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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Unusual AM detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur In [1], a description of two miniature radio receivers was published. The receivers had the same radio frequency (RF) part and differed only in 3H amplifiers. Experienced radio amateurs must have noticed the absence of a conventional diode detector in the design, and some of those who decided to repeat it "corrected the mistake" and got a normally working receiver. The less experienced simply repeated the design and also received well-functioning receivers. Detectors without diodes are well known since the days of lamp technology - these are grid and anode detectors. In the grid detector, the diode is still implicitly present - it is the grid-cathode gap of the radio tube. The audio frequency voltage rectified by him is applied to the same grid of the lamp and is amplified by it, so the transmission coefficient of the grid detector is higher than that of the diode one. In the anode detector, the operating point of the lamp was set near the lower bend of the anode-grid characteristic, in the region with a large nonlinearity. The amplification of the lamp at this point is less, therefore, and also because of other disadvantages, anode detectors were rarely used. These technical solutions were subsequently partially transferred to transistor technology - detectors made on transistors appeared. To understand their work, let us turn to the basics of detection theory. Like all basics, they are quite simple. An introduction to amplitude modulation (AM) can be found in [2]. A simplified diagram of a diode detector is shown in fig. 1a. AM signal from source G1 is connected to diode VD1. At large signal amplitudes, the detector acts as a rectifier. The detected AF signal is allocated at the load R1. Capacitor C1 serves to smooth out the ripple of the rectified voltage. The current-voltage characteristic (CV) of a diode at large signals is usually approximated by the broken line shown in Fig. 1b. The bottom of the graph shows the voltage waveform of the AM signal applied to the diode, and the waveform of the current through the diode is shown on the right. It can be seen that the diode passes only the positive half-waves of the signal, and their average value corresponds to the oscillations of the audio frequency (3H). At sufficiently large values of R1C1, the voltage at the load corresponds to the envelope of the current pulses.
Peak detectors are very effective, providing an output voltage nearly equal to the amplitude of the input RF voltage. The same happens in rectifiers - radio amateurs know this. Therefore, it was peak AM detectors that were mainly used in tube radios, and subsequently they "transferred" to transistor technology. Due to the direct proportionality of the output voltage to the amplitude of the input, they were often called "linear" detectors. As a result, quadratic detectors have long been safely forgotten, leaving them for the simplest detector receivers. At the same time, peak detectors also have a serious drawback: they work well only at high RF signal amplitudes. Semiconductor diodes are characterized by the presence of a certain "threshold" voltage, below which a very small current flows through the diode, therefore, the diode itself remains practically closed. Its value is determined by the properties of the semiconductor material and is about 0,15 V for germanium, about 0,5 V for silicon, and somewhat less for Schottky diodes (metal-semiconductor junction). It is quite clear that if the input voltage of the detector is less than the threshold, the diode will remain closed and the receiver with such a detector will be unable to receive weak radio signals. For this reason, they try to use only germanium diodes in detectors. In some designs, the problem is solved by applying an initial bias voltage to the diode, but in this case the circuit becomes more complicated and has its own problems, so this solution is rarely used. The situation changes if the CVC can no longer be represented by a broken line (Fig. 1c). This is a smooth curve of diode current i versus diode voltage u. Like any mathematical function, it can be expanded into a series and limited to only two terms, since the contribution of the higher terms of the series at low voltages across the diode is negligible. For detection, the curvature of the characteristic (the second term of the series expansion) is essential. It is thanks to her that detection occurs. This is clearly seen in the oscillograms in Fig. 1, in. Mathematical analysis shows that the detected signal is proportional to the curvature of the characteristic and the square of the amplitude of the input signal. This is where the name "square detector" comes from. At sufficiently small signal amplitudes, any detector becomes quadratic and its useful product - constant without modulation or changing with audio frequencies, the current in the load quickly decreases in proportion to the square of the signal amplitude. The quadratic detector introduces some distortion. It can be calculated that the coefficient of non-linear distortion is m / 4. It is significant only at the modulation peaks, reaching 25% at m = 1, and with an average modulation coefficient m = 0,3 it is about 2,3%. Distortions consist in the enrichment of sound vibrations by the second harmonic and are hardly noticeable by ear. Historically, the quadratic detector was the basis of the very first detector radio receivers. Modern radio amateurs probably had to read about enthusiasts who spent hours looking for a "sensitive point" with a needle on a homemade crystal. Subsequently, the industrial production of semiconductor diodes began, which made it possible to create stably operating detectors. Note that semiconductor diodes began to be produced long before the advent of transistors - a bipolar transistor was discovered in 1948 during laboratory studies of a semiconductor diode. Analyzing a quadratic detector, it is easy to notice its main drawback - low conversion efficiency, since the amplitude of the output signal in it is much less than the amplitude of the input. A quadratic detector, the scheme of which is shown in fig. 2a is able to work reliably with a signal in a rather significant range of levels. Above, we found out that the detector needs an element with a large curvature of the CVC. And the base-emitter junction of the transistor has such a characteristic, because at its core it is an ordinary diode. But the transistor not only detects the signal, but also amplifies it. Thus, in accordance with the terminology adopted in radio engineering, the device can be called an active quadratic detector. With a minimum number of parts, it combines the advantages of quadratic and linear detectors. A few words about the choice of mode. As is known, the initial section of the input characteristic of the transistor, close to the "threshold" point, has the greatest non-linearity, as shown in Fig. 2b, therefore, the current of the initial bias of the base-emitter transition of the transistor must be much lower than in conventional amplifying stages. At the same time, you should not get carried away, setting the current almost at the very "threshold", since in the microcurrent mode, the stability of operation and the gain of transistors are reduced.
Since several years have passed since the publication of [1], in order not to bore readers with a search for descriptions, we present a diagram of the RF receiver assembly (Fig. 3). As can be seen from the figure, this is the most common input part of a direct amplification receiver with a WA1 magnetic antenna, the coil of which, together with KPI C1, forms the only circuit that is tuned to the frequency of the received signal. The first stage on the field-effect transistor VT1 serves as an RF amplifier. The second stage, assembled on a bipolar transistor VT2, is a detector one. An audio frequency signal is already removed from its output, and the radio frequency currents are closed to a common wire by capacitor C3.
In conclusion, it remains only to answer the question implicitly put in the title of the article - what is unusual about this detector? According to the author, the most unusual thing is that for a very long time the detector remained unnoticed. This is rather surprising, since all transistor amplifier stages "in combination" are such detectors, having some non-linearity. It is also possible to discover the detection effect purely by chance, for example, by listening to a radio transmission from a powerful station to the playback amplifier of a tape recorder. Nevertheless, the usual psychological stereotype worked - not to notice what cannot be. Literature
Author: D.Turchinsky, Moscow
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