This circuit of a simple FM receiver is quite compact, it can be easily built into a small speaker, flashlight, old equipment that does not support the FM range, and so on. The circuit diagram is shown in Figure 1. This circuit is built on a specialized TDA7088T microcircuit, which is a low-frequency superheterodyne. The receiver input circuit, consisting of coil L1 and capacitors C2, C3, is tuned to a frequency of 87...108 MHz. By changing the inductance of coil L1 (increasing or decreasing the distance between the turns), maximum sensitivity of the receiver is achieved. The search for radio stations is carried out by briefly pressing the SB2 “Start” button. When you reach the end of the range, you return to the beginning by pressing the SB1 “Reset” button. Automatic frequency adjustment is carried out by varicap VD1, coil L2 and capacitor C7. By increasing the distance between the turns of coil L2, you can adjust the range, and by increasing the number of turns of the coil by 1.5 times, you can adjust it to a frequency of 66...73 MHz. Capacitor C1 serves to protect the receiver; it will not allow the positive component to pass through. This is necessary if you will build the receiver into the equipment and use the device body as an antenna. The DA2 chip is a 3V voltage stabilizer. The 1.2 W output amplifier consists of a DA3 chip. The amplifier supply voltage varies from 4.5 to 18V, so the amplifier power is turned on before the DA2 stabilizer. The volume is adjusted by resistor R4.
To make coils we need PEV-2 wire with a thickness of 0.51 mm. and mandrels with a diameter of 4mm and 2.5mm. Coil L1 is 5.5 turns on a 4mm mandrel. And the L2 coil is 5.5 turns on a 2.5mm mandrel.
The current consumption of the receiver with this amplifier does not exceed 25 mA. Therefore, a dissipative radiator for the DA2 voltage regulator is not required. The antenna is connected to connector XS1.
The parts of this receiver are mounted on two boards made of single-sided fiberglass. On Printed circuit board No. 1 the radio receiver itself is presented, and on Printed circuit board No. 2 amplifier and stabilizer. This is done so that this radio receiver can be built into equipment with a ready-made amplifier.
That's all, if you have any suggestions or comments, write to the site administrator.
Workshop for beginners.
From detector receiver to superheterodyne.
Homemade radio constructor. Part 6.
It so happened that the 3rd part of the amateur radio designer, which was dedicated to VHF receivers, took the lead, since it was an optional activity. Therefore, I will remove this gap and in this post I will talk about the simplest detector and direct amplification VHF (FM) receivers.
In Moscow, radio broadcasting stations operate in two bands. VHF 1 occupies a frequency of 65.9 -74 MHz and VHF 2 radio stations operate in the frequency range 87.5 - 108 MHz. Frequency modulation (FM) is used in two bands, and on all foreign-made receivers this type of modulation is abbreviated as FM (frequency modulation). In translation there is also such a combination of letters FM.
Since the 90s, imported radio receivers with the VHF 2 (FM) band have thoroughly flooded the market, and at the moment the airwaves have been completely mastered by radio broadcasting companies and more than 40 stations are already operating on this wavelength.
| Rice. 1. Detector VHF (FM) receiver. |
The simplicity of the design of the VHF detector receiver is seductive. You connect three or four parts together, and several radio broadcasting stations can be heard in the headphones. In urban environments, where there is a lot of interference, this receiver will work better than one made on medium or long waves, provided that the VHF broadcasting transmitter or repeater is located near your home. In my case, the reliable reception range was six kilometers.
Is such a receiver needed? Detector, the simplest, made according to the classical scheme? To answer these questions, assemble this structure, and when you assemble it, you will understand that you did not waste your time. Many interesting experiments can be carried out with a simple receiver. You may want to improve it, add a gain stage, improve selectivity, make an antenna with a higher gain, etc. The fact that you will not stop there is already good.
VHF detector receiver.
It was something similar to an old frigate. Its body, a volumetric resonator, 0.75 meters long (4th part of the wavelength = 3 meters, which corresponds to 100 MHz), screwed together from two galvanized troughs, with masts of directional antennas of the wave channel type, was raised on ropes thrown over blocks on roof of a country house. I would attribute this episode to an April Fool's joke, but in the city this pile of metal will work, you just need to connect a germanium diode with high-impedance headphones to it.
