Schemes of detector receivers. Radio circuits electrical circuit diagrams

The concept of a detector receiver is strongly associated with huge antennas and radio broadcasting on long and medium waves. In the published article, the author provides experimentally tested circuits of VHF detector receivers designed for listening to transmissions of VHF FM stations.

The opportunity itself detector reception on VHF was discovered completely by accident One day, while walking in Terletsky Park (Moscow, Novogireevo), I decided to listen to the broadcast - luckily I took with me a simple loopless detector receiver (it was described in R2001, No. 1, pp. 52, 53, Fig. 3) .

The receiver had a telescopic antenna about 1.4 m long. I wonder if reception is possible with such a short antenna? It was possible to hear, rather faintly, the simultaneous operation of two stations. But what was surprising was that the reception volume periodically increased and dropped to almost zero every 5-7 m, and differently for each station!

It is known that in the Far East, and even in the Northeast, where the wavelength reaches hundreds of meters, this is impossible. I had to stop at the point of maximum reception volume of one of the stations and listen carefully. It turned out to be “Radio Nostalgie”, 100.5 FM, broadcasting from nearby Balashikha.

There was no direct visibility of the radio center antennas. How could a FM transmission be received by an amplitude detector? Subsequent calculations and experiments show that this is quite possible and completely independent of the receiver itself.

The simplest portable detector VHF receiver is made in exactly the same way as a field indicator, only instead of a measuring device you need to turn on high-impedance headphones. It makes sense to provide for adjusting the connection between the detector and the circuit in order to select it according to maximum volume and quality of reception

The simplest detector VHF receiver

The circuit diagram of a receiver that meets these requirements is shown in Fig. 1 It is very close to the one used to make the receiver mentioned above and which made it possible to discover the very possibility of detector reception. Only the VHF band circuit has been added.

Rice. 1. Schematic diagram of the simplest VHF detector receiver.

The device contains a telescopic whip antenna WA1, directly connected to the L1 C1 circuit, tuned to the signal frequency. The antenna here is also an element of the circuit, therefore, in order to extract maximum signal power, it is necessary to regulate both its length and the circuit tuning frequency. In some cases, especially when the antenna length is close to a quarter of the wavelength, it is advisable to connect it to the tap of the loop coil, and select the position of the tap according to the maximum volume.

Communication with the detector is regulated by trimming capacitor C2. The detector itself is made on two high-frequency germanium diodes VD1 and VD2. The circuit is completely identical to the rectifier circuit with voltage doubling, however, the detected voltage would double only with a sufficiently large capacitance of the coupling capacitor C2, but the load on the circuit would be excessive, and its quality factor would be low. As a result, the signal voltage in the circuit and the sound volume would decrease

In our case, the capacitance of the coupling capacitor C2 is small and the voltage doubling does not occur. For optimal matching of the detector with the circuit, the capacitance of the coupling capacitor must be equal to the geometric mean between the input impedance of the detector and the resonant resistance of the circuit. Under this condition, the maximum power of the high-frequency signal, corresponding to the maximum volume, is delivered to the detector.

Capacitor C3 is a blocking capacitor; it closes the high-frequency components of the current at the output of the detector. The load of the latter is provided by telephones with a DC resistance of at least 4 kOhm. The entire receiver is assembled in a small metal or plastic case. A telescopic antenna with a length of at least 1 m is fixed at the top of the case, and at the bottom there is a connector or sockets for connecting telephones. Note that the telephone cord serves as the second half of the receiving dipole, or counterweight

Coil L1 is frameless, it contains 5 turns of PEL or PEV wire with a diameter of 0.6-1 mm, wound on a mandrel with a diameter of 7...8 mm. You can select the required inductance by stretching or compressing the turns when tuning.

Variable capacitor (VCA) C1 is best used with an air dielectric, for example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacitance is small and can be 7-15 pF. If there are more plates (and therefore a larger capacitance), it is advisable to either remove some of the plates or connect a permanent or tuning capacitor in series with the KPI, thus reducing the maximum capacitance. Small-sized “smooth-tuning” capacitors from transistor receivers with the HF range are also suitable as C1.

