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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Loud-speaking detector receivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The interest of radio amateurs in powering the simplest radio receivers with "free energy" does not weaken, i.e. energy drawn by the receiver antenna directly from the ether. The detector receiver designed by the author can provide reception not only on headphones. The question of how much signal power can be obtained from the antenna and how to build a loud-speaking detector receiver has already been discussed in the author's articles [1,2, XNUMX]. However, questions remain about how much power is needed for loudspeaker reception and how to optimally use the radio signal power received by the antenna? After digging into old reference books and magazines and converting non-systemic units to the SI system, it can be established that for normal listening to the speaker's voice at a distance of 1 m, a sound emitter volume of approximately 60 dB is required. In this case, the emitted acoustic power is 12,6 μW. We find the required electrical power by dividing the acoustic power by the loudspeaker efficiency. For common household sound heads and low power loudspeakers, it is about 1%. Then we get an electric power of the order of 1 mW. Curious to calculate how much electrical power is needed for specific heads to get a volume of 60 dB? The results of calculations for sound heads with different returns are: 0,025GD-2 - 3,6, 0,05GD-1 - 1,8, 1GD-5, 1GD-28, 2GD-7 - 1, 5GD-1, 6GD-1RRZ , 6GD-30 - 0,25 and 8GD-1RRZ - 0,2 mW. Even this small selection clearly shows that loudspeakers with high output are needed, and it is on them that one should be guided. The acoustic design of dynamic heads also has a huge impact on the return, in particular, the larger the size of the case, the better. In the experiments, the author used two 4GT-2 heads in a wooden case with a volume of about 50 liters. Horn loudspeakers have a higher efficiency and, accordingly, three times greater return, firstly, due to better matching of the electromechanical system with the environment, and, secondly, due to some radiation directivity. This is confirmed by the amateur radio experience of describing all kinds of horns made of paper, cardboard and plywood and very successful speaker designs with high returns [3]. A horn speaker with a phase inverter folded into a "horseshoe" provided an efficiency of about 6% with a 1GD-2,3 loudspeaker, and up to 3,4% at low frequencies. So, we have found that with a highly sensitive speaker, a 3-hour signal power of about 0,2 mW is enough for us. The second part of our "research" will relate to the electrical circuits of a loud-speaking detector receiver. An analysis of the operation of the detector leads to the conclusion that it is not necessary to amplify the voltage of the detected 3H signal, but mainly the current, since the amplification of the voltage will inevitably lead to limitation of the signal peaks. This led to the idea of the expediency of using a push-pull emitter follower on a complementary pair of transistors, operating in class AB mode and well known from the transistor ultrasonic circuitry. It is more efficient and consumes less current during quiet sounds and pauses, which allows you to store the energy of the detected carrier and then use it at the peaks of the 3H signal. A receiver circuit with such an amplifier is shown in fig. one.
The variable component of the detected signal is fed through the isolation capacitors C3, C4 to the bases of the amplifier transistors, and the constant component is fed through the inductor L2 to the storage capacitor C5. It is impossible to connect it directly to the output of the detector, since in this case sound vibrations would be smoothed and suppressed. The choke parameters are not critical, any choke or transformer with a winding containing at least 2000 turns with a magnetic circuit cross section of at least 1 cm2 is suitable. The optimal transformation ratio T1 turned out to be about 30 for a four-ohm load. It is convenient to use a small "silovik" - a power transformer for transistor receivers with a primary winding of 220 and a secondary of 6,5 ... you have to rewind the secondary winding. The dimensions of the device with two rather large and heavy magnetic cores of the transformer and inductor should not be embarrassing, since a large antenna and a floor-standing speaker system already determine the status of the structure - it is obviously stationary! A full-wave detector-rectifier with voltage doubling allows you to increase the supply voltage. At the same time, distortions at the peaks should decrease, and in order to load the detector diodes quite symmetrically and further reduce distortions, it was decided to build an amplifier according to a bridge circuit. This option made it possible to get rid of the isolation capacitor at the output. The receiver circuit with a full-wave detector, bipolar power supply and a bridge amplifier is shown in fig. 2.
