Electronic tuning of the receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic tuning of the receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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I propose a diagram of a device for electronic tuning of the receiver in the VHF band. In my opinion, it is interesting for its simplicity, small size, and is convenient when building VHF receivers on popular microcircuits (K174XA34, CXA1238, etc.). Usually, you have to output a variable multi-turn resistor to the front panel to adjust the range. This resistor wears out quite quickly during operation, which makes tuning difficult later.

Electronic tuning of the receiver

The principle of operation of the proposed device is smooth adjustment using two buttons - "+" and "-". At the same time, the setting voltage gradually increases or decreases at the output of the device within the range of 2 ... 4,5 V. When the control buttons are released, the selected voltage is stored. The role of the regulating element is performed by the source follower on the transistor VT1. A storage capacitor C1 is connected to its input. When the power is turned on, the capacitor is not charged, and the gate voltage of the FET is equal to the supply voltage. At the output, the voltage is equal to the maximum value, this corresponds to the beginning of the VHF band. If you press the "+" button, the storage capacitor C1 starts charging, the transistor VT1 gradually opens, as a result, the control voltage Uynp decreases. When the "-" button is pressed, the capacitor C1 is discharged, and the control voltage increases.

The entire process of tuning in the range takes approximately 8...10 s. Reworking the receiver is to remove the multi-turn resistor. The control output in the receiver from the variable resistor engine is connected to the output of the device (Uynp).

It is desirable to power the device with a stabilized voltage. Therefore, it is advisable to use the device in stationary receivers with mains power supply. With a supply voltage of 12 V, the adjustment of the device consists in setting the tuning limits in the VHF band with resistor R4. The value of R4 is 27 ... 33 kOhm. With reduced power, reduce the resistance R3 to 12 kOhm and also select the range limits with the resistor R4. Resistor R2 can be made up of several resistors connected in series (for example, 3 pieces of 3,3 MΩ each). Particular attention should be paid to the capacitor C1. It can also be made up of several connected in parallel (for example, 0,47 uF and 0,15 uF). It is desirable to use film or metal film, which have greater stability (K73-17, etc.). Otherwise, the setting will "float away". Buttons "+", "-" - type PKN 125.

Author: S. Malyshev

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