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| Rice. 2 VHF (FM) detector receiver with ULF, 0 - V - 1. |
The simplest VHF FM detector receiver circuit is no different from the amplitude detector of the ranges: LW, SV, HF, but in design it will differ in the inductor coil, it will have only a few turns of wire. Such a circuit with a variable capacitor of about 30 pF covers 2 ranges at once with a margin of 65 to 108 MHz.
In order to increase the quality factor, given that RF currents flow along the surface of the wires, I chose a diameter of 2 mm, using copper wire for electrical wiring, removing the insulation from it and winding 4 turns on a mandrel with a diameter of 1.2 cm.
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| Photo 1. Inductor. |
Detection of an FM signal at an audio frequency occurs in two stages. The FM signal is first converted to AM, due to the fact that tuning to the radio station occurs at the slope of the frequency response of the circuit, which leads to a change in the amplitude of the FM signal (the higher the frequency or filling density, the more the signal amplitude changes and vice versa). The converted AM signal is converted into audio frequency by an amplitude detector on a diode.
But it is possible to hear the broadcast from such a receiver in the immediate vicinity of the transmitter, so it is advisable to immediately connect a ULF with a low-impedance telephone or a computer speaker, since the slope of the circuit at the received frequency is very flat and the change in amplitude as a result of converting the FM signal to AM is very small. When I connected all this, I myself was wondering what I would hear. After all, the oscillatory circuit has a bandwidth of about 5 MHz at this frequency, which means that I should hear about 10 stations at the same time.
This is practically the first time I have assembled such a simple radio receiver for this frequency for an FM signal.
A detector receiver made according to the voltage doubling circuit (according to Villard) Fig. 3 will not give in practice a significant gain in volume (2 times or 6 dB). When the diodes are connected in this way, the circuit will be more heavily loaded, and to restore its quality factor, it will be necessary to change its switching factor or capacitive coupling, and in the best case, the gain in sound level will be 4 dB better, which is almost imperceptible to the ear. Instead of germanium diodes, which have long been discontinued, microwave PIN diodes have worked well in this circuit. I have been using them for a long time; their characteristics are closer to germanium diodes. See “Simple DIY microwave field indicators.”
The toy turned out to be funny. I managed to count up to five radio stations. Of course, they interfered with each other, the music of one overlapped with the speech of the other station, but on the whole the receiver received the air, and it was even possible to find a section in the range when a powerful radio station, suppressing more distant ones, sounded comfortable. And the best antenna in urban conditions turned out to be a building rule, such an aluminum strip for leveling walls. Its length is 1.5 meters, than a non-linear continuous vibrator for VHF 2 band. The VHF detector no longer needed grounding, and this was an advantage compared to the AM receiver, if we compare it with the same number of parts.
But so far there was one significant drawback, this was poor selectivity or selectivity on the neighboring channel, well, just a communal apartment, some kind of retro-style toy, a memory of childhood, of a public kitchen filled with neighbors with their gossip and stories. On the other hand, it’s convenient, you listen to music, and at the same time you find out the news and weather from another radio station.
I tried to improve the quality factor of the circuit in order to increase the gain and achieve good selectivity in the adjacent channel, for which I made a coil from an aluminum tube, securing it in a “jam bowl”, constructing some kind of resonator. Even though the radio stations were being received, there was no real gain.
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There was also an idea to attach a directional helical antenna with a high gain to the basin, using a copper water pipe with a coil diameter of 0.5 meters and a pitch winding length of up to 5 meters, but during a period of sharp decline in demand for alcohol as a result of rising prices for it, such a design would resemble a production-scale moonshine still. The idea had to be abandoned.
Application Several dozen such receivers, consisting of vibrators in the form of pieces of wire directed to the nearest transmitter, oscillating circuits tuned to a powerful radio station, and the same number of diodes, and an inexhaustible source of energy is ready, which will take up much less space than similar detectors - storage devices of DV and MV ranges.
I tried to get rid of annoying neighbors and installed another tunable resonant amplification stage in front of the detector, thus making
VHF receiver (FM ) direct gain 1 –V – 1.
