FM radio receiver on one chip (CXA1238S). VHF receiver (FM tuner) with analog tuning VHF circuits on IMS SHA 1538

There was a lot of work all week. The antivirus license has expired and I had to update it on many computers in the office. Plus selection of materials and discussion of the design of the company’s future website. Well, the “turnover” hasn’t gone away. So in the evenings I only had enough time to wear a headscarf or look at messages in the forums.
On Saturday I started searching at home for the materials and parts I needed, as a result of which I “dug up” my old design - a VHF tuner on the SXA1238. For some time now I have started writing the manufacturing date on my boards. This one has the date "02.IV.2002". Yes, it was a long time ago... In general, I wanted to tell you a little about this design. Nostalgia...
Somewhere at the beginning of 2002 I bought a book: B.Yu. Semenov "Modern tuner with your own hands."

I really liked the book, I wanted to make the tuner described in it on the SXA1238 with the SAA1057 synthesizer and microprocessor control.

All diagrams and drawings of printed circuit boards from the book are posted with the permission of the author.

Schematic diagram of the tuner and photo of the SХА1238 IC.

I found the parts without difficulty, even the KR1878BE1 microprocessor. I copied the board from the book, almost without changes.

The photo shows the assembled board with and without the RF compartment cover. The K561TM2 IC is installed in the empty socket, the white connector is for the indicator.

The photo shows a view of the processor and synthesizer and a view of the board from the solder side.

The HF part is covered with a screen made of tinplate; a shielding plate is fixed below, which is insulated from the board with thin fiberglass.

To program the microprocessor, a programmer had to be made. Diagram from the same book. I developed my own printed circuit board.

Schematic diagram of the programmer and the finished product.

Of course, KR1878BE1 is far from the most best processor, but there was nowhere to go. With great grief I managed to stitch it through.

Then the fun began. The receiver itself worked without problems. Using a regular variable resistor instead of a synthesizer, I adjusted the range. I liked the receiver, it works quite well, has good sensitivity, and the IC already has a stereo decoder built into it. But nothing worked with the synthesizer. What have I not tried! Well, except for one thing - write the program yourself. Unfortunately, PIC programming is still unknown territory for me.
I even made a “test bench” to test the KR1878BE1.

The photo shows a “test bench” for KR1878BE1.

I was looking for reference materials, I studied the designs of other radio amateurs with this IC, I implemented some of them, but I was never able to operate the SAA1057 normally. The only option with which she “started up” is described here:

But this synthesizer is designed for a transmitter, the frequency is set using DIP switches and the IF frequency is not taken into account. But at least the synthesizer worked with this circuit! Other designs have been tried:

The photo shows other designs with which I tried to get the SAA1057 to work.

For the latter, I even made a receiver based on TEA5710 according to the scheme from the same book by Semenov. The board was redesigned, a stabilizer and a buffer stage were added to field effect transistor(For digital scale or synthesizer output). True, this was much later, when I made a second tuner on the TEA5710 and with the TSA6057 synthesizer. There were no problems with this synthesizer.

Schematic diagram and revised printed circuit board receiver on TEA5710.

If anyone has positive experience with SAA1057, please share. Maybe I’ll finish another “unfinished construction” of mine.

Yes, I almost forgot - rarity, datasheet on SХА1238 . It is for this IC, and not for SHA1538. A rare and exotic thing, only on Japanese. Can be downloaded here.

Since in modern equipment the AM path has become additional, and the FM path is the main one, the main attention is paid to its design. The structure of this path is as follows: resonant UHF (AGC or discrete gain control is possible), frequency converter, piezo-IF filter, wideband IF, frequency detector, stereo decoder. The number of configurable circuits is from two to four, depending on the requirements for selectivity of the receiver. The UHF and frequency converter are usually made on one chip (for example, TA7358AP or KA22495), less often - on discrete elements (in high-end models). The amplifier and stereo decoder are also separate microcircuits, although there are also combined ones that combine these two units.

As an example, let's consider the FM IF path and the stereo decoder of the "Road Star" car radio produced in 1993 (Fig. 3). From the output of the frequency converter, the IF signal with a frequency of 10.7 MHz is supplied to the aperiodic first stage of the amplifier. Its task is to match the converter with the ZF1 piezoceramic filter and compensate for losses in it. The signal then goes to a broadband amplifier. The phase-shifting circuit L1C3, tuned to the IF, is part of the frequency detector. After the detector, the complex stereo signal is fed to a stereo decoder. Its operating mode is set using resistor R7. Capacitors C11, C12 together with the elements of the signal switch (not shown in the diagram) form pre-emphasis compensation circuits.

Structure input stages FM path - resonant UHF and frequency converter with a separate local oscillator - is also traditional. In older models, the VHF unit is made on discrete bipolar transistors and is a single design with a ferrovariometer. Currently, tuning circuits with varicaps is widely used, and exclusively in radio receiving paths with frequency synthesizers (in the PLL loop). In domestic car receivers, multi-turn resistors are often used for tuning. Tuning with capacitors is now used only in cheap models made with a combined AM-FM path on microcircuits. Since with this design there is only one tunable circuit at the output of the UHF in the VHF path, the selectivity along the mirror channel is low.