Capacitor C2 is a ceramic tuning capacitor, type KPK-1 or KPK-M with a capacity of 2...7 pF. It is permissible to use other tuning capacitors, as well as install a KPI similar to C1, placing its handle on the receiver panel. This will allow you to regulate communication “on the go”, optimizing reception

Diodes VD1 and VD2, in addition to those indicated in the diagram, can be of the types GD507B, D18, D20. The blocking capacitor C3 is ceramic, its capacitance is not critical and can range from 100 to 4700 pF.

Setting up the receiver is not difficult and comes down to tuning the circuit with capacitor C1 to the station frequency and adjusting the connection with capacitor C2 until maximum volume is obtained. The configuration of the circuit will inevitably change, so all operations must be carried out several times in succession, while simultaneously choosing the best place for reception.

By the way, it does not necessarily have to coincide (and most likely will not) with the place where the field strength is maximum. We should talk about this in more detail and finally explain why this receiver can receive FM signals at all.

Interference and conversion of FM to AM

If the L1C1 circuit of our receiver is adjusted so that the carrier of the FM signal falls on the slope of the resonance curve, then the FM will be converted to AM. Let's see what the quality factor of the circuit should be for this. Assuming the circuit bandwidth is equal to twice the frequency deviation, we obtain Q = fo/2*f = 700 for both the upper and lower VHF bands.

The actual quality factor of the circuit in the detector receiver will probably be lower due to the low intrinsic quality factor (about 150...200) and the shunting of the circuit by both the antenna and the input impedance of the detector. However, weak FM to AM conversion is possible, and thus the receiver will barely function if its circuit is slightly detuned up or down in frequency.

However, there is a much more powerful factor contributing to the conversion of FM to AM - interference. Very rarely is the receiver in the line of sight of the radio station's antenna; more often it is obscured by buildings, hills, trees and other reflective objects. Several rays scattered by these objects arrive at the receiver antenna.

Even in the line of sight zone, in addition to the direct beam, several reflected ones arrive at the antenna. The total signal depends on both the amplitudes and the phases of the adding components.

The two signals are added if they are in phase, that is, the difference in their paths is a multiple of a whole number of wavelengths, and subtracted if they are out of phase, when the difference in their paths is the same number of wavelengths plus half a wavelength. But the wavelength, like the frequency, changes during FM! Both the path difference of the rays and their relative phase shift will change. If the path difference is large, then even a small change in frequency leads to significant phase shifts. Elementary geometric calculation leads to the relationship:

where, delta t is the beam path difference required to shift the phase by ± Pi/2, i.e., to obtain the full AM total signal; tdeltaf - frequency deviation. By total AM we mean here the change in the amplitude of the total signal from the sum of the amplitudes of the two signals to their difference. The formula can be further simplified if we consider that the product of frequency and wavelength fo*(lambda) is equal to the speed of light c; delta t = c/4*delta f.

Then, during one period of a modulating sound oscillation, the total amplitude of the interfering signal will pass through maxima and minima several times, and the distortions during the conversion of FM to AM will be extremely strong, up to the complete unintelligibility of the sound signal when received by an AM detector.

It is always better to use a directional antenna as it increases the direct signal and reduces reflected signals coming from other directions.

Only in our case of the simplest detector receiver did interference play a useful role and make it possible to listen to the transmission, but the transmission can be heard weakly or with large distortions not everywhere, but only in certain places. This explains the periodic changes in the reception volume in Terletsky Park.

Detector with frequency detector

A radical way to improve reception is to use a frequency detector instead of an amplitude detector. In Fig. 2 shown circuit diagram of a portable VHF detector receiver with a simple frequency detector made on one high-frequency germanium transistor UT1.

The use of a germanium transistor is due to the fact that its junctions open at a threshold voltage of about 0.15 V, which makes it possible to detect rather weak signals. The junctions of silicon transistors open at a voltage of about 0.5 V, and the sensitivity of the receiver with a silicon transistor is much lower.

Rice. 2. VHF detector receiver with frequency detector.