When using 2 resonant circuits, the band should narrow by 1.4 times, and the suppression of the adjacent channel should increase by 2 times, which is what happened in practice, but the remaining fairly wide band (3.5 MHz) covered two stations each. This design only worked in the city, but in the countryside, 70 km from the city and 20 km from the repeater, I could not catch a single station, only smooth white ULF noise. True, as soon as I connected to a television antenna with an amplifier, something began to appear at the noise level, but there was still a long way to go for the device to function properly. For the normal operation of such a receiver, I needed to go back to the 50s of the last century and borrow the circuit of the KVN-49 TV; the receiving path of this device was made according to a direct amplification circuit. The receiver had only two channels. This was a line of lamps with circuits that were switched by a switch lever that closed contact blades along the entire length of the chassis. And just 20 years ago, when the FM band had not yet been mastered, such a homemade receiver would have been quite acceptable for use, at least in urban conditions. I didn’t want to go back to the past to complicate the scheme.
Application. The given circuit of a tunable resonant amplifier (Fig. 5) has stood the test of time and is quite successfully used to this day as preselector V superheterodyne receivers. In more serious devices, all tuning and variable capacitors are replaced with varicaps, and tuning to the station is carried out using a microprocessor.
Non-tunable resonant RF amplifier finds application for ultra-long-distance communications, being used as an antenna amplifier, installed directly in the antenna. Due to its narrow reception band, it will have a lower noise figure and better immunity to interference compared to a wideband aperiodic stage, which is mainly used in standard antenna amplifiers.
Returning to the topic of simple direct amplification VHF receivers, I will probably give up building up circuits in order to narrow the bandwidth, and will assemble a super-regenerative detector stage for the VHF-2 range
Super regenerative VHF receiver (FM) band.
I have never seen a happier person at the moment when he demonstrated the work of his super-regenerative receiver. Just three transistors on a piece of cardboard, a whip antenna and several very long-distance stations, choking on foreign speech, interrupting each other.
I also collected similar HF receivers for radio-controlled models and simple intercoms. This type of signal detection is captivating in its simplicity, but at the moment it is becoming retro, giving way to a superheterodyne receiver, which, thanks to a modern element base, will have an advantage.
But we must pay tribute to this device, because once you assemble it, you will not be able to tear yourself away from it, turning the tuning capacitors, selecting modes, achieving coordination with the circuits, etc. in an attempt to get something supernatural out of this radio, as its name suggests. I won’t disappoint anyone, since I myself assembled such a receiver for the VHF - 2 range (88 - 108 MHz) and have been conjuring it for several evenings.
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| Rice. 6. VHF (FM) receiver with super-regenerative detector. 1 - V - 1 |
This receiver has better selectivity over the adjacent channel, I practically moved to a separate apartment. The sensitivity is better, I can already listen to it at the dacha. But I’d better keep quiet about the other parameters. Otherwise, all interest in him will disappear and no one will be destined to see the happy face demonstrating the work of the receiver.
The design of the receiver is similar to the previous one, but you will have an irresistible desire to shield the super-regenerative detector because, already by bringing your hand to the demodulator coil, its setting changes, because it includes a high-frequency generator that emits high-frequency generation in flashes thanks to a second generator of a lower frequency, and that’s all this is done on a single transistor. I deliberately slightly changed the previous circuit, turning the UHF resonant cascade into an aperiodic one, so that such a design could easily be redone. It is mainly the detector that is subject to change. However, cascode UHF will provide better isolation from the antenna. Everything is written about it in the 3rd part of the amateur radio designer.
It is advisable to make such a simple VHF radio receiver in the form of a model in a retro style, which can be used at a school creativity exhibition as a practical task for the holidays. As a demonstration radio receiver, it will be more efficient in urban environments, where there is a lot of interference, compared to the MF and LW bands.
Look continuation of this post"Tube regenerative FM detector."
This post contains a prototype of a direct amplification receiver according to the 0 – V – 1 circuit. An active speaker is connected to a tube (high-frequency pentode 6Zh5P) regenerative detector and the receiver is ready. In the city, reception is carried out using a whip antenna without grounding. Buy a ticket to your childhood or the past and assemble this retro design. You will not regret!
Everyday Practical Electronics, January 2006
For a long time, a huge number of circuits for simple HF receivers were published in magazines, but there was nothing similar for the VHF range. We will try to correct this omission. The scheme was originally developed as part of a school project.
Circuit diagram of a simple FM receiver shown in the figure. It consists of a regenerative RF stage on transistor TR1 and a two- or three-stage audio amplifier on transistors TR2-TR4. In some areas with reliable reception there is no need for a three-stage ULF; in this case, transistor TR3 and the corresponding components can not be installed, and the free terminal of capacitor C5 can be connected to the collector of transistor TR2.
