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The control unit of the tuner "Laspi-003-stereo" allows you to make a preliminary fixed tuning for five programs in the range of 65,8 ... 73 MHz and a push-button selection of any of them.

A few years ago, when only five or six radio stations operated in this range, the carrier frequencies of which differed by at least one megahertz, the tuner tuning system seemed very convenient and functioned reliably. But now, when in Moscow, for example, 1 radio stations are already operating in the VHF-13 band, the situation has changed. Many of them became difficult to tune into, and the presets were no longer stable. It is clear that further expansion of the network of radio stations (according to some reports, their number will increase to 20 in the near future) will further worsen the situation.

R. Kunafin's article [1] was once devoted to the solution of this problem. He proposed to limit (halve or triple) the frequency bands covered by each of the tuning resistors - to perform a kind of electronic "stretching" for each of the fixed tuning buttons. Due to such a stretch, it is possible to increase the angle of rotation of the resistor engine, which is necessary for tuning from one station to another, thereby facilitating the use of the tuner when tuning to a radio station. This proposal practically solves the issue of dividing the voltage used to rebuild the varicap capacitance into equal intervals with mutual overlap (0 ... 0,33, 0,17 ... 0,51, etc.).

But equal intervals of tuning voltages with a nonlinear character of the capacitance-voltage characteristic of the used varicap lead to subranges unequal in frequency interval. With a very large number of stations (which is now the case in large cities), there may be more of them on the high-frequency subband, and the tuning advantages may not be realized.

In this case, it seems more correct to divide the entire VHF band into intervals equal in frequency. Schematic diagram of the tuning unit of the "Laspi-003-stereo" tuner, which implements this option, is shown in the figure. Resistors R3, R5, R7, R9, R11, R16 and R17 (numbering according to the factory scheme) are standard, left unchanged. Resistors with three-digit indices (R101-R112) - additional, designed to form adjustable voltages in order to obtain frequency tuning in subbands at equal intervals.

With this refinement, the frequency subrange on each fixed-tuning button is 3 ... 4 MHz instead of the previous 7,2 MHz.

When reworking, you can use MLT, C2-6, C2-23 or C2-33 resistors with a power of 0,125 W. All additional resistors are hinged mounted directly on the terminals of the tuning resistors.

The proposed version of the device practically does not require adjustment. It turned out to be sufficient to check the possibilities of restructuring and setting the desired five stations for a fixed choice.

The finalization of the settings block of the "Laspi" tuner (any model) seems to be very useful. But the options proposed by R. Kunafin and myself are only part of the problem described above. It can be completely solved only by improving the selectivity of the tuner (increasing the squareness of the frequency response of the IF path), by replacing the lumped selection filter with a SAW filter. A cardinal solution would be to replace the voltage-controlled VHF unit with a frequency synthesizer-controlled unit. This would make it possible to abandon unstable variable preset resistors, achieve the necessary stability of the tuning frequency, expand the upper limit of the received range from 73 to 74 MHz, increase the number of fixed settings, introduce the VHF-2 range (87,5 ... 108 MHz), in which today there are a lot of radio stations (in Moscow there are, for example, 20). But this, as they say, is a topic for a separate discussion, which, judging by the publications [2], is already beginning.

Literature

1. Kunafin R. Improving the accuracy of VHF tuner tuning. - Radio, 1996, No. 12, p. 33.
2. Gorodetsky I. Increasing the number of fixed settings in the tuner "Laspi-001stereo". - Radio, 1996, No. 9, p. 24, 25.

Author: V.Brylov, Moscow

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