As in the previous design, the antenna is connected to the input circuit L1C1, which is tuned to the signal frequency using KPI C1. The signal from the input circuit is supplied to the base of the transistor. Another one is inductively connected to the input circuit - L2C2, which is also tuned to the signal frequency.

The oscillations in it, due to inductive coupling, are shifted in phase by 90° relative to the oscillations in the input circuit. From the output of coil L2, the signal is supplied to the emitter of the transistor. The collector circuit of the transistor includes a blocking capacitor C3 and high-resistance telephones BF1.

The transistor opens when positive half-waves of the signal act on its base and emitter, and the instantaneous voltage at the emitter is greater. At the same time, a detected and smoothed current passes through the phones in its collector circuit. But the positive half-waves overlap only partially when the oscillation phases in the circuits are shifted by 90°, so the detected current does not reach the maximum value determined by the signal level.

During FM, depending on the frequency deviation, the phase shift also changes, in accordance with the phase-frequency characteristic (Ф4Х) of the L2С2 circuit. When the frequency deviates to one side, the phase shift decreases and the half-waves of the signals at the base and emitter overlap more, resulting in an increase in the detected current.

When the frequency deviates in the other direction, the overlap of half-waves decreases and the current drops. This is how frequency detection of a signal occurs.

The transmission coefficient of the detector directly depends on the quality factor of the L2C2 circuit; it should be as high as possible (in the limit, as we calculated, up to 700), which is why the connection with the emitter circuit of the transistor was chosen to be weak. Of course, such a simple detector does not suppress the AM signal received; moreover, its detected current is proportional to the signal level at the input, which is an obvious drawback. The only justification is the exceptional simplicity of the detector.

Just like the previous one, the receiver is assembled in a small case, from which a telescopic antenna extends upward, and telephone sockets are located at the bottom. The handles of both control units are located on the front panel. These capacitors should not be combined into one block, since by setting them separately, it is possible to obtain both higher volume and better reception quality.

The receiver coils are frameless; they are wound with 0.7 PEL wire on a mandrel with a diameter of 8 mm. L1 contains 5 turns, and L2 - 7 turns with tapping from the 2nd turn, counting from the grounded terminal. If possible, it is advisable to wind coil L2 with silver-plated wire to increase its quality factor; the diameter of the wire is not critical.

The inductance of the coils is selected by compressing and stretching the turns so that clearly audible VHF stations are in the middle of the tuning range of the corresponding KPI. The distance between the coils within 15...20 mm (the axes of the coils are parallel) is selected by bending their leads soldered to the KPI.

With the described receiver, you can conduct a lot of interesting experiments, exploring the possibility of detector reception on VHF, the peculiarities of the passage of waves in urban areas, etc. Experiments on further improvement of the receiver are not excluded.

However, the sound quality when received on high-impedance headphones with tin membranes leaves much to be desired. In connection with the above, a more advanced receiver was developed that provides better sound quality and allows the use of various outdoor antennas connected to the receiver by a feed line.

Field Powered Receiver

While experimenting with a simple detector receiver, I repeatedly had to make sure that the power of the detected signal was quite high (tens and hundreds of microwatts) and could ensure fairly loud operation of phones.

But the reception is poor due to the lack of a frequency detector (FD). The second receiver (Fig. 2) solves this problem to some extent, but the signal power in it is also used inefficiently due to the quadrature supply of the transistor with high-frequency signals. Therefore, it was decided to use two detectors in the receiver: amplitude - to power the transistor; frequency - for better signal detection

The diagram of the developed receiver is shown in Fig. 3. The external antenna (loop dipole) is connected to the receiver with a two-wire line made of VHF ribbon cable with a characteristic impedance of 240-300 Ohms. Coordination of the cable with the antenna is obtained automatically, and coordination with the input circuit L1C1 is achieved by selecting the location where the tap is connected to the coil.

Generally speaking, an asymmetrical connection of the feeder to the input circuit reduces the noise immunity of the antenna-feeder system, but given the low sensitivity of the receiver, this is not of particular importance here.