The most critical part of the circuit is the first stage, TR1/VC1, here connections should be made with the shortest possible conductors. Coil L1 contains 8 turns of 1 mm (20 SWG) enameled copper wire on a 6 mm mandrel. After winding, the coil should be stretched to a length of 13 mm for further adjustment.
The tuning capacitor VC1 is used from a two-section capacitor with a trimmer from a pocket receiver, one section is used. The “ground” of capacitor VC1 is connected to capacitor C1, whose capacitance is 22 pF. The inductance of the RF choke L2 is not critical and can be in the range of 1 µH...10 µH.
The output stage is designed to work on headphones from the player, the windings of which are connected in series to obtain a resistance of 64 Ohms (connect to the outermost and ring-shaped contact of the plug).
Settings
To configure the receiver, potentiometer VR1 must be slowly rotated (towards the output connected to the “+” power supply) until somewhere in the middle there is a sharp increase in noise, signaling the start of generation. After this, the potentiometer should be turned slightly in the opposite direction, very slowly, so that the generation stops. Now you can tune in to a radio station. The frequency range 87 MHz...108 MHz should be set using the VC2 trimmer at the upper limit (108 MHz) of the range, and by compressing/stretching the L1 coil at the lower limit (87 MHz).
The receiver was tested in different locations in three different countries, including England, and was always able to receive several radio stations with good volume.
Francis Hall, Meinershagen, Germany.
FM radio circuit diagram
This fm diagram You can easily enlarge the radio receiver by clicking on it. This is a simple radio picks up frequencies in the FM range, which is where most radio stations now broadcast..
The article will show the appearance of the parts and the printed circuit board for this radio.
Beginners may be confused by the less common elements labeled on the diagram as KB109 and 10.7 MHz. In the first case, KV109 is understood as a varicap element, which changes its capacitance when the voltage at its terminals changes. Below is a photo of the varicap and overall dimensions.
Depending on the letter index, varicaps KV109 are marked with a corresponding colored dot. For example, KV109A is marked with a white dot. In our scheme, you can use varicaps with any letter index. The leg on the marking side is the anode, and the leg on the convex mark side is the cathode.
If you look closely at the diagram, the elements marked 10.7 MHz differ from each other in the number of pins. With two terminals, the element can rightly be called a quartz resonator, but it is more correctly called a discriminator filter.
Some may have a question about multi-turn variable resistor. This is a variable resistance whose motor moves slowly and smoothly, allowing for precise adjustments. You could see such variable multi-turn resistances in old TVs, in channel tuning units. The most common type of resistance is SP3-36. Below is his photo.
At the end of the resistance there is a convenient twist handle. The position of the engine can be easily controlled visually using the slider on the shaft. The resistance can be glued into the housing, and adjusted through a slot in the housing using a twist knob.
About the coils
Now let's talk about coils. The coils are very easy to make. Previously, a detector receiver required thin winding wire, a ferrite rod, and patience to wind 100-120 turns of a loop coil. The phrase in the circuit diagram for the coil “11 turns / 0.5 / 2.5” indicates that we will need to wind 11 turns with wire with a diameter of 0.5 mm on a 2.5 mm mandrel. Winding copper wire with a diameter of 0.5 mm in varnish insulation (PEL) can be found in workshops for repairing electric motors and household appliances, or in other places. The mandrel is a drill with a diameter of 2.5 mm. We first align the wire using the pulling method. We wind it onto the mandrel tightly, turn to turn. Before starting winding, we strip the end of the wire by 2-3 mm and immediately tin it with solder. After winding, cut the wire, leaving a lead of 2-3 mm; We also clean and service it. Similarly, we make a second coil of 10 turns. You should end up with something similar.
Capacitor markings:
1000 pf – marking 102
10 pf – marking 100
Resistor markings (approximately):
47 kOhm – yellow, purple, orange
4.7 kOhm – yellow, purple, red
As a rule, the receiver of a novice radio amateur is made by “aerial installation”, because there is no sufficient skill in making printed circuit boards. At least, my first receiver was exactly like that, like many other designs. For those who are familiar with the topic of printed circuit boards, I offer a drawing of a printed circuit board. I recommend using this drawing as a guide when assembling the receiver on circuit boards.
