Detector receiver with UZCH. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Detector receiver with UZCH. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The receiver offered to the attention of beginner radio amateurs allows you to listen to the signals of powerful radio stations in the LW, MW and even KB bands on high-impedance phones. The receiver is simple, does not require scarce parts, and with proper assembly and serviceable parts, it starts working immediately, without needing to be adjusted. Nevertheless, after spending some time and effort, it is possible to improve the parameters of the receiver, while at the same time deepening one's knowledge of radio engineering.

For the receiver to work, grounding or a counterweight and an indoor or outdoor antenna with a length of 2 ... 10 m are required. A longer outdoor antenna can also be used, thereby improving the reception of distant stations. Powerful local radio stations are also listened to by a magnetic antenna. The sensitivity of the receiver from the antenna input is no worse than 5 mV.

The receiver is powered by one galvanic cell of any type with an EMF of 1,5 V. The current consumption does not exceed 0,35 mA, therefore it is permissible to use an old and already expired element with increased internal resistance in other devices, provided that its voltage at load receiver will be at least 1,2 V.

The receiver circuit is shown in the figure. The signal from the antenna goes directly to the L1C1 oscillatory circuit, which is necessary for tuning to the frequency of the received radio station and attenuating the signals of other stations. It determines the selectivity of the receiver. Next, the radio frequency (RF) signal is fed through a small coupling capacitor C2 to a half-bridge detector assembled on two silicon diodes VD1, VD2. Diodes with the help of resistor R1 are ajar with a small initial bias current. This increases the sensitivity of the receiver to weak signals, since the operating point of the diodes is in the section of their current-voltage characteristic (CVC) with maximum curvature, and it is precisely due to the curvature of the CVC that detection occurs.

Detector receiver with ultrasonic frequency converter

After detection, the already audio-frequency signal is filtered from RF ripples by blocking capacitor C3 and, through an isolation capacitor C4, is fed to a three-stage audio frequency amplifier (UHF) assembled on transistors VT1 - VT3 according to a circuit with a direct connection between the stages. The amplifier is covered by negative feedback (NFB) for direct current through the integrating circuit R2R5C5. The transistors of the amplifier operate in microcurrent mode, which contributes to its efficiency and stability. Despite the low power consumption, the UZCH gain exceeds 1000. The load of the output stage is high-resistance (3,2-4,4 kOhm) BF1 headphones.

The coil of the oscillatory circuit L1 can be used ready-made (a magnetic antenna from any broadcasting receiver will do) or made independently. A segment of a ferrite rod of a magnetic antenna 50 ... 100 mm long will serve as a magnetic circuit in the LW and MW ranges. The DV coil contains about 200 turns of any thin wire, and the SV coil contains 60 turns of a stranded litz wire. Winding is carried out in one layer turn to turn on a tubular frame made of waxed paper or cardboard.

It is desirable to make the KB coil with the maximum quality factor. To do this, you need to find or glue a cylindrical frame with a diameter of 30 mm and wind seven turns of copper wire with a diameter of 1 mm on it with a step of 2 mm. It is permissible to use a winding wire of the PEL brand, and the insulation does not need to be removed.

A variable capacitor (KPI) will fit any type, preferably with an air dielectric. It is advisable to connect sections of a double or triple KPI in parallel to increase the frequency overlap. The remaining capacitors are ceramic, of any type. Resistors can also be any.

Mount the receiver on a suitable mounting strip with petals or on several strips fixed to a common base. The location of the elements is uncritical and installation takes no more than half an hour with a certain skill. Nevertheless, it is still better to follow some rules: do not use long connecting wires, place the detector near the oscillatory circuit and the UZCH input, and the UZCH output away from the input.

Setting up the receiver begins with checking the UZCH. Multimeter check the voltage on the collector of the transistor VT3. With a supply voltage of 1,5 V, it should be 0,7 ... 0,9 V. Otherwise, it is necessary to carefully check the installation, the serviceability of transistors, resistors, telephones, the absence of leakage currents in capacitors and correct the defect. Next, with one probe of the multimeter, touch the base of the transistor VT1. When the amplifier is working properly, the noise and hum of the AC should be heard in the phones.

The health of the detector diodes VD1 and VD2 is checked by measuring the voltage drop across each diode. It should be about 0,5 V. When the antenna and ground (counterweight) are connected, the noise of the air will be heard in the phones, and when the circuit is tuned using the KPI to the frequency of the radio station, it will be transmitted. It is very useful to choose the resistor R1. With an increase in its resistance to 33 ... 47 kOhm, the sensitivity of the receiver increases several times, and with an excessive increase in resistance, it decreases. In this experiment, it should be borne in mind that when the resistor R1 is disconnected or when the UZCH input is disconnected from the detector, the receiver self-excites. More about this is written in the article by V. Polyakov "Radio engineering calculations - 12" ("Radio", 2003, No. 8, pp. 51, 52).

The desired range of received frequencies is set by selecting the number of turns of the L1 coil, guided by the signals of the received radio stations. Some stations may be received so loudly that the UHF will distort the sound. In this case, it is useful to weaken the connection with the antenna by connecting it through a small capacitor (10 ... 20 pF). Note that when the connection of the circuit with the antenna and with the detector is weakened (reducing the capacitance of the mentioned capacitor, as well as C2), the selectivity of the receiver increases. It is even better to connect the antenna through a KPI with a maximum capacitance, for example, 180 pF. This will make it possible to regulate the connection with the antenna in the process of receiving radio stations, achieving the optimal volume and the desired selectivity of the receiver.

Author: S. Kovalenko, Kstovo, Nizhny Novgorod Region

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