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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Universal VHF FM receiver (70-150 MHz). Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception A few years ago, the author was faced with the task of creating a miniature mobile single-channel receiver capable of tuning in a wide frequency range and receiving both wideband and narrowband FM, either by switching, or, in extreme cases, with minimal alterations. The study of technical descriptions and experiments with single-chip FM receivers based on K174XA34 and the like showed the complete failure of the latter for use in serious designs - low sensitivity and selectivity, the inability to control the bandwidth, the problematic use of an external stable local oscillator, etc. Then the author looked through almost all the magazines "Radio" and "Radio Amateur" for the previous years, hoping to find something ready. Unfortunately, as expected, nothing ready to be found. However, the constructions [5,8,9] aroused the greatest interest. Moreover, the most optimal design looked like the following - HF and converter from [9], IF and detector from [5], and HPF and VLF from [8]. At the same time, the design turned out to be rather cumbersome. The next stage of the search was a review of the Internet sites of chip manufacturers. It was here, on the MOTOROLA website, that the author discovered [13] a receiver circuit that actually included all the ideas of the above designs. The scheme of this receiver, with minor additions and explicit "blunders" excluded, is shown in Fig. one.
Having creatively worked on the above scheme, the author implemented the following version of it (Fig. 2). The receiver circuit is built taking into account the recommendations of [13] and other designs listed and not listed in the list of references, as well as the theory set forth in [1]. It is worth noting that the concept of universal is probably not entirely correct. Rather, the receiver can be called the base, because. the design makes it easy to add a frequency synthesizer and a second frequency conversion, turning it into a decent communication receiver. For a more detailed acquaintance with these issues, I suggest downloading the necessary documentation from the MOTOROLA website [11,12,13]. In passing, I note that it is possible to make the receiver narrow-band without resorting to the second frequency conversion, which will be discussed later. The receiver can be rebuilt in the range from 70 to 150 MHz without changing the values of the tuning elements. The real sensitivity of the receiver is about 0.3 μV. Supply voltage - 9 volts. It should be noted that the supply voltage of the MC3362 is from 2 to 7 volts, and the MC34119 is from 2 to 12 volts. Therefore, the MC3362 is powered through a 78L06 voltage regulator, with an output voltage of 6 volts.
The input stage of the receiver is made according to the traditional resonant circuit. The signal from antenna A1 through the coupling coil L1 enters the input circuit L2. The inductive connection with the antenna was not made by chance, because this is the only way to ensure good matching with various antennas and over a wide frequency range [1,6,7]. To reduce the effect of shunting the L2 circuit by the input circuits, and increase its quality factor, and consequently narrow the bandwidth and increase selectivity, an incomplete inclusion of the circuit was applied. The KP307G field-effect transistor is used as an amplifying element. The specified transistor has a high slope characteristics and acceptable noise performance. The double-gate KP350 has the same characteristics, but it is very afraid of static electricity, and it also requires additional elements to provide bias on the second gate. All other transistors showed worse results both in terms of gain and noise. The amplified signal is allocated on the L3 circuit, which, for the same reasons as L2, has an incomplete inclusion. From the circuit L3, through the coupling coil L4, the signal enters the mixer. Such a scheme provides a minimum mutual influence of UHF and a mixer, increases selectivity, and provides maximum matching with the input stage of the mixer, made according to a differential scheme. The reference frequency is supplied from the internal local oscillator to the mixer. The reference elements of the local oscillator are C7L5 and the built-in varicap matrix, by changing the voltage on which the resistor R6 can be used to slightly tune the frequency. Resistor R5 is designed to create a "stretch". In principle, R5, R6 and C6 can be eliminated by connecting the 23rd leg of the MC3362 to the positive wire, and the restructuring is carried out by elements C7 and L5. From the 20th leg, the local oscillator signal can be applied to the