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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Receivers for direct conversion of AM and FM signals. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception In recent years, shortwave radio amateurs often use so-called direct conversion receivers to receive telegraph and single-sideband signals. Unlike superheterodynes, they do not have an IF path and a detector - there is only a frequency converter that transfers the spectrum of the received high-frequency signal directly to the audio frequency region (in other words, filtering and the main signal amplification occur at low frequencies). Due to this, the direct conversion receiver turns out to be much simpler than the superheterodyne one both in manufacturing and in setting up. High sensitivity and selectivity inherent in superheterodynes are easily obtained using modern low-noise transistors (the level of noise they create, reduced to the input of a low-frequency amplifier, can be 0,1 ... 0,2 μV) and fairly simple but effective low-pass filters ( LPF). Added to this is the "natural" selectivity of human hearing, telephones (loudspeakers), the sensitivity of which decreases with increasing frequency. These advantages of direct conversion receivers are increasingly attracting the attention of designers of broadcasting equipment. However, a conventional direct conversion receiver cannot demodulate AM and FM signals. The fact is that its mixer does not detect the received vibrations, but converts their frequency. Therefore, when tuning, for example, to the frequency of a radio station transmitting from AM, a whistle is first heard (carrier beats with oscillations of the local oscillator), the tone of which decreases as the frequency difference between the signal and the local oscillator decreases. It is almost impossible to disassemble the transmission under these conditions. With more precise tuning, the tone of beats with a frequency F becomes very low, inaudible, however, the transmission is accompanied by periodic changes in volume with a frequency of 2F. This happens because the phase of the local oscillator changes continuously relative to the phase of the received signal. If the phases coincide, the transmission volume is normal, if their difference is 90 ° or 270 °, it drops to zero, if it is shifted by 180 °, the signal appears again, but its polarity is reversed. The point here is the beats of the two sidebands of the AM signal, which, being converted to audio frequency, either add or subtract at the output of the mixer. With frequency modulation, the signal frequency changes in time with sound vibrations in the range from fc-Δf to fc + Δf (fc is the carrier frequency, Δf is the transmitter frequency deviation). The beat frequency F at the output of the mixer of the direct conversion receiver in this case, even with fine tuning, does not remain constant - it varies from 0 to Δf. - so it is impossible to disassemble the transmission at all. Good reception quality of AM and FM signals is obtained by synchronizing the oscillation of the local oscillator with the carrier frequency of the signal, which can be done in several ways. The easiest way is to use the phenomenon of capturing oscillations of the local oscillator of the signal carrier. To do this, part of the signal voltage from the input circuit or from the output of the RF amplifier is introduced into the local oscillator circuit. The capture band is determined by the formula 2Δfз=fcUc/QUг (fс is the signal frequency coinciding with the local oscillator frequency, Uc is the input signal voltage, Q is the quality factor of the local oscillator circuit, Ug is the voltage across it). It should be set (by adjusting the signal voltage introduced into the circuit) to the minimum necessary for reliable synchronization (approximately 200 ... 400 Hz). This improves the noise immunity of the receiver, reducing the chance of noise penetrating the clock chain. With the quality factor of the circuit Q = 35, voltage Ug = 0,1 V and capture band 2Δfz = 400 Hz, the synchronization voltage in the CB band (at a frequency of 1400 kHz) is about 1 mV, in the KB band (14 MHz) - about 100 μV. More complex and advanced synchronous receivers contain a phase locked loop (PLL). Articles [1,2, XNUMX] were devoted to the description of such receivers. There are other ways to receive modulated signals using a direct conversion receiver. They have been proposed for a long time, but, probably, due to little fame, they have not yet received distribution. The purpose of this article is to draw the attention of people's laboratory enthusiasts to asynchronous receivers in order to practically solve the problem of their use in amateur radio communications and for broadcast reception. The simplest way to detect AM oscillations in a direct conversion receiver is to detun it by 2 ... 3 kHz relative to the carrier, and turn on a full-wave detector at the output, as shown in Fig. 1. Here U1 is a mixer, G1 is a local oscillator, Z1 is a low-pass filter, A1 is a low-frequency amplifier. At the output of the latter, a frequency beat signal of 2 ... 3 kHz is formed. amplitude modulated by the transmitted information. Through the coupling capacitor C1, this signal is fed to the detector (V1 - V4). At its output, a voltage pulsating with a double beat frequency is emitted, the envelope of which changes according to the law of modulation of the received signal. As a result, both a radio transmission and a continuous whistle with a double beat frequency (4 ... 6 kHz), somewhat weakened by the blocking capacitor C2, are heard in the headphones. You can get rid of this interference by connecting between the output of the detector and the telephones a low-pass filter with a cutoff frequency of about 3 kHz.
