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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Receiver range 0-18 MHz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The receiver, the circuit of which is shown in the figure, has parameters that do not differ from the parameters of the so-called "all-band" transistor receivers of the past, which received LW, MW and HF frequencies up to 20 MHz with amplitude modulation. Due to the "low budget" of this circuit, the tuning indicator had to be abandoned and the whole design was made as simple as possible. Nevertheless, the name "Mini All-Wave Receiver" would be quite applicable to this scheme.
In the range up to 30 MHz, most stations are mainly at frequencies below 18 MHz. It is entirely possible to build a receiver to receive them with relatively simple circuitry. The simplicity of the circuit is its main advantage, but this does not mean that the parameters of the circuit will be bad. This receiver is a superheterodyne with a single frequency conversion, and tuning to any station in the range of 0-18 MHz can be done without any additional switching. The circuit uses a high intermediate frequency (IF). As a result, the frequency of the image channel is very high, and the image channel can be easily suppressed. In addition, the ratio of the maximum frequency of the GPA to its minimum frequency is relatively small. The circuit's input circuit contains a NE612 IC mixer and oscillator (IC1). The chip generator uses a Colpitz-type circuit and the frequency setting is made by a double varicap (D1). The mixer is followed by a crystal filter with a center frequency of 45 MHz and a bandwidth of 15 kHz. The bandwidth is a bit too much for AM, but the advantage of such a filter like 45M15AU is its low price. With an intermediate frequency of 45 MHz and a frequency of received signals of 0 ... 18 MHz, the GPA frequency should be equal to IF + F0 = 45 ... 63 MHz. The mirror channel will be 90 MHz higher in frequency than the received signal and will be in the 90-108 MHz range. A single coil connected in series with the antenna provides ample suppression of the image channel. Behind the IF filter is an LC circuit to suppress the fundamental frequency of the 45M15AU filter (this filter operates on the third harmonic). A logarithmic detector is used as an IF, the main advantage of which is the presence of a minimum number of external components. The detector is an AD8307 (IC2) with a sensitivity of about -75 dBm, which is about 40µV. Considering the mixer gain (about 17 dB), the receiver sensitivity is about 5 µV. Due to the logarithmic response of the detector, the receiver does not need AGC (Automatic Gain Control). Next is a simple LC filter to suppress the fundamental frequency of the IF and noise. Behind the filter is an ultrasonic frequency converter with a gain of about 200. This is enough to work on a loudspeaker. Volume control is carried out by potentiometer P1. In order to tune into a radio station, a multi-turn potentiometer would have to be used in a receiver with such a large frequency overlap. But since this design is low budget, two potentiometers are used - for coarse and fine adjustment. The transistor, switched on as a current generator, provides a constant voltage of about 1V to the fine adjustment potentiometer (P2). The coarse adjustment potentiometer (P3) has a negligible effect on the voltage across potentiometer P2, but allows the voltage across both potentiometers to be varied. In this case, the coarse adjustment potentiometer can be used to select a "window" within which the fine adjustment potentiometer P2 can be adjusted. The ratio of the setting ranges P2 and P3 is about 1:5. If you need to change this ratio, say, to 1:10, then for this you need to increase the resistance of the resistor in the emitter circuit from 4.7kOhm to 10kOhm. Because the frequency of the GPA must be stable, then only for the mixer and the GPA voltage stabilization is used. The supply voltage for the AD8307 microcircuit is reduced with the help of a ballast resistor to the required value, while the UZCH is powered directly from the battery. The current consumed by the circuit in the absence of a signal is about 20 mA and at an average volume increases to 50 mA. The circuit is operational when the supply voltage drops to 6.5 V. This means that the 9V battery will last for a long time. Setting up the circuit is quite simple. Adjustment potentiometers should be set to the lower position according to the diagram. Using the trimmer capacitor C7, a frequency of 50 Hz is obtained from the network. This means that the receiver frequency is 0 Hz. In addition, you can also tune in to a powerful LW station, as the lowest frequency that the receiver will receive. The receiver needs at least a 50cm telescopic antenna to make the receiver portable. With such an antenna, dozens of stations will be heard, especially in the evenings when radio wave propagation is good. A wire a few meters long however allows for increased sensitivity, especially during the day, but this is not particularly necessary. Author: Gert Baars, Netherlands
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