frequency synthesizer, and the control voltage must be applied in this case to the 23rd leg. A separation frequency signal of 6,5 MHz (but it can be 10,7 MHz and 5,5 MHz, this was checked) is fed to the piezoceramic filter Z1 and further, bypassing the first IF and the second converter, to the second IF, limiter and phase detector. From the phase detector, through the high-pass filter on C13R9, which provide a cutoff of frequencies above 5 kHz [2,3], the signal is fed to the LF amplifier, made according to the bridge circuit, on the MC34119 microcircuit. Unlike the 174 series, this amplifier has significant amplification, high resistance to self-excitation, low self-noise, very high efficiency and a small number of add-on elements. The output power into a 20 ohm load is about 0,2 watts. If the receiver is planned to be used as a broadband broadcaster, then I recommend changing the values of C13R9 based on the recommendations of [2,3], or eliminating this circuit altogether. Details and design. Unfortunately, the receiver version was not brought to the "boxed" version. Firstly, this was not required, and secondly, the author is much more interested in the process of "knowledge and creation" than "combing and licking". Therefore, those wishing to repeat this design will have to breed the printed circuit board themselves. By the way, this has to be done even if there is a drawing, because often there are no those elements that the author used. And the scheme is quite simple, so there should not be any difficulties with this. The breadboard that the author used has dimensions of 100x30 mm. and is made of double-sided foil fiberglass, 1,5 mm thick. All parts are located on the side of the printed conductors (there is no need to drill holes), and the second side is used as a screen. How good it is, I can't say. I have a suspicion that this contributes to the appearance of parasitic capacitances. If you look at industrial VHF and UHF units, then for some reason they are all made on one-sided foil. Resistors, capacitors and electrolytic capacitors can be of any type. Trimmer capacitors of the PDA type, but there may be others. Resistor R6 is desirable to use a multi-turn. The contour of the LC frequency detector is taken from an imported receiver (Chinese) and should be green or blue. The capacitance of such a circuit at a frequency of 10,7 MHz is 90 pF. Therefore, for a frequency of 6,5 MHz, an additional capacitance Ca is 150 pF, and for a frequency of 5,5 MHz, 250 pF.[14] The piezoceramic filter Z1 can be of any type. Although the microcircuit is designed for an output impedance of 300 ohms (for 10,7 MHz) and 1,5 kΩ at the input (455 kHz). However, all filters work fine. It is only necessary to note that the filters are different even for the same frequency and have different bandwidths, somewhere around 10-20% of the operating frequency, and therefore the selectivity will be different. In addition, at frequencies of 6,5 MHz and 5,5 MHz, in addition to band-pass filters, notch (suppression) filters are also produced. They are usually marked with one dot, and striped with two. Inductors L2, L3, L5 have the same design. They are wound on frames with a diameter of 5 mm (such frames are used in SKM and SKD TVs of the 3rd and 4th generations), with 0.7 mm silver-plated wire and have 5 turns each. Winding length 6 mm. The coils are arranged vertically. Inside the coils is the core. Brass for upper band operation (140 MHz), or ferromagnetic for lower band operation (70 MHz). The communication coil L1 has 4 turns (turn to turn) with a PEL 0,3 wire at the top terminal L2. The communication coil L4 has 2 turns (turn to turn) with a PEL 0,3 wire at the upper terminal L3. The branch at L2 and L3 is made from the middle. All contours were calculated using [14], based on the following considerations. The winding length is 6 mm, the number of turns is 5 + 1 (an additional turn takes into account the length of the taps and the inductance of the tracks), the winding diameter is 5.5 mm (0.5 mm takes into account the looseness of the winding). After the calculation, we get L=0.13µg. To tune to a frequency of 108 MHz, the capacitances of the capacitors should be as follows: C1=C4=17 pF. The local oscillator operates below the received frequency, and a varicap matrix with a minimum capacitance of about 5 pF is additionally connected to the circuit, hence C5 \u19d 5-14 \uXNUMXd XNUMX pF. The calculated results almost perfectly coincided with practice when taking into account the mounting capacitance of 2-3 pF and the source-drain capacitance of 2 pF. (17 - 3 - 2 \u12d 1 pF. It was this capacitance that C4 and C140 showed.) The limiting frequency of the local oscillator is 150 MHz, and taking into account the brass core, XNUMX MHz. For those who wish to use a