector also made it possible to expand the reproducible frequency band to 10 kHz, i.e., to fully realize the spectrum of modulating signals transmitted by radio stations on the air.

Main Specifications

• Sensitivity (with an output power of 50 mW), mV / m not worse ...... 1
• Reproducible frequency range, Hz......50...10 000
• Selectivity at detuning +20 kHz, dB not less than......26

The receiver is powered by a voltage source of 12 ... 15 V, the current consumption (at low volume) does not exceed 40 mA.

The receiver is powered by a voltage source of 12 ... 15 V, the current consumption (at low volume) does not exceed 40 mA. In the described embodiment, the receiver is designed to receive transmissions from radio stations operating at frequencies of 549, 846, 873 and 918 kHz. By changing the capacitance of the capacitors, you can tune in to the frequencies of other radio stations in the MW and LW bands

The circuit diagram of the receiver is shown in the figure. Reception is carried out on the built-in magnetic antenna WA1. The input circuit consists of a coil L1 and capacitors C1-C8 connected to it, tuning capacitors C2, C4, C6 are used to fine-tune to the frequencies of the selected radio stations, resistors R1-R3 reduce the quality factor of the magnetic antenna circuit, expanding its bandwidth to about 20 kHz.

The radio frequency (RF) amplifier is assembled on transistors VT1, VT2 and serves not so much to amplify the signal as to match the relatively high resonant resistance of the oscillatory circuit of the magnetic antenna with the low input impedance of the key mixer. In addition, the RF amplifier protects the input circuit from the penetration of RF voltage from the digital part of the receiver.

The local oscillator is assembled on a field-effect transistor VT3 and is tuned (in each position of the switch SA1) to quadruple the frequency of the received signal. The local oscillator circuit includes the L2 coil, the capacitors C1.2 - C9 connected by the SA13 section of the switch, and the VD1 varicap, which adjusts it exactly to the quadruple signal frequency.

From the drain of the transistor VT3, the local oscillator signal is fed to a digital frequency divider by four, assembled on the triggers of the DD1 microcircuit (as practice has shown, the K176 series triggers work normally with an input signal frequency of up to 4 MHz). At the outputs of the triggers, a four-phase (0, 180, 90 and 270 °) voltage is formed with the frequency of the received signal. It has a rectangular shape and a duty cycle (the ratio of the period to the pulse duration) equal to 2. The DD2 logic chip generates pulses with a duty cycle of 4, which in turn open the keys of the balanced mixers assembled on the DD3 chip. The signal inputs of the keys are connected together, and they are supplied with the voltage of the received signal from the output of the RF amplifier. The two lower ones according to the key circuit form a balanced mixer (phase detector) of the phase locked loop (PLL) system. It generates an error voltage proportional to the deviation of the phase shift between the signal and local oscillator voltages from 90°. The error voltage is smoothed out by capacitors C21 and C22, amplified by the operational amplifier DA1.1, and through a proportionally integrating filter R10R11C27 it enters the varicaps VD1, VD2, adjusting the local oscillator frequency.

If, when the receiver is turned on or the settings are switched, the signal frequency is within the capture band, the PLL system captures it, setting the exact equality of frequencies and the phase shift of the signals at the mixer inputs by 90 °. At the same time, at the inputs of a balanced mixer formed by two upper (according to the scheme) keys, the signal phases coincide, which is necessary for synchronous demodulation of AM oscillations.

The demodulated audio signal (AF) from the output of the synchronous detector is fed to a symmetrical low-pass filter (LPF) L3C17-C20 with a cutoff frequency of 10 kHz. This filter, which determines the selectivity of the receiver, attenuates the signals of radio stations adjacent in frequency, which, after conversion in the detector, fall into the ultrasonic frequency range. To simplify the design, both coils of a symmetrical filter are placed on the same magnetic circuit, which is quite acceptable provided that the order of connecting their conclusions, shown in the diagram, is observed. The associated slight decrease in the attenuation of common mode noise does not matter, since they are well suppressed by the operational amplifier DA1.2, on which the AF preamplifier is assembled. The R12C24 circuit equalizes the input resistances of the inverting and non-inverting inputs of the op-amp.

