A simple high quality 70-110 MHz FM stereo receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple high quality 70-110 MHz FM stereo receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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I bring to your attention a circuit of a high-quality stereo FM receiver in the 70-110 MHz range, accessible for repetition even to those who have little design experience. The whole device actually consists of two parts, each of which can be used separately. The receiver, for example, can be mounted in place of a free 3.5-inch bay in a computer, and the output can be connected to a sound card. In general, this is where it all started. Then I wanted to make ULF too, after a microcircuit caught my eye, replacing the dual volume level resistor with a fashionable push-button setting.

The receiver itself is assembled on a SONY CXA1238M chip. This is a high-quality single-chip low-voltage stereo receiver designed to receive AM / FM signals from radio broadcasting stations. The receiver contains: high-frequency amplifiers and mixers of the AM and FM bands, AM and FM intermediate frequency amplifiers, AM and FM demodulators, a stereo signal output decoder for a coding system with a pilot tone. We are only interested in the FM part of the chip.

Chip features:

High sensitivity, mkv - 3-5
Separation between channels, dB - 30
Output voltage, mV - 100
Low supply voltage, V - 3-6
Low current consumption, mA - 12.5
Station Tuning LED
LED indication of STEREO mode
Switchable silent setting
Few external components
ULF is assembled on DA1 - KA2250 and DA2 - BA5406 microcircuits.

The first is an electronic volume control, the second is a stereo ULF with a low supply voltage and an output power of up to 5 watts per channel at a load of up to 3 ohms and low distortion - 0.3% with an output power of 0.5 W.

stereo receiver

Fig.1 Receiver circuit (click to enlarge)

The high-frequency radio signal received by the antenna connected to the X2 connector is fed to the L3C26VD3C23 oscillatory circuit and then through the UHF on the VT1 KT368B transistor to the UHF microcircuit input (pin 18). The amplified signal is allocated to the UHF load, the L1C24VD2C19 tunable circuit, and enters the microcircuit mixer. A local oscillator signal is also supplied to the mixer, the frequency of which is determined by the L2C25VD1C20 circuit.

The tuning of this circuit is always greater than the frequency of the input signal by 10.7 MHz. Tuning over the range is carried out by changing the voltage on the varicaps VD1, VD2 and VD3 with a variable resistor RP2 "TUNING". From pin 10 to pin 24 of the microcircuit, through the filter R11R12C13, an auto-tuning voltage is supplied, the threshold of which can be adjusted by changing the capacitance C3. From the output of the mixer (pin 16) through the bandpass filter ZQ1, the intermediate frequency signal is fed to the built-in limiting amplifier and demodulated by the phase detector of the microcircuit. The complex stereo signal is decoded by the built-in stereo decoder and outputs 5 and 6 of the DA1 chip already have a full low-frequency stereo signal. The signal level at the output of the microcircuit is about 100 mV, which is enough for almost any ULF.

The microcircuit is powered by a stabilized + 5V voltage from the DA2 stabilizer on the 7805 microcircuit. 78L05 could also be used (as a transistor), but I used the first one for reliability. It also powers the indicator LEDs. During installation, I drowned it, and sawed off the mounting hole.

Details of the tuner are selected the most miniature. This made it possible to obtain small dimensions - 65 * 75 * 15 mm and minimal pickup on the receiver, which is positive for its stable operation.

Imported resistors half the size of our MLT-0,12. You can use them vertically. Piezo filters ZQ1, ZQ2 and ZQ3 - SFE-10.7 (I used from some dead Chinese receiver). Varicaps type KV109V, but you can use any suitable parameters. I used imported BB639.

Coils L1, L2, L3 do not have a frame, are wound with PEL-0.5 wire on a mandrel with a diameter of 3 mm (I used a ballpoint pen) and contain 7, 6, 3 + 3 turns, respectively. After winding the coil should be slightly stretched. To adjust the range, a multi-turn resistor SP3-36 was used. You can use any other one by connecting to the X5 connector (not indicated on the diagram, see the board picture). Trimmer capacitors have a value of approximately 5-15 pF. The L4 inductor has a rating of 50-100 μH, any small-sized one.

Simple high quality 70-110MHz FM stereo receiver
Fig.2. Location of elements on the board

Simple high quality 70-110MHz FM stereo receiver
Fig.3. Detail Side Drawing of PCB

Simple high quality 70-110MHz FM stereo receiver
Fig.4 - drawing of the printed circuit board on the reverse side

Setting. Before switching on, it is necessary to carefully check the installation, especially for the presence of "snot" between the tracks. I assure you, this will save you from many incomprehensible troubles. Don't be lazy!

Connect to the output of the ULF stereo receiver - connector X1 and after power is supplied to connector X3 you can hear a characteristic hiss. With the help of a tuning resistor, by rotating the rotor of the capacitor C25 and stretching and compressing the turns of the L2 coil, we tune the tuner to receive any station. It is advisable to immediately adjust the overlap of the desired section of the range with the same elements. This is easy to do using some kind of radio receiver for control. If the overlap is too large, then you can connect a resistor to the right terminal of the RP2 resistor in the wire break and, by selecting it and R13, set the range limits. Next, we connect a voltmeter to the control point X4, and by adjusting the capacitors C24, C20 and coils L1, L3 we achieve maximum readings. with somewhat less accuracy, you can adjust the circuit without a voltmeter according to the maximum volume of the received stations.

