Increasing the dynamic range and sensitivity of the radio station ALAN-100+. Scheme, description

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Increasing the dynamic range and sensitivity of the radio station ALAN-100+. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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One of the most common radio stations "ALAN-100+" has a number of shortcomings that are well known to many users. Meanwhile, a simple refinement of this station will help increase the sensitivity of its receiver and expand the dynamic range.

For a noticeable increase in the dynamic range, you can, for example, replace the broadband piezoceramic filter for the first IF with a narrow-band quartz filter, as recommended in the article "Refining CB radio stations" published in Radio. 1996, No. 12 of 1996. One of the reasons for the low dynamic range of the radio is also that the second mixer uses a microprocessor reference oscillator signal through a small capacitor and resistor. This was done in order to exclude the effect of the mixer on the reference oscillator. But such a circuit design has led to the fact that in most cases the local oscillator signal level for the second mixer is very low. As a result, the mixer gain is not high, which often leads to a decrease in sensitivity, and the dynamic range of the receiver is narrowed. If the radio station is redesigned to work in Russian and European standards, then during mode switching, as a rule, the signal level of the reference local oscillator changes, and hence the sensitivity when operating in mode "0" or "5".

Increasing the dynamic range and sensitivity of the radio station ALAN-100+

To expand the dynamic range and at the same time increase the mixer transfer coefficient, which in some cases allows you to increase the sensitivity, can be done in a simple way - to raise the level of the local oscillator signal. However, this cannot be done by increasing the capacitance of the capacitor C72 or reducing the resistance of the resistor R95. Installing a separate local oscillator is inconvenient, since it requires a quartz resonator, and besides, there are difficulties with the placement of additional elements. In addition, the newly introduced fixed local oscillator may degrade the operation of the radio, because when switching the "0" or "5" mode, the value of the second IF will change. And this will lead to noticeable distortion of the received signal.

The way out of this situation is to install a simple buffer amplifier, which is best done on a field effect transistor. It amplifies the reference oscillator signal. entering the mixer, and at the same time reduces the influence of the mixer on the generator. A diagram of such an amplifier is shown in the figure. Transistor VT1 is connected according to the source follower circuit.

Amplifier parts are installed on the radio station board from the print side as follows. The PCB track from the B95 resistor to the jumper wire is cut in two places like this. to divide it into about three parts. In place of the first cut (counting from R95), the transistor VT1 is soldered, and in place of the second, capacitor C1. Then the resistors R1 and R2 are soldered, and R3 is installed instead of the jumper wire.

In the amplifier, field-effect transistors KP303 with letter indices G and D can be used, preferably in a plastic case, since they have smaller dimensions, or KP313 with indices B and C. All resistors are MLT. capacitor C1 - KM. CT. KD

The adjustment amplifier does not require, but it is desirable to select the optimal signal level at its input. Instead of the resistor R95 (according to the radio station scheme), a small-sized variable resistor with a resistance of 22 ... 47 Ohm is temporarily installed. for example SDR-19 By rotating the rotor of this resistor, they achieve maximum sensitivity when receiving a weak signal. Then measure the resistance of the variable resistor with an ohmmeter and replace R95 with a constant of approximately the same rating.

Author: I. Nechaev, Kursk

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