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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING High quality stereo decoder for a pilot tone system. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications In our country, stereophonic broadcasting on a system with a pilot tone is becoming increasingly widespread. The foreign equipment used to receive transmissions through this system has stereo decoders (SD) of a key type in a microcircuit design. They are technologically convenient for mass repetition, but, however, they are inferior, according to the author, to matrix-type stereo decoders. Radio amateurs wishing to improve the performance of their stereo receivers are encouraged to build a stereo decoder system with a pilot tone (PT) with spectrum separation, also sometimes called sum-difference or matrix, quite rarely used in this stereo broadcast system. In our country, where, as is known, a stereo broadcasting system with polar modulated oscillations (PMC) [1] has been adopted, stereo matrix decoders (SD) are widely used. This is explained by the fact that the subcarrier suppressed during transmission by 14 dB can be relatively easily restored in the SD. In this case, an overtone signal with a "normal" ratio of the subcarrier and its sidebands is detected by a full-wave diode detector. The detected difference signal is added (subtracted) with the total signal on the resistive matrix, where the channels are separated. Abroad (and recently in Russia when radio stations operate in the range of 88 ... 108 MHz), the so-called system with a pilot tone (PT), equal to half the value of the subcarrier frequency, is widely used, i.e. 19 kHz. The subcarrier in this system is almost completely suppressed during transmission, leaving only the sidebands of the supertone signal, which cannot be detected without distortion by conventional diode detectors. For this reason, the vast majority of SDs for a system with PT are classified as key ones. In the first models of such LEDs based on discrete elements, to obtain pulses that control switches (usually diode ones), doubling the frequency of the FET was used [2]. In the microchip-based LEDs that appeared later, control pulses are obtained by dividing the frequency of a voltage-controlled reference oscillator (VCO), which is covered by a PLL system. The FET is compared in a PLL system with a VCO frequency divided up to 19 kHz and provides frequency and phase stabilization of the control pulses. Recently, similar key LEDs in microchip design (microcircuits A290, TA7342, TA7343, etc.) have also appeared on the domestic market. This allows radio amateurs to create simple LEDs for receiving stereo transmissions in the 88 ... 108 MHz band, broadcasting in which began 5 - 6 years ago and is becoming more widespread in our country. However, with the well-known advantages of key LEDs, such as simplicity of circuit implementation (especially in a microcircuit design), good channel separation - this class of LEDs, according to the author's deep conviction, still cannot provide truly high-quality stereo reception. The fact is that total information prevails in a real musical signal - in [1] it is indicated that the modulation coefficient of a subcarrier rarely exceeds 30% at the maximum possible 80%, and in the first approximation, the signal passing through the LED can be considered monophonic. The constant signal switching that takes place in key LEDs, in fact, causes the low-frequency component to be sampled at a very low frequency (38 or 31,25 kHz), while according to [3], in order to eliminate the influence of the sampling frequency on the low-frequency signal, it must be greater than the highest frequency of the low-frequency signal (15 kHz for a system with polar-modulated oscillations) at least 4 - 5 times, i.e. be 60...75 kHz. The consequence of such "processing" of a low-frequency signal is the deterioration of the sound at higher frequencies, while the formal quality indicators of LEDs obtained on a sinusoidal test signal can be very high - the coefficient of nonlinear distortion is 0,2 ... 0,3% or less. In matrix LEDs, the sum signal is not sampled, while the difference signal, whose value, as mentioned above, is small, with full-wave detection turns out to be "sampled" with a frequency twice that of the subcarrier, i.e. 76 or 62,5 kHz. This improves the quality of the restored difference signal and, accordingly, the signals at the output of the LED. The above considerations were experimentally verified by the author when comparing the sound of the matrix [4] and key [5] LEDs. Despite the very primitive circuitry and elemental base of the matrix LED, its sound, in the author's opinion, significantly exceeded the sound of the key LED, which was distinguished by fuzzy, blurry high frequencies. The only advantage of the key LED was, perhaps, only a slightly higher quality of channel