Reception is possible when tuning the local oscillator both above and below the signal frequency. The local oscillator frequency must be 10.7 MHz higher than the signal frequency. This can be determined by the response of the AFC to the received station. If the local oscillator frequency is lower than the received one, then the AFC will, as it were, "repel", if higher - "attract". To do this, it will be necessary to stretch the turns of the coil L3 (reduce its inductance) until the signal of the same station appears again.

The adjustment of the input circuit L3C26 and the UHF circuit L1C24 must be done until small changes in their settings lead to a voltage drop at the X4 test point. Next, with a trimmer resistor RP1, we achieve the ignition of the VD5 LED, which indicates that the stereo decoder is triggered. By rotating the slider left and right until the LED goes out, we find out the limits of rotation of the resistor axis when the LED is on, and set the position of this section in the environment.

The VD4 LED is used to indicate the presence of power, VD5 to indicate the "stereo" mode, and VD6 to indicate fine tuning to the received radio station.

The SONY CXA1238M chip used in the design has a very small size and is designed for surface mounting. How unexpectedly it turned out to make a printed circuit board for it even easier than for a conventional type of microcircuit. The microcircuit is also available in a version with conventional outputs - СХА1238S.

NPO "Integral" produces an analogue of this microcircuit - ILA1238NS.

In the case of using these microcircuits, and indeed parts of other sizes, in the manufacture of the board, it is necessary to take into account the following recommendations for the layout of the printed circuit board, taken from the proprietary description for the microcircuit.

The inductors that make up the input circuit FMIN, the local oscillator of the FM path, the load circuit at the FM output of the RF FM amplifier, should be located at right angles to each other to minimize mutual coupling.

It is advisable to introduce a dividing screening track connected to pin 21 on the printed circuit board between the coils connected to pins 22 (output of the local oscillator of the FM path) and 20 (output of the RF FM amplifier). The value and parameters of the tuning elements C24, C25, C26, L1, L2 and L3 are given for a specific printed circuit board and, therefore, it may be necessary to clarify their parameters for other wiring options.

Pin 17 is a common pin for RF circuits (HF amplifiers, local oscillators and mixers) of AM and FM paths, pin 11 is for IF amplifiers and demodulators of AM and FM paths, pin 30 is for stereo decoder circuits. Capacitors C15 and C21, connecting pins 21 and 17, should be located as close as possible to pin 17 of the microcircuit. The PCB track connecting the ZQ1 filter and pin 13 (FMIFIN) must be of the minimum length.

Low frequency amplifier

Since the design consists of two parts, there is no continuous numbering of elements.

Fig.5. ULF scheme (click to enlarge)

Chip DA1 - KA2250 is a two-channel (stereo) digital-analog volume control with output signal adjustment from 0 to -66dB in 2dB increments.

The volume of the input signal is increased by pressing the "UP" button, and decreasing - by the "DOWN" button. When turned on, the microcircuit is initialized and the level is set to -40dB. The microcircuit has a bipolar power supply and the chain R5, R6, C2, C26 is used to transfer it to the unipolar mode. Resistors R1 and R2 are necessary only if ULF is used as an independent design.

When used together with the receiver described above, they are not needed.

The rate of volume change can be adjusted by selecting the capacitance of the capacitor C3. An increase (decrease) in the capacitance leads to a slowdown (acceleration) in the change in the signal level.

From the outputs of the DA1 chip, the signal is fed to a two-channel amplifier on the DA2 chip - BA5406. The microcircuit has a power supply of 12 volts and at a load of up to 3 ohms it allows you to get an output power of up to 5 watts. The voltages at the outputs of DA1 and the inputs of DA2 are approximately equal potential (+/- 0.1 volt difference), which led to the need to use chains C6R9C12 and C5R10C11, which can be replaced, if available, with non-polar electrolytic capacitors.

Diodes VD1 and VD2 are any low-power, buttons SB1 and SB2 which you like. On the layout used from dead computer mice. The DA2 requires a heatsink to operate properly, the size and shape of which is chosen based on the maximum power output and cooling conditions. The microcircuit case is connected to the ground and does not require isolation from the heatsink.

The presented version of the printed circuit board was developed only as a layout to test the idea and select elements.

To power the receiver and amplifier, it is better to use a stabilized voltage of +12 volts, using, for example, a stabilizer on a 7812 chip, powering the latter from a 16-18 volt rectifier at a current of up to 1A. Somewhat worse performance will be when using only a 10-14 volt rectifier for power. Maybe it will be more phony, I have not tried it. And the receiver doesn't care, it has its own stabilizer.

You just need to remember that according to the passport data, the maximum supply voltage of the BA5406 chip is 15 volts! For the KA2250 microcircuit, in this embodiment it is much more - 24V (+/- 12V)

You can also use a 12 volt battery for power.

If the installation is done correctly and the parts are all in good order, the amplifier does not need to be tuned, except for the selection, to your taste, of the rate of change in volume by the capacitor C3.

Simple high quality 70-110MHz FM stereo receiver
Fig.6. Location of elements on the board

Simple high quality 70-110MHz FM stereo receiver
Fig.7. Detail Side Drawing of PCB

Simple high quality 70-110MHz FM stereo receiver
Fig.8. PCB drawing on the reverse side

For the curious: Pin 8 of the DA1 chip is designed to control the signal level, and 7 seems to be for putting the chip into sleep mode. For some reason I didn't get it.

Maybe I misunderstood the purpose of the output, but I don’t need it. On the board, they are divorced for experiments.

If necessary, you can do without the DA1 chip, replacing it with a conventional variable dual resistor of 10-50 kOhm. But then it will be an uninteresting banal scheme, which is enough without this one.

Author: Chernov Sergey

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