The positive half-waves of the high-frequency signal are detected by the diode VD1, smoothed by the capacitor C2 and filtered by the low-frequency inductor L2 with the storage capacitor C8, creating a positive supply voltage. Similarly, elements VD2, L3, C3 and C9 create a negative supply voltage. Composite emitter followers on transistors VT1, VT2 and VT3, VT4 are excited in antiphase from different detectors, creating an antiphase 3H signal at the terminals of the primary winding of the matching transformer T1. Just like in the previous design, its optimal transformation ratio turned out to be about 30, but due to the anti-phase excitation of the primary winding by the bridge amplifier, the output power is greater. The purpose of the remaining elements of the circuit fig. 2 is the same. as in fig. 1. Remain in force and recommendations for the choice of chokes. Setting up receivers powered by "free" energy has a number of features. Unlike conventional, this receiver does not work until it is tuned to a powerful radio station, since there is no supply voltage. But even after tuning, some time should pass until the storage capacitors are charged (C5 - in Fig. 1 and C8, C9 - in Fig. 2). The charge time is directly proportional to their capacity, so during the first experiments it should not be large. But at the same time, in the case of prolonged loud sounds (especially during musical passages), the supply voltage and the detected 3H voltage drop noticeably due to the increasing amplifier current, which leads to a limitation of the dynamic range. This does not lead to any special undesirable consequences and even improves legibility. When the receiver is "put into permanent operation", the capacity of storage capacitors can be increased even up to several thousand microfarads, this will improve the dynamics of the receiver and allow "working out" the peaks of the 3H signal. In any case, all receiver capacitors must have a small leakage (checked with an ohmmeter) so as not to load our weak etheric "power source" with excess current. The selection of bias resistors in receivers is made taking into account the following considerations: the greater the resistance, the lower the current consumed (quiescent current in receivers - Fig. 1 and 2), the worse the amplifying properties of the transistor, but the higher the supply voltage! A compromise can only be found empirically for this particular antenna, in terms of maximum volume and sound quality, In receivers according to the diagrams in Fig. 1 and 2, the bias resistors do not have to be the same at all, especially if the transistors were not selected in pairs with the same current gain and initial collector current. It is necessary to proceed from the fact that the constant voltage at the emitters (measured by a high-resistance voltmeter relative to the common wire - "ground") is equal to half the supply voltage (Fig. 1) or zero (Fig. 2). It is better to start the experiment without installing resistors at all, then try to set the values from 2,7 to 1 MΩ and, only having a "powerful" antenna, move on to hundreds of kΩ, since the supply voltage noticeably "sags" in this case. If the transistors of the complementary pair have a large initial current. you can reduce it by turning on a resistor between the bases or even connecting the bases together, while releasing one of the coupling capacitors. It makes no sense to include any thermal stabilizing resistors and diodes, as is usually done in similar ultrasonic frequencies, with our powers of units of milliwatts. In conclusion, we note that when tested in a country house (33 km southeast of Moscow), the receivers provided a volume that was quite sufficient to sound a small quiet room. Particularly good results were shown by the receiver according to the scheme of Fig. 2. The antenna was an "oblique beam" only about 12 m long, stretched from the window of the house to a neighboring tree. The pipes of the water well served as grounding. The receiver was tuned to "Radio Russia" 873 kHz, the radio stations "Radio-1" and "Mayak" were also loudly received. The sound cannot even be compared with the sound of ordinary portable and pocket "rattles" - you will no longer want to listen to the latter. Literature
Author: V.Polyakov, Moscow
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Comments on the article: Leo Thin And what is the power of the transmitters, if the receiver must deliver 1 mW of electrical power? Sanya 2 Leo Thin Depends on the distance, as well as the height of the transmitting and receiving antennas.
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