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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Bidirectional frequency scanning in a VHF radio receiver based on the TDA7088T chip. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The typical circuit for switching on the TDA7088T microcircuit provides only unidirectional scanning "up" over the operating frequency range. The article proposes a simple way to introduce a "down" scan function into the radio receiver on this chip. The TDA7088T chip and its analogues are widely used in inexpensive portable VHF FM radio receivers, radio amateurs also use it in their designs [1, 2]. A distinctive feature of this chip is the ability to automatically tune to a station by scanning the range. In addition to the purely constructive external difference between a receiver with automatic tuning and a traditional receiver with manual tuning (two buttons instead of a variable capacitor knob), this solution has an important advantage: with a sufficient signal level at the receiver input, tuning to the selected station is automatically maintained using the auto-tuning system built into the microcircuit frequency (AFC). However, there is a certain drawback - in the typical scheme for switching on this microcircuit [3], in the automatic tuning mode, only unidirectional scanning "up" in the range is possible. As a result, it is inconvenient to use such a receiver. For example, in Moscow, several dozen radio stations operate in the VHF-2 band (87,5 ... 108 MHz). Therefore, in order to tune in to a neighboring radio station operating at a lower frequency, it is necessary to reset the setting to the beginning of the range with the "Reset" button and only then, successively pressing the "Scan" button several dozen times, tune in to the required radio station. Often, when this button is pressed briefly, the receiver does not have time to rebuild, and the AFC system captures the same station again. If you press too long or if the signal is weak, the receiver may "skip" the radio station, and then the tuning process has to be started again. To eliminate this shortcoming, it is proposed to exclude automatic tuning and instead introduce manual tuning using a variable resistor [4]. In this case, the tuning at the station turns out to be so sharp that it requires the use of a multi-turn variable resistor, and since pin 16 of the microcircuit remains free, the AFC system no longer “holds” the selected station. Therefore, the requirements for the stability of the local oscillator frequency, and hence the supply voltage, increase. The reference data for the TDA7088T chip [3] also provides a circuit with manual tuning by a variable capacitor and an AFC system on an additional varicap, however, in this case, it is advisable to provide for the possibility of disabling this system. It should be noted that VHF receiver circuits with analog range scanning and subsequent frequency capture have already been offered, including before the widespread use of the TDA7088T chip. A semi-automatic receiver tuning system was proposed in [5], and the practical design of a rather complex VHF receiver with such a tuning system was proposed in [6]. In [7], a description of the system for automatic tuning of a VHF receiver is given. These designs provide for bidirectional scanning, but they are difficult to implement and are not suitable for upgrading the receiver on the TDA7088T chip.
At the same time, it turned out to be easy to introduce the ability to scan "down" the range into such a radio - for this it was necessary to add a few details to it. The figure shows a fragment of the receiver circuit to be finalized. The numbering of parts corresponds to the scheme of the "Posson" receiver [1], the newly introduced elements are shown by thickened lines. The printed conductor connecting pin 16 of the TDA7088T chip and the pin of the SA1 button (the former "Reset" button) with the junction point of the capacitor C13 and resistor R2 must be carefully cut and the VD1 * diode soldered into the gap - cathode to the button and chip. The location of the conductor break is shown in the diagram with a cross. After this refinement, the SA1 button will act as a "down" scan button. When you press it, the capacitor C13 is slowly discharged by the reverse current of the diode VD1 'and the varicap VD1, as well as through its own leakage resistance and the leakage resistance of the capacitor C5. The voltage on the varicap gradually decreases, its capacitance increases, and the tuning frequency of the receiver decreases. Thus, the tuning of the receiver when scanning "down" is semi-automatic, that is, the button must be held until it tunes in to a radio station. After that, you can release it to stop at the desired radio station, or hold it down to continue scanning. The operation mode of the "Scan" button does not change after the modification. Capacitor C1' is installed to suppress interference induced on the high-resistance output circuit 16 of the microcircuit when the button is released. The interference voltage can be rectified by the VD1 'diode, which can "knock down" the tuning of the variable capacitor), this solution