VHF FM receiver on the K174XA34 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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In the radio engineering literature of recent years [1-8], a lot of materials have been published on the topic of VHF radio receivers, which work quite confidently within the line of sight. Unfortunately, older models do not allow you to listen to radio stations in the upper FM band (88-108 MHz, or VHF-2, as it is also called).

As you know, in Ukraine and the CIS countries, until recently, only a system with polar modulation (66-74 MHz) was used for stereo broadcasting. In foreign countries, a pilot tone system is used for stereo broadcasting. So, in the USA and European countries, the range of 88-108 MHz is allocated for these purposes, in Japan - 76-90 MHz. In recent years, the broadcasting system with a pilot tone has also been used in Ukraine (the range of 100-108 MHz has been allocated for these purposes). Having appreciated the advantages of working in the new FM band, many radio stations operating in stereo began to actively master it. Only in the last few years, the number of such radio stations in many large cities exceeded the number of those operating in the old VHF-1 range by an order of magnitude [8].

(click to enlarge)

The diagram of a VHF receiver with capacitor tuning is shown in the figure. The basis is a single-chip VHF FM receiver on a K174XA34 chip (KR174XA34, a foreign analogue of TDA7021), included according to a typical circuit [10-11]. The installation is done on a simple printed circuit board made of double-sided fiberglass, which can be made at home in a couple of hours. The mounting process is described in [10]. According to this publication, the serial numbers of the elements are also preserved. During installation, special attention should be paid to the minimum length of the connecting conductors. During debugging, the shortcomings in the described developments were corrected, and some design changes were made. For ease of installation, a 1-pin DIP-socket is used for the DA16 chip. As DA2, the authors used the imported microcircuit MC34119R, but you can also use the analog K1436UN1. Elements R1 R4, C1 C18, VT1 and DA1 are placed on the printed circuit board [10, Fig. 2]. The size of the board is slightly increased in order to place a variable capacitor C18 on it, as well as additional elements VT2 and VLF on the DA2 chip. The load of the DA2 chip is a 16 ohm phone (or a suitable speaker).

To increase the sensitivity in the circuit used UHF transistor VT1. Reception is carried out on a whip telescopic antenna WA1. For tuning, a variable capacitor C18 is used. To suppress signals with frequencies below 60 MHz, a C1L1C2 RF filter is used at the UHF input. The output of the DA1 chip (pins 14, 15) is connected to a broadband amplifier on a VT2 transistor, after which the low-frequency signal enters the low-pass filter. To minimize noise when receiving weak signals at the amplifier output on VT2, a simple passive low-pass filter (LPF) was used on R10C25 elements with a cutoff frequency of 70-80 kHz. After the low-pass filter, the signal is fed to the ULF (DA2), which is included according to a typical circuit for a 16-ohm load. You can adjust the volume with resistor R9. The receiver is powered from a 4,5-9 V source. Measurements have shown that at maximum volume and a voltage of 8 V, the device consumes a current of about 60 mA.

Motorola's DA2 microcircuit has a wide supply voltage range (2-16 V) and low current consumption (3 mA at Upp = 3 V). Output power not less than 55 mW at a load of 16 ohms and upit. = 3 V. In blocking mode, the current consumed by the microcircuit does not exceed 65 μA. Due to the presence of a differential input in the DA2 chip, it can be switched on both according to a typical inverting amplifier circuit and according to a non-inverting circuit with a high input impedance (about 125 kOhm). In this case, the gain is about 50, and the harmonic coefficient is not more than 0,5% [9].

The following details are used in the scheme. Chip DA1 type K174XA34, KR174XA34, TDA7021. Chip DA2 - MS34P9R, K 1436UN1. Transistors VT1 - KJ372, KT368; VT2 - KT3102, KT342. Resistors such as MLT, OMLT, S2-13 with a power of 0,25-0,125 W, R11 - with a power of 0,5 W. Capacitors C12, C21-C23 type KM or K53; C19, C20/ C27, C28 type K50 or K53. The capacitance of capacitors C20 / C27 and C28 is from 100 to 500 microfarads. The remaining capacitors are of the KG, KLS, KM or KU type. Capacitor C18 with a capacity of 10-150 pF. The lower (according to the diagram) capacitor plate (common with C17) must be "case". Variable resistor R9 type SP4-1. The coils are wound with PEV-2 wire with a diameter of 0,4 mm and contain: LI - 8-9 turns on the frame 0 5,5 mm, L2 - 5-6 turns on the frame 0 3,5 mm for VHF1 (66-74 MHz) or 4-5 turns for VHF2 (88-108 MHz).

Establishment. First, set the DC modes for the RF amplifier on VT1, check the voltage on pin 4 of DA1, the DC mode of the broadband amplifier on VT2. With a maximum power supply voltage of 8-9 V, the VT1 collector should be 3-4 V, pin 4 of the DA1 microcircuit - a maximum of 6 V and 2-3 V on the VT4 collector. Before adjusting, a piece of wire 1-2 m long must be connected to the receiver and the debugging procedure should be carried out in the line-of-sight mode of the radio signal. Insert DA1 and DA2 into sockets. Capacitor C20 should be located near the DA2 chip. An oscilloscope can be used for setup. By connecting the probe to the "base" or "collector" VT2, rotate the capacitor C18 ("tuning at the station") and try to tune in to the working stations, while checking on the screen for the presence of amplitude disturbances on the oscilloscope that appear synchronously with the sound signal of the radio station. The boundaries of the restructuring are set by selecting the values ​​​​of C15, C16 and the coil L2. Tuning to radio stations in the FM band (88-108 MHz) will be easier if you use a smaller capacitor C18 (for example, 10-60 pF).

Shielding of receiver parts is performed with thin copper or brass foil. At the same time, the screen of the coil L2 is round in shape with an area of ​​about 3 cm, which simultaneously "covers" L2 and adjacent capacitors. Having assembled and debugged the monophonic version of the receiver / you can try to assemble a stereo receiver. To do this, you can use the recently developed stereo decoder KR174XA51 [12].

Literature

1. Makarov D. VHF receiver in a Marlboro pack, Radio. - 1995 No. 10.
2. Polyakov V. On the operation of the receiver on the K174XA34 chip, Radio 1999 No. 9.
3. Gerasimenko K. AM-FM receiver on two ICs of the K174XA series, Radio hobby 2000 No. 3.
4. Pominov A, V. Tuner for VHF-FM radio stations, Radioamator 1998 No. 6.
5. Biryukov S. Microcircuit K174XA35, Radio 1996 No. 4.
6. Karanda Yu. L. Universal stereo decoder, Radioamator 1998 No. 8.
7. Potachin I. VHF receiver, Radio 2000 No. 6.
8. Vyhodets A. V., Dudka N. P. Stereophonic radio broadcasting, Radioamator 2000 No. 1.
9. Fedyaev V. E. Imported microcircuits in radio telephones and radio stations, Radioamator 1997 No. 12.
10. Nechaev I. VHF prefix to the DV-SV receiver, Radio 1999 No. 10.
11. Gvozdev S. Microcircuit K174XA34, Radio 1995 No. 10.
12. Alenin S. Two-system stereo decoder KR174XA51, Radio 1999 No. 5.

Authors: V. G. Nikitenko, O. V. Nikitenko, Ukraine, Kyiv; Publication: cxem.net

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