There are well-known methods for symmetrically connecting a feeder using a coupling coil or a balun transformer. In the author’s conditions, the loop dipole was made of ordinary insulated installation wire and placed on the balcony, in a place with maximum field strength. The length of the feeder did not exceed 5 m. With such short lengths, losses in the feeder are negligible, so telephone wire can be successfully used.

The input circuit L1C1 is tuned to the signal frequency, and the high-frequency voltage released on it is rectified by an amplitude detector made on a high-frequency diode VD1. Since during FM the amplitude of oscillations is unchanged, there are practically no requirements for smoothing the rectified DC voltage.

Rice. 3. Circuit diagram of a VHF receiver powered by field energy.

The quadrature black hole receiver is assembled on transistor VT1 and phase-shifting circuit L2C2. The high-frequency signal is supplied to the base of the transistor from the tap of the input circuit coil through coupling capacitor C3, and to the emitter from the tap of the phase-shifting circuit coil. The detector operates in exactly the same way as in the previous design.

To increase the black hole transmission coefficient and more fully utilize the amplifying properties of the transistor, a bias is applied to its base through resistor R1, which is why it was necessary to install a separating capacitor C3. Pay attention to its significant capacitance - it was chosen to short-circuit low-frequency currents to the emitter, i.e., to “ground” the base at audio frequencies. This increases the gain of the transistor and increases the receiving volume.

The collector circuit of the transistor includes the primary winding of the output transformer T1, which serves to match the high output resistance of the transistor with the low resistance of the telephones. You can use high-quality stereo phones TDS-1 or TDS-6 with the receiver. Both phones (left and right channels) are connected in parallel.

Capacitor C5 is a blocking capacitor; it serves to close high-frequency currents penetrating the collector circuit. The SB1 button is used to close the collector circuit when setting up the input circuit and searching for a signal. The sound in the phones disappears, but the sensitivity of the indicator increases significantly.

The design of the receiver can be very different, but you need a front panel with KPIs C1 and C2 installed on it (they are equipped with separate tuning knobs) and the SB1 button. To prevent hand movements from affecting the adjustment of the contours, it is advisable to make the panel metal or foil material.

It can also serve as a common wire for the receiver. The KPI rotors must have good electrical contact with the panel. The antenna and telephone connectors X1 and X2 can be installed either on the same front panel or on the side or rear walls of the receiver housing. Its dimensions depend entirely on the parts available. Let's say a few words about them.

Capacitors C1 and C2 are KPV type with a maximum capacity of 15.25 pF. Capacitors SZ-C5 are small-sized ceramic ones.

Coils L1 and L2 are frameless, wound on mandrels with a diameter of 8 mm and contain 5 and 7 turns, respectively. Winding length 10... 15 mm (adjustable during setup).

PEL wire 0.6...0.8 mm, but it is better to use silver-plated one, especially for coil L2. Taps are made from 1 turn to the electrodes of the transistor and from 1.5 turns to the antenna.

The coils can be positioned either coaxially or parallel to each other. The distance between the coils (10...20 mm) is selected during installation. The receiver will work even in the absence of inductive coupling between the coils - capacitive coupling through the interelectrode capacitance of the transistor is quite sufficient. Transformer T1 was taken ready-made from a broadcast loudspeaker.

Any germanium transistor with a cutoff frequency of at least 400 MHz is suitable as VT1. When using a pnp transistor, for example, GT313A, the polarity of the dial indicator and diode should be reversed. The diode can be any germanium, high-frequency.

Any indicator with a total deviation current of 50-150 µA is suitable for the receiver, for example, a dial indicator of the recording level from a tape recorder.

Setting up a receiver comes down to tuning the circuits to the frequencies of clearly audible radio stations, selecting the position of the coil taps for maximum volume and reception quality, as well as the connection between the coils. It is also useful to select resistor R1, also based on maximum volume.

With the described antenna on the balcony, the receiver provided high-quality reception of two stations with the most powerful signal at a distance to the radio center of at least 4 km and in the absence of direct visibility (blocked by houses). The collector current of the transistor was 30...50 μA.

