Encyclopedia of radio electronics and electrical engineering / radio reception
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For several years, small-sized radio receivers (manufactured by the People's Republic of China) with automatic (scanning) tuning to received stations have been sold on the Russian market with various trade names (see "Radio", 2001, No. 6, pp. 33, 34). The receiver has three controls: a scan enable button, a reset button to the lower frequency range, and a volume control. Despite their simplicity and reliability, receivers have a number of disadvantages, the main of which should be called low sensitivity. However, this drawback has serious consequences - with a weak signal, the receiver spontaneously tunes to another radio station.
To increase the sensitivity of such receivers, several solutions have already been proposed on the pages of the magazine for the introduction of a radio frequency amplifier. But there is another way to reduce the consequences of the noted disadvantage: to exclude automatic tuning and use the manual one, traditional for most receivers. This method of refinement allows you to simultaneously solve another problem - it becomes possible to tune not only "up" in the frequency range (as laid down in the basis of the receiver), but also "down", which is an undoubted convenience.
Naturally, when finalizing, it will be necessary to disable the automatic tuning circuits (however, if desired, they can be restored, they are not completely dismantled). A variant of a circuit solution based on a "Posson" type receiver is shown in the figure.
The exclusion of the scanning mode is achieved by turning off pin 16 of the DA1 chip (enough to cut the printed track close to the soldering of this pin) - shown in the diagram by a dashed line. From the upper variable resistor R3 (volume control), disconnect all circuits except ground, and connect the circuits disconnected from it to each other. In this case, the receiver will only be set to maximum volume. Connect the upper output of the variable resistor to the plus of the power source (it can be near output 4 of the DA1 microcircuit), and the middle one to the cut point of the printed conductor from the side of the resistor R2.
After this refinement, the resistor R3 changes the voltage on the receiver tuning varicap, and thereby a change in the receive frequency will be achieved. And now, even with the weakest signal of the received radio station, the receiver "holds" the tuning frequency. And the absence of a volume control at a strong signal level can be compensated for by the spatial location of the receiver itself (changing the direction of the receiver antenna) or by shifting the headphones relative to the listener's auricle.
Author: V. Kvashenko, Moscow
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