receiver at 144 MHz or higher, I recommend reducing the number of turns of coils L2, L3, L5 to 4. this chain in general. ULF tuning is not required. It may be necessary to select the value of R12 for the optimal value of gain and bass bandwidth as recommended in [4]. To adjust the PD, the piezo filter is disconnected from pin 19 and a frequency-modulated signal is applied to it at the frequency of the selected IF. For example, I used a conventional three-point crystal oscillator, with a varicap connected in series with quartz, modulating it with a conventional AF generator on a single transistor from [2]. To tune the local oscillator to a given range, I used the same RF generator, converting it into an LC generator, and the same single-transistor RF. The generator is located next to the receiver, at which the UHF is turned off (the resistor R4 is soldered) and the capacitor C7 is tuned to the frequency of the generator. Then the UHF is connected, the capacitance C1 is set to the minimum, and L3 is adjusted by the capacitor C4 to the maximum signal volume. Then the antenna is connected (a piece of wire 50-100 cm) and the L2 circuit is tuned with capacitor C1. The final fine tuning of the contours is made by tuning cores. If the UHF starts to get excited when fine tuning L2, I recommend leaving it somewhat detuned, above the received frequency. A few notes. The specified receiver can be converted into a narrow-band version. This can be done in several ways: 1) Enable the second transformation. This is easy to do by looking at the diagram shown in Fig. 1. Quartz must be selected 465 kHz above or below the first IF. It is desirable to make the first IF 10,7 MHz to increase the selectivity of the image channel. The LC circuit must be used from the IF of Russian transistor SV-DV-KB receivers. Using contours from imported (Chinese) receivers with yellow coloring is problematic, because they have a tuning frequency of 455 kHz, and it is not always possible to reach it up to 465 kHz. As a filter Z2 (Fig. 1), you can use FP1P-024, FP1P1-60.1 or something similar; 2) You can also use a single conversion if you replace Z1 (Fig. 2) with a ready-made quartz filter FP1P1-307-18 with a frequency of 10,7 MHz and a bandwidth of 18 kHz and very large sizes, or with MCF-10,7-15 with the same frequency and 15 kHz bandwidth. The dimensions of this filter are much smaller than 15x10x10 mm. However, there are serious problems with this option. The essence of which is that the output low-frequency voltage of the frequency (phase) detector is the smaller, the wider the band of the BH contour and the smaller the frequency deviation. (This further explains why narrowband FM uses a low IF.) Therefore, to obtain sufficient volume, it is necessary to narrow the bandwidth of the LC circuit (which is very difficult), or put an additional amplifier in front of the ULF. And those are noises! There is one more option. Instead of LC, use a 10,7 MHz quartz resonator, as implemented in [5]. However, the MC3362 was not designed for this application and the author has not tested it. For those who want to do this, I recommend using an almost similar MC13136 chip, but designed for a quartz resonator in a black hole, instead of an LC. In addition, both options have a common drawback. With a narrow bandwidth, fluctuations in the local oscillator frequency become very noticeable, i.e. either a synthesizer or quartz stabilization is required. One more observation. In the receiver (Fig. 2), the author performed a double conversion, making the first 10,7 MHz IF and the second 6,5 MHz. The result was depressing. The receiver barely received a radio station with a power of 1,5 kW located at a distance of 2-3 km. Replacing the microcircuit did not give any results, I did not conduct further proceedings. For those who want to further reduce the size of the receiver, I recommend using the MC3363, which has a UHF transistor built into the case, as well as a noise reduction system. But it is produced only in a planar package, which complicates its installation, and is much more expensive, about 200-250 rubles, against 25 rubles MS3362. The MC34119 costs the same. Some passing conclusions. I am experimenting with the given receiver, as well as with the RF and IF blocks of the Chinese receiver, Ural-Auto, Melody-106, i.e. I use HF from the developed receiver, and IF from another and vice versa, the author made the following few conclusions, perhaps already known: 1) the quality of the receiver (sensitivity and selectivity) is mainly determined by the quality of the IF-FR block and is practically independent of the RF block;
Literature 1. Barkan V.F., Zhdanov V.K. Radio receivers. 1972.
Author: Alexey Bolshakov; Publication: cxem.net
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