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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING FM receiver at 144 MHz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Using foreign microcircuits available today on the Russian market, it is possible to manufacture a simple but high-quality VHF FM receiver for receiving amateur radio stations in the 2-meter range. The prototype of this receiver was created by Austrian radio amateurs ten years ago for the flight of an Austrian cosmonaut on the Mir orbital station (project OE-AREMIR). The receiver is well developed, as it was released in the form of a set in a small series (several hundred copies) by one of the technical colleges in Vienna. The sets were distributed among schools, giving schoolchildren the opportunity to join amateur radio (the receiver had to be assembled and adjusted!), And the great mystery of space exploration (the Austrian cosmonaut worked on the air for his young compatriots). The receiver is a super heterodyne with double frequency conversion, which allows you to receive signals from amateur VHF FM radio stations in the band 145...146 MHz. The "heart" of the receiver is the MC3362R chip, which contains two mixers with local oscillators, a limiting amplifier for the second intermediate frequency, an FM demodulator and a noise suppressor. Double frequency conversion makes it possible to obtain good suppression of the image reception channel in the VHF bands. At a minimum, for a functionally complete receiver, only an audio frequency amplifier needs to be added to the node on this chip, but to implement high sensitivity (this is required in amateur radio), a high frequency amplifier is still needed. The circuit diagram of the receiver is shown in fig. 1.
With the exception of connectors (antenna, power supply), controls (switch, variable resistors) and a dynamic head, all its elements are placed on one board. The signal from the antenna connector mounted on the receiver body is fed to the input of the ANT board and amplified by the UHF cascade, made on a low-noise double-gate field-effect transistor VT1. The resonant frequency of the input circuit is set by the coil trimmer L1, and the output circuit is set by the trimmer capacitor C18. From part of the turns of the UHF output circuit, the signal is fed to the input of the first mixer of the DA1 chip. The input of the microcircuit is symmetrical, therefore its second input (pin 24 of the microcircuit) is connected to a common wire through capacitor C6. To simplify the receiver, the first local oscillator is made according to a scheme with parametric frequency stabilization. It is quite enough to monitor the work of amateur radio stations and to conduct short radio communications. If necessary (and with the appropriate capabilities), a more complex local oscillator or even a frequency synthesizer can be introduced into the receiver. The frequency of the first local oscillator is determined by the inductance of the coil L3, the capacitance of the capacitor C8 and the capacitance of the varicap, which is part of the DA1 microcircuit. The control voltage for this varicap is applied to pin 23 of the microcircuit. To ensure good suppression of the image channel, the first intermediate frequency is chosen sufficiently high - 10,7 MHz. The ceramic filter Z1 for the first IF (connected between the first and second mixers) is used from broadcast VHF FM receivers and has a relatively wide bandwidth. The second local oscillator has quartz frequency stabilization. It uses a Z3 resonator at a frequency of 10,245 MHz, which corresponds to the second intermediate frequency of 455 kHz. Ceramic filter Z2 at 455 kHz (connected between the second mixer and limiting amplifier) - from broadcast AM receivers. The Z4 filter at a frequency of 455 kHz, which is part of the demodulator, is an ordinary oscillatory circuit, and is shown in the diagram as a filter only because in the original design the capacitor is not installed on the board, but is located inside the coil screen. An IF filter from a miniature AM broadcast receiver is used here. The audio frequency output signal is taken from pin 13 of the DA1 chip and through the volume control (R24, located outside the board) is fed to the ultrasonic frequency converter on the DA2 chip. The LM386 chip is very popular in amateur designs of foreign radio amateurs. It is miniature (made in an eight-pin DIP package), has an output power of 0,5 W and requires a minimum of body kit. Moreover, it has a control input (pin 8), which prohibits the signal from passing to the output of the microcircuit. This allows, in this case, to organize the operation of the noise suppressor without problems and unnecessary elements. The noise suppressor control signal from pin 11 of the DA1 chip is fed to the key transistor VT2 connected to pin 8 of the DA2 chip. The level of operation of the squelch is regulated by applying voltage to pin 10 of the DA1 chip. It