The receiver according to the considered functional diagram (essentially, a superheterodyne with a very low - equal to the beat frequency - IF) is suitable for experiments, but is not suitable for broadcast reception, because due to the large detuning, which cannot be less than 1,6 kHz, the band The bandwidth of the path does not coincide with the spectrum of the signal, and this worsens the noise immunity and leads to distortions. The task of receiving AM signals, as is now clear, is in order to isolate the envelope at a very low, lying in the audio range, "carrier" frequency, and the oscillations of the latter must be suppressed. This is possible in a receiver with two so-called quadrature LF channels, the signals in which are shifted in phase by 90 °. In this case, after two-half-wave detection of quadrature signals, the same pulsating voltages (also with a doubled frequency) will be obtained, but the ripples themselves will turn out to be antiphase (when the frequency is doubled, the phase shift also doubles), and they can be eliminated by simply summing the detected signals. The block diagram of such an AM signal receiver is shown in fig. 2 [3]. It contains two mixers - U1 and U2. The local oscillator voltage G1 is fed to them through a high-frequency phase shifter U3, which creates a phase shift of 90 °. Each channel of the receiver has a low-pass filter (Z1 and Z2), a bass amplifier (A1 and A2) and a full-wave detector - a quadrator (a full-wave detector operating in the quadratic detection mode performs a squaring operation, so it is also called a quadrator) U4 and U5. The signals from the outputs of the detectors are fed into the summing device U6.
The part of the receiver, consisting of the detectors U4, U5 and the adder U6, can be made according to the circuit shown in Fig. 3. The detectors are balanced (they achieve suppression of beats with a frequency F = fc-fg) with trimming resistors R1 and R2. The detected signals are added in the primary winding of the transformer T1, which, if desired, can be replaced by an op-amp.
The degree of suppression of the signal with a frequency of 2F depends on the balancing of the channels and the error in setting the phase shift. With a gain imbalance in the channels of + -1% and an error in the phase shift setting of + -1 °, it reaches 40 dB. Such suppression is sufficient for radio communication and broadcasting reception under conditions of weak signals or interference. For high-quality reception, it should be at least 60 dB, which, of course, requires an order of magnitude reduction in the adjustment error. The simplest way to receive FM signals is essentially the same as described for AM signals (see Fig. 1). The only difference is that the capacitance of the isolation capacitor C1 in this case should be small (to ensure signal differentiation before detection). Under this condition, the detected voltage will be proportional to the beat frequency between the received signal and the oscillations of the local oscillator. A similar method of receiving FM signals is used in known devices with a low IF and a detector operating on the principle of a pulse counter |4| The disadvantage of this method is the presence of a low-frequency mirror channel, which doubles the receiver bandwidth compared to the required one. The asynchronous receiver of FM signals with quadrature channels [5] contains the same input part as the device for receiving AM oscillations, but the signals from the outputs of the LF amplifiers A1 and A2 are fed to the processing device, the block diagram of which is shown in Fig. 4. It consists of differentiating circuits U7 and U8, multipliers U9, U10 and a subtractor A3 (numbering of circuit elements continues that started in Fig. 2). The bandwidth of the filters Z1, Z2 in this case is taken corresponding to the maximum deviation Δfmax of the FM signal (50 kHz - in broadcasting and 6 ... 12 kHz - in radio communications) or somewhat larger. The time constant of differentiating circuits is chosen from the same considerations: RC=(0,5....0,7)/ 2πΔfmax. Ring diode mixers or integrated circuits can be used as multipliers, and a differential amplifier can be used as a subtractor.
Consider the operation of the receiver. Assume that signal S2 lags signal S1 by 90°. In this case, the differentiated signal S'2 is in phase with the signal S1, and its amplitude is proportional to the frequency F. At the output of the multiplier U10, a positive voltage appears, proportional to this frequency, and its second harmonic. Similar processes take place in the multiplier U9, but since the differentiated signal and the signal S2 are out of phase, a voltage of negative polarity appears at its output. In subtractor A3, the second harmonics cancel each other out. A change in the sign of the signal frequency detuning relative to the local oscillator frequency changes the phase of the signal S2 by 180 ° for fc>fg, the phase of the signal S2 is -90 ° (in the U2 mixer, the frequency and phase of the local oscillator oscillations are subtracted from the frequency and phase of the signal, respectively), and for fc The discrimination curve of the receiver (dependence of the output voltage on the detuning) is shown in fig. 5. Its "zero" corresponds to the fine tuning of the local oscillator to the carrier frequency of the signal. It is easier to provide good suppression of beats with frequency F and its harmonics in the receiver under consideration, since interference can only be heard at F
Direct conversion asynchronous receivers with quadrature channels have certain advantages over superheterodynes. In them, for example, high selectivity is easily achieved - an effect equivalent to the use of a three-circuit FSS in the superheterodyne IF path is provided by a simple U-shaped low-pass filter, consisting of one coil and two capacitors. If active RC filters are used for filtering, then the number of coils in the receiver can generally be minimized. The main advantage of such receivers is that all amplification and all signal processing occur at low frequencies, where integrated circuits can be widely used without taking any special measures for shielding and decoupling cascades. The disadvantages include some complexity of the circuits (however, they go for a double complication of the path in stereophonic systems!) And, perhaps, somewhat worse than with traditional methods, the reception quality with insufficiently careful channel balancing. In conclusion, it is interesting to note that adding to the AM signal receiver (Fig. 2) a device made according to the block diagram in Fig. 4, turns it into a device for receiving signals from both AM and FM, and the introduction of an additional low-frequency phase shifter into a single-sideband receiver [6]. Literature
Author: V.Polyakov, Moscow
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