Of course, possible designs of VHF detector receivers are not limited to those described. On the contrary, they should be considered only as the first experiments in this interesting direction. If you use an effective antenna placed on the roof and aimed at the radio station of interest, you can obtain sufficient signal power even at a considerable distance from the radio station.

This opens up very attractive prospects for high-quality reception on headphones, and in some cases it may be possible to obtain loudspeaker reception. Improvement of the receivers themselves is possible by using more efficient detection circuits and high-quality volumetric, in particular, spiral resonators as oscillatory circuits.

V. Polyakov, Moscow. R2001, 7.

After making a bug, the question arises of what to listen to it with. Naturally a radio receiver. Just what? Good receivers cost good money, and the average user often only has access to cheap Chinese models, the sensitivity of which is very poor, and the range of signal reception from the bug depends on the sensitivity of the receiver as well as on the power of the bug. We will talk about correcting this shortcoming.

The most common of these receivers is the “scanner”, where the settings are made using two buttons - “reset” and “scan”. Its basis is the TDA7088 mikruha (). There are many design options, but the design is the same everywhere, down to the part numbers. The antenna in the receiver is the headphone wire, which is connected to the output of the AF amplifier through an isolating circuit, which makes it possible to separate the RF signal induced in the wire by the field of the radio station. This is achieved by connecting two 10 µH chokes in series with the headphones, which are clearly not enough for good operation of the receiver. The first modification is to increase the inductance of these chokes. To do this, you need to take a small ferrite ring, wind 40-60 turns of PEV-0.1 wire on it and replace it with the inductor going to the positive power supply. After this, the sensitivity should increase to 7-8 µV/m, i.e. to the chip's own sensitivity. Although this is already good compared to the 15 µV/m that the receiver gave before, it is still not enough. To further increase sensitivity you can’t get by with passive elements, you need to assemble an amplifier. Based on the concept of sensitivity, an amplifier can be either HF or AF. I think there will be no problems with the second one - you can, for example, connect active speakers from your computer to the receiver. The first one will have more problems. First you need to tear off the input circuit from the receiver - coil L2, capacitors C10, C11, C7 and resistor R2. All this is shown in the figure:

Now we need to assemble the amplifier. There are many options for circuits, the best results are obtained with an amplifier based on field-effect transistors, but here is the simplest option:

The transistor can be replaced with KT316, KT325. The amplifier current consumption is about 3 mA. It should be taken into account that the antenna in the diagram is only implied; in fact, it is a tap from the choke (see above), into the gap of which the UHF is switched on. Don't forget to cut this track on the board, otherwise nothing will work! In conclusion, I would like to say that this is not the end of all the bullying of the receiver. We will also change the range, attach micro-earphones, and even turn the receiver into a radio intercom!

Here's part 2. So let's get started. We take a receiver we already know, spin it up... If the receiver is not the same, it doesn’t matter. Having unscrewed your receiver, you should see something like the following: a lot of parts, including two buttons and a volume control, a microcircuit and two coils. Sometimes there is only one coil. That's what we need. It is not difficult to distinguish it - usually the coils are unbent and the coil itself is filled with paraffin.

Oh yes... I forgot to tell you about the purpose of the whole idea... Here I should make a small lyrical (or not so much) digression. So far on this site we have been talking about devices using the standard FM band (standard FM band is 88-108 MHz). Of course it’s cool, for example, to install a bug on your neighbor and broadcast his telephone conversations to the whole house. But if you don’t need anyone to be able to catch a signal from a beetle on their receiver, then you won’t be able to get by with this standard.

So you see the coil... This is good, it means your brain is not completely swollen yet. So you take this coil, unwind 1-2 turns from it and solder it in place. Then, by compressing/stretching the turns, you ensure that the first station scanned is the last one in the range. This will be a kind of marker. I do not recommend completely removing radio stations from the range, because... sometimes you don’t understand whether the receiver is working or not... That’s it. The receiver is ready... Now you need to patch the bug in the same way and that’s it! You don’t have to worry that someone (except you) will hear your neighbor’s conversations... Although we shouldn’t forget about the FSB, FAPSI and other services - they can hear and see whatever they want

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