is installed with a resistor R19 (located outside the board). A dynamic head with a power of 0,5 W and a resistance of 4 ohms is connected to the LS output of the board. The supply voltage (+9 V) is applied to pins A, B and C of the board, the GND pins are connected to the common wire of the receiver. The remaining pins of the board (with digital designations) are used to connect resistors and switches that are outside the board. In order not to create confusion, the original designations of these conclusions have been retained, coinciding with their numbering on the original printed circuit board. For the same reason, a somewhat complicated circuit for controlling the voltage supplied to the varicap of the first local oscillator has been retained. Switch S1 selects one of two reception options: with tuning within the selected frequency band (variable resistor R23) or reception at a fixed preset frequency. The last version in the original design "keeps in memory" the frequency of the amateur radio station of the Mir orbital complex. When repeating the receiver, the fixed receive frequency may be selected differently. For example, this could be the general calling frequency for radio amateurs in your area. A fixed receive frequency is set with a tuning resistor R18. Variable resistor R21 - fine tuning to the frequency of a working radio station. It functions in both reception options. Trimmer resistor R22 serves to "lay" the reception band (setting its lower limit). Switch S1 in the diagram is shown in the position corresponding to tuning in the selected frequency band. The power supply of the DA1 chip is stabilized by the DA3 integral stabilizer. The printed circuit board of the receiver and the arrangement of parts on it are shown in fig. 2.
When repeating the design, it may need to be slightly adjusted if it is not possible to obtain trimmed resistors and trimmed capacitors with the same mounting dimensions as used in the original design. It is even less likely that a radio amateur will be able to get inductors identical to those used by the creators of the receiver. However, such improvements to the board should not cause difficulties for a radio amateur of average qualification. Inductors L1 and L3 - from VHF FM receivers. Their parameters are not given in the description of the receiver. Both of these coils are placed in screens (they are not shown in Fig. 1). Coil L2 - frameless. It contains 5 turns of silver-plated copper wire wound on a blank with a diameter of 6 mm. The tap is made from the second turn, counting from the "hot" (lower according to the diagram in Fig. 1) end of the coil. Filter Z1 is SFE 10.7MA and filter Z2 is CFW 455U. Instead, you can use the appropriate filters of domestic production, but the value of the second IF will be 465 kHz. This must be taken into account when choosing the frequency of the Z3 quartz resonator. Since the bandwidth for the first IF is relatively wide (about 100 kHz), and for the second IF is not more than 10 kHz, the requirements for the accuracy of choosing its frequency are relatively low. For the Z4 filter, you can use the coil from the IF circuit of the transistor receiver by installing a surface-mounted capacitor under the board. The capacitance of this capacitor must ensure resonance with the used inductor at a frequency of 455 or 465 kHz (depending on the operating frequency of the filter Z2). The rest of the details are normal. It may only be necessary to select the capacitor C8 according to the temperature coefficient of capacitance to ensure the least instability of the first local oscillator. As a starting capacitor, a negative TKE M330 capacitor is recommended. The receiver is assembled in a case, which is soldered from double-sided foil material 2 mm thick. The holes for the conclusions of parts that are not connected to a common wire are countersunk from the side where the parts are located. Holes that are not used for installing parts and not countersunk from the side of their installation are designed to connect common wires on both sides of the board (solder short pieces of tinned wire). The same function is also performed by the terminals of elements (microcircuits, transistors, screens, etc.) connected to a common wire, which are soldered on both sides of the board. The body of the receiver is made of double-sided foiled material 2 mm thick. Drawings of body parts are shown in fig. 3. They are connected by soldering. Threaded corners are soldered in the corners of the case to secure the back cover.
The photo (Fig. 4 and Fig. 5) shows the view of the receiver from the front panel and the placement of the board in the receiver case.
The knot in the lower part (Fig. 5) is homemade (not included in the set). This is a prescaler by 10 and buffer stages. It provides control of the operating frequency of the receiver using a relatively low frequency external frequency counter.
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