Details and design. The magnetic antenna of the receiver is made on a round magnetic circuit with a diameter of 8 and a length of 160 mm from ferrite grade 600NN. Coil L1 contains 52 turns of wire LESHO 21x0,07, wound round to round on a sleeve glued from cable paper. For the local oscillator coil L2 (8 + 24 turns of PEL wire 0,15), unified fittings from IF filters of portable receivers were used. Coil L3 low-pass filter (2x130 turns of PEL 0,15 wire) is wound in two wires on a ferrite (2000NM) ring of size K16X8X5.

Capacitors KT-1 and tuning capacitors KPK-M are used in the input and heterodyne circuits of the receiver. The remaining capacitors are KLS and K50-6. Fixed resistors are any small-sized. Instead of the KP303A transistor, other transistors of this series can be used in the RF amplifier if an automatic mixing resistor shunted by a capacitor with a capacitance of 0,01 ... 0,5 microfarads is low cutoff voltage in the source circuit). Transistor VT2 - any high-frequency p-n-p structure. With the same success, a high-frequency transistor of the n-p-n structure (for example, the KT315 series) will work in this cascade if its collector is connected to the power wire, and the emitter (through resistor R5) to a common wire. The local oscillator can be assembled on the KP303A transistor. The resistance of the resistor R7 in this case must be increased to 1,8 ... 2,2 kOhm.

Chip K176TM2 (DD1) can be replaced by K176TM1. In the absence of the K176LE5 chip, you can do without it. In this case, the outputs of the frequency divider triggers (DD1) are connected directly to the control inputs of the balanced mixers (DD3), and 2 kΩ resistors are included in the output circuits of the keys (pins 3, 9, 10 and 2,2) (otherwise the simultaneous opening of two keys will violate operation of balanced mixers). However, it should be taken into account that due to the introduction of these resistors, the transmission coefficient of the mixers will decrease somewhat. Other varicaps of the KB104 series can also be used for autotuning. Zener diode VD3 - any with a stabilization voltage of 9 V.

The design of the receiver can be any, you just need to make sure that the length of the wires connecting the board to the SA1 switch is minimal, and the magnetic antenna is located as far as possible from the digital circuits.

Establishing the receiver begins with measuring the voltage at the emitter of the transistor VT2 of the RF amplifier. It should be about 4,5 V. If necessary, this is achieved by selecting the resistor R4. Then, using an oscilloscope, they check the operation of the local oscillator and the digital part of the receiver. At the source of the transistor VT3 there should be a sinusoidal voltage, at the outputs of the triggers of the DD1 microcircuit - a rectangular one with a duty cycle of 2, and at the outputs of the DD2 microcircuit - the same shape, but with a duty cycle of 4. If the local oscillator generates and the triggers do not switch, it is necessary to select the resistor R7. The operating modes of the op-amp are checked by measuring the voltage at pins 9 and 13 of the DA1 microcircuit: on the first of them it should be equal to 4,5 V, and on the second - within 3 ... 7 V. If the op-amp DA1.1 entered saturation ( voltage at pin 13 is close to zero or to the supply voltage), it is necessary to check the operation of the digital part of the receiver and, if necessary, balance the amplifier by including a resistor with a resistance of several megaohms between the inverting input (pin 3) and the common wire or +9 V power wire.

Next, tune the receiver to the frequencies of the radio stations. This can be done by applying RF voltage from a standard signal generator through a communication loop to a magnetic antenna, or simply by receiving radio signals. Tuning starts with the longest radio station (549 kHz). By rotating the trimmer of the L2 coil, they find the station by a characteristic whistle and, by rebuilding the local oscillator in the direction of lowering its height, they achieve frequency capture by the PLL system (the beats of the audio frequency disappear, and the transmission is heard cleanly, without distortion). The input circuit is adjusted by capacitor C8 according to the maximum reception volume. Similarly, the receiver is tuned for other positions of the switch SA1, but the coil trimmer L2 is no longer touched (the local oscillator frequency is set by trimmer capacitors C9, C10 and C12).

In the presence of interference of the local oscillator signal to the magnetic antenna, the tuning of the receiver is complicated. The fact is that the phase of the pickup voltage is unpredictable and, moreover, depends on the setting of the input circuit. Synchronously detected in the mixer of the PLL system, the pickup voltage shifts the frequency of the local oscillator, so the settings of the input and heterodyne circuits are interconnected. This harmful effect practically does not manifest itself if the voltage of the received signal at the magnetic antenna is greater than the interference voltage.

Author: V.Polyakov, Moscow

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