separation. The weak link of the known matrix LEDs is the diode subcarrier detector, which is performed using a high-frequency transformer with a large number of turns of the secondary winding, since in order to obtain an acceptable level of distortion during diode detection, the input voltage of the detector must be several volts [1]. The parasitic capacitances of the high-frequency transformer turn out to be significant, which causes amplitude and phase distortions of higher frequencies and worsens the channel separation. The difference signal distortion can be significantly reduced by using synchronous detectors, in particular, those based on CMOS switches. Such detectors make it possible to detect (unlike diode) signals of minimum amplitude, including those with a completely suppressed carrier, which takes place in a FET system. They introduce extremely small distortions, which are practically determined by the ratio of the resistance of the open channel of the key to the input resistance of the next stage, which is advisable to perform in the form of an emitter (source) follower. Absolutely the same circuit solutions can be used to form the pulses controlling the CMOS keys as in the "standard" key LEDs, i.e. VCO with PLL and frequency divider. Taking into account the above considerations, the proposed SD for a system with a FET was developed, the schematic diagram of which is given below. Main technical characteristics of SD
The device consists of four functional blocks:
The input signal (directly from the output of the FM demodulator of the receiver or tuner), which, as a rule, has a value of 60 ... 90 mV, is fed to the amplifier block A1, made on transistors VT1, VT2 (Fig. 1). From the output of the amplifier, the KSS goes to the R11 C6 circuit, which corrects the pre-distortion of the total signal (t = 50 μs). The overtone part of the signal (sidebands of the subcarrier plus FET) through the capacitor C5, which together with the resistors R12 and R14 forms a high-pass filter, which partially suppresses the total signal, enters the base of the transistor VT5. Transistors VT5 and VT6 amplify the side bands of the 38 kHz subcarrier modulated by the difference signal, which are selected on a low-quality oscillatory circuit (Q = 6), consisting of the winding of the transformer T1 and capacitor C8, and fed to the full-wave key detector on the keys of the DD1 microcircuit.
The selected difference signal of positive and negative polarity from the outputs of emitter followers VT7, VT8 and VT9, VT10 through trimmer resistors R21 and R26 (adjust the channel separation) is fed to the matrices R24R25, R28R29. Here, through the resistor R11, the total signal is supplied. The signals of channels A and B selected on the matrices are fed to an active low-pass filter (LPF), made according to the scheme common for such devices (Fig. 3), and then to the output of the LED.
The control pulse shaper A2 (Fig. 2) consists of a VCO on transistors VT1, VT2 (f = 76 kHz) with a PLL on the key DD1.1 and op amp DA1 [6] and a frequency divider on the triggers of the DD2 microcircuit, which generates "meander" pulses with a frequency of 38 kHz to control the keys of the detector and a square wave with a frequency of 19 kHz for the PLL system. It should be noted that the applied RC generator has a very high thermal stability, determined practically only by the TKE of the capacitor C9, however, it is very sensitive to the instability of the supply voltage, which should be as low as possible.
To force the LED to switch to the "Mono" mode with the switch SA2 (Fig. 5), for example, in the case of uncertain reception, a transistor key VT4 (Fig. 1) is provided, which locks the input of the differential channel when a positive (opening) voltage is applied to its base. The second key on the transistor VT3 allows you to "turn off" the total channel with the SA1 switch installed directly on the board of the A1 unit (this may be required when adjusting the device). In this case, only the difference signal passes to the output of the LED, which is convenient to control "by ear" when setting up the decoder or for subjective control of the quality of the received signal, since unsatisfactory reception conditions primarily affect the difference signal.
The stereo indication and stereo automatics unit A4 is assembled according to the scheme shown in fig. 4. The principle of operation of the prototype of this device, which is a synchronous FET detector with a threshold element (comparator), is described in detail in [6]. The proposed device differs from the original one by the presence of an input signal amplifier on a VT1 transistor and an output signal inverter amplifier on a VT2 transistor. Instead of a specialized comparator K521CA1, as practice has shown, general-purpose op-amps with bipolar transistors at the input (UCM = 5 ... 10 mV), corrected for unity gain, can be used.