has an important advantage: with a sufficient signal level at the receiver input, tuning to the selected station is automatically maintained using the built-in automatic frequency control system (AFC). However, there is a certain drawback - in the typical circuit for switching on this microcircuit [3], only the receiver is possible in the automatic tuning mode. If this button is located on the receiver board, it is not necessary to install this capacitor. The author modernized the souvenir receiver, made in the form of a reduced copy of a personal computer connected by a cable to a separate unit in the form of a reduced "mouse" manipulator, on which two settings buttons were located. In this case, the installation of capacitor C1' was necessary. If desired, you can leave the unidirectional scan "up" in the range. As a result, it is inconvenient to use such a receiver. For example, in Moscow, several dozen radio stations operate in the VHF-2 band (87,5 ... 108 MHz). Therefore, in order to tune in to a neighboring radio station operating at a lower frequency, it is necessary to reset the setting to the beginning of the range with the "Reset" button and only then, successively pressing the "Scan" button several dozen times, tune in to the required radio station. Often, when this button is pressed briefly, the receiver does not have time to rebuild, and the AFC system captures the same station again. If you press too long or if the signal is weak, the receiver may "skip" the radio station, and then the tuning process has to be started again. To eliminate this shortcoming, it is proposed to exclude automatic tuning and instead introduce manual tuning using a variable resistor [4]. In this case, the tuning at the station turns out to be so sharp that it requires the use of a multi-turn variable resistor, and since pin 16 of the microcircuit remains free. It should be noted that VHF receiver circuits with analog range scanning and subsequent frequency capture have already been proposed, including before the widespread use of the TDA7088T chip. A semi-automatic receiver tuning system was proposed in [5], and the practical design of a rather complex VHF receiver with such a tuning system was proposed in [6]. In [7], a description of the system for automatic tuning of a VHF receiver is given. These designs provide for bidirectional scanning, but they are difficult to implement and are not suitable for upgrading the receiver on the TDA7088T chip. At the same time, it turned out to be easy to introduce the ability to scan "down" the range into such a radio - for this it was necessary to add a few details to it. The figure shows a fragment of the receiver circuit to be finalized. The numbering of parts corresponds to the scheme of the "Posson" receiver [1], the newly introduced elements are shown by thickened lines. The printed conductor connecting pin 16 of the TDA7088T chip and the pin of the SA1 button (the former "Reset" button) with the connection point of the capacitor C13 and the resistor R2, it is necessary in the receiver to quickly tune to the beginning of the range by adding the SB2 '"Reset" button connected in parallel to the capacitor C13 . You can also add the SB 1 'button for a quick transition to the end of the range by connecting it in series with a resistor R1' with a resistance of 10 ... 100 kOhm between the lower (according to the circuit) output of the capacitor C13 and the common wire of the receiver (minus power supply). If you install a resistor R1' with a resistance of several tens of megaohms, then when you press the button SB1', a slow scan "up" in the range will be performed (similar to what happens "down" when you press the button SA1). Diode VD1' - any low-power rectifier (series KD102, KD103) or pulsed (series KD521, KD522) silicon diode, it is only necessary that its reverse current be as small as possible. Capacitor C1 '- ceramic, for example, K10-17 or surface-mounted K10-17v (or imported), resistor R1 * - any low-power. Buttons can be used any small-sized with self-return. The diode, capacitor and resistor are mounted directly on the board, and additional buttons are mounted on the case. The upgraded receiver does not require adjustment. If the downscan is too fast, it may be necessary to select a VD1' diode with less reverse current. It is possible to conclude whether the installed diode is suitable only after it cools down after soldering, since the heated diode has an increased reverse current. In addition, it is recommended to check the operation of the receiver before installing the board in the case, avoiding light on the diode VD1' and the varicap VD1 (if they are in transparent glass cases). Light increases the reverse current and the downward scan may be too fast. If you cannot find a suitable diode, you can increase the capacitance of capacitor C13 by installing an additional capacitor in parallel with it. After the proposed revision, it became much more convenient to use the receiver. Literature 1. Dahin M. Receivers with automatic tuning. - Radio, 2001 No. 6, p. 33, 34
Author: P. Maksimov, Moscow; Publication: radioradar.net
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