The radio receiving path of almost all cheap Asian-made car radios with analog tuning is made according to the same or similar scheme. The AM, FM and stereo decoder paths are made on a single CXA1238 chip from Sony, connected according to a standard circuit. The receiver is tuned using a quadruple block of variable capacitors. Range switching is internal at pin 15, the only control is switch SA1. The CB range signals are isolated by the input circuit L1C2L5CP2.1 and are fed to the input of the AM path (pin 19). The L7C6CP2.2 local oscillator circuit is completely connected to the microcircuit. The wideband VHF input circuit is formed by the L2C3C1 circuit, then the signal after the resonant UHF (load - circuit L3C5CP1.1) goes to the frequency converter. The wideband amplifier is common to both paths; the selectivity is determined by the piezofilters ZF1 and ZF2. The ZF3 resonator is part of an FM detector with a PLL. In addition to its main function, the stereo decoder performs the functions of a linear amplifier in the AM path. Trimmer resistor RP1 sets the operating mode of the stereo decoder (subcarrier frequency - 38 kHz, synchronized by the pilot tone). Capacitors C21, C22 together with resistors R10, R11 form distortion compensation circuits.

Received frequency range, MHz............... 88...108
Supply voltage, V................................6...30
with stabilizer, V................................... 2...7.5
Sensitivity (no worse), µV................... 1.5...3
Signal-to-noise ratio (not less), dB........... 50
Transitional separation between channels, dB... 42
Output voltage, mV................................... 250
Current consumption, mA.................................... 15
Reproducible frequency range, Hz........... 40...16000

Not long ago we managed to put together an excellent stereo tuner on the CXA1238S chip, all famous manufacturer SONY. If we look at this chip from the inside, we will see that it is a complete set of AM/FM receiver in one housing.

We will only use the part that is responsible for the World Cup.

The circuit does not contain rare or expensive elements. Polar capacitors are electrolytic, the rest are ceramic. Loop coils L1, L2, L4 are wound on a mandrel with a diameter of 3 mm (a ballpoint pen rod) with PEL-0.5 wire. Resistor R12 is intended for tuning, you can use any variable resistor to avoid a verner device and improve tuning, it is better to use a multi-turn resistor (such a resistor can be found in old TVs, it is used to tune TV channels to the desired frequency).

L3 - 10...14 turns of PEL-0.5 on a frame with a diameter of 5 mm with a tuning HF core.

Table with elements:

During assembly, it is not recommended to use active fluxes such as L-5 and others.
- With a supply voltage of 2...7.5 V, a jumper should be installed instead of DA2.
- R14 AFC resistor - determines the degree of tuning retention at the station (indicated in red in the diagram).

1. Input circuit L4, C26 is not installed;
2. With circuit L1, C17, the local oscillator is tuned to receive any powerful FM station;
3. Circuit L2, C19 adjusts the UHF at maximum volume;
4. The circuits are adjusted by compressing or stretching coils L1, L2 or changing the number of turns;
5. When replacing the ZQ1 filter with an LC circuit, it must be tuned to resonance at a frequency of 10.7 MHz. By ear, this is determined by the minimum distortion and maximum volume;
6. By moving the slider of the tuning resistor R12 to the extreme position, adjust the local oscillator frequency with circuit L1C17 until the range is completely covered;
7. Use resistor R4 to adjust the VCO frequency until the pilot tone is reliably captured, which is determined by the ignition of VD1 when receiving stereo transmissions (switch SA1 is open);
8. Install the L4C26 input circuit and adjust it to the maximum volume of stations located at the edges of the range.

It is recommended to repeat the setup when the signal is weak to obtain maximum reception quality. After completing the adjustment, the turns of the coils must be secured with cotton wool soaked in paraffin. If, even after the final setup, noise remains when receiving powerful stations, it is recommended to ground pins 19, 24 to the common wire through capacitors with a capacity of 0.1 μF (indicated C* in the diagram). The LF signal level at the output is too low, so you should not try to tune the tuner by connecting headphones to the output.

During operation, a flaw in the operation of the AFC was discovered: uneven action of the AFC from the center of the range to its edges, therefore it is recommended to remove the resistor R14 and replace it with a chain R14", and C27 (in blue in the diagram). Capacitance C27 - determines the degree of tuning retention at the station You can expand the overlap range of the tuner by reducing the capacitance or removing capacitors C17 and C19. In this case, you will need to re-adjust sections of the range, adjust the local oscillator (L1-6 vit.), adjust the UHF (L2-7 vit.), adjust the input circuit to the middle of the range (L4), you will need to change the number of turns of the loop coils.

It should also be taken into account that it is advisable to carry out all settings in the middle of the RF (broadcast) range. The capacitance of capacitor C27 must be reduced to 1...3 pF. On the 12th pin you can “hang” an LED relative to the “+” power supply, through a current-limiting resistor similar to R5. Varicaps VD2, VD3 can be replaced with a dual KPI.

As an amplifier for working with headphones, we use the simplest amplifiers on TDA7050, TDA2822 microcircuits or other great variety of TDAs capable of operating a more powerful load. Unfortunately, I don’t provide the board topology.

Anisimov Ivan, Ukraine, Kiev. region, Vyshgorod | www.radiokot.ru