Details. Capacitors C6, C8 of block A1 and C9 of block A2 must be mica, polystyrene or glass enamel with a tolerance of ± 5%. The resistor R11 of block A1 must have the same tolerance. Instead of the applied transistors KTZ102V, you can use others of the same series, as well as KT315B, KT342A with h21e> 200. KT209 transistors can be with any letter index. It is undesirable to replace them with high-frequency p-n-p transistors. If such transistors (KT3107, KT361, etc.) still have to be used, then capacitors with a capacity of 68 - 100 pF should be installed between their base and collector. Transformer T1 of block A1 is wound on a standard four-section frame with a trimmer made of 400NN ferrite from heterodyne coils of MW and LW radio receivers. The windings are wound simultaneously with three wires: two PEV 0.1 and one PELSHO 0,09. The number of turns is 410. The winding from the PELSHO 0,09 wire is the primary, secondary winding (PEV 0,1 wires) with a tap from the middle is obtained by connecting the end of one winding to the beginning of the other. The design of the device is not critical - during prototyping, the blocks were connected to each other by unshielded conductors up to 20 cm long without any undesirable effects in the operation of the LED. When installed in the receiver, the LED must be placed as far as possible from the circuits of the output units of the audio frequency or placed in the screen to avoid high-frequency interference from the VCO and frequency dividers. Establishment. In the case of using serviceable parts for the manufacture of the device, the modes of the elements for direct current are set automatically. If the supply voltage differs from the nominal one (within 12 ... 15 V), the value of the resistor R1 of the A2 block is selected so that the voltage at the junction point of the resistors R1 and R2 is 3 ... 3.3 V. By selecting the resistor R1 of the A4 block, the voltage is set to collector of the transistor VT1 equal to half the supply voltage. Transformer T1 of block A1 is tuned to a frequency of 38 kHz by applying a voltage of this frequency from an external generator (15 ... 20 mV) to the input of the LED. The voltage is controlled on the secondary winding of the transformer T1. The required quality factor (Q=6) is set by the trimming resistor R15. Next, the LED is connected to the output of the detector of the receiver with a range of 88 ... 108 MHz (to the correction circuits, if any) and the receiver is tuned to a confidently received station. The sum channel is switched off by the switch SA1 of block A1. The stereo automation unit, of course, should be disabled. By adjusting the resistor R14 (and also, if necessary, R13 - roughly), the devices of the control pulse shaper A2 achieve the appearance of a detected difference signal at the output of the SD - this is easy to do "by ear". Then check the stability of the reception of the difference signal (i.e., the clarity of the PLL) when changing the range. The capture (and hold) band of the PLL can be adjusted within certain limits by changing the value of the resistor R8. After that, the sum channel is turned on and, with the help of trimmer resistors R21 and R26 of block A1, the maximum channel separation is achieved. The easiest way to do this operation is when receiving recordings of rock bands of the 60s and 70s, when almost complete separation of instruments by channels was practiced. It is possible to further improve the channel separation by changing the quality factor of the transformer T1 of block A1 within certain limits by selecting the resistor R15, which makes it possible to compensate to a certain extent the frequency-phase distortions introduced by a specific FM path. However, it should be noted that this adjustment is interdependent with the channel separation adjustment described above. You can determine the outputs of the LED channels (left-right) using a "reference" stereo receiver (radio). It should be noted that it is difficult to accurately tune the transformer T1 according to the received signal to a frequency of 38 kHz, since, as already noted, the subcarrier in the FET system is completely suppressed and is absent in transmission pauses. Here you can use the following trick: with the receiver tuned to the station (there is a PLL capture mode), temporarily unsolder the capacitor C5 from the base of the transistor VT5 of block A1. Then, to the base of this transistor, through a capacitor with a capacity of 10 ... 15 pF, apply pulses with a frequency of 1 kHz from output 2 or 2 of the DD2 microcircuit of block A38 and, controlling the voltage at T1 with an oscilloscope, adjust the transformer T1 to the maximum signal. In this case, the transformer T1 will be finely tuned to a frequency of 38 kHz. Lastly, the A4 stereo indication / stereo automation unit (if installed) is adjusted. Resistor R8 of this block regulates the threshold of the comparator so that in the presence of a stereo signal, the HL1 LED lights up clearly. In the absence of a signal and when changing the range of illumination (and "blinking"), the LED should not be. If the voltage at the LED input differs from the recommended one (60 ... 90 mV), it may be necessary to adjust the gain of the cascade on transistor VT1 by selecting resistor R4 (in this case, you will again need to set the DC mode of this transistor). The sound quality of an amateur receiver with the described LED was compared with the sound quality of receiving stereo paths with LED on TA7342 and TA7343 microcircuits. Listening was carried out using a tube amplifier with an output power of 2x15 W and acoustic systems 25AC-033, as well as stereo phones. A higher transparency, natural sounding of the proposed LED was noted. Channel separation practically did not differ from that of "reference" LEDs. Literature
Author: A.Kiselev, Moscow
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