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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Quadrature counter-wave mixer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer In a number of practical cases of designing radio equipment, frequency converters are required that provide two quadrature signals at the output. They are widely used in single-sideband signal conditioners for communications, in synchronous priming devices (direct conversion receivers), and in digital processing equipment. The author of this publication offers another way to easily build a quadrature mixer. For a complete description of the radio signal, it is necessary to set two of its parameters: the current amplitude A and the current phase Ψ. On the complex plane, the signal is represented by the vector A, rotated by an angle Ψ (Fig. 1).
However, the practical representation of such heterogeneous parameters in the form of electrical quantities is extremely inconvenient. It is much better to use projections of the signal vector onto the real axis I = A cosΨ and onto the imaginary axis Q = A sinΨ. These parameters are homogeneous and are displayed by DC voltages (but changing with modulation) current when converted to zero frequency, or AC voltages when Ψ = ωt + φ. From known I and Q one can always find A and Ψ: A2 = I2 + Q2, Ψ = arctg(Q/l). Signal designations accepted in foreign literature: I - in phase and Q - quadrature. The traditional technique for constructing quadrature converters involves the use of a high-frequency (HF) phase shifter installed in the circuit for supplying a heterodyne voltage to mixers (Fig. 2a). At the outputs of the mixers, differential frequency signals are formed, and since the phases of the signals are transformed in the same way as the frequencies, these signals will have a relative phase shift π/2. Sometimes, for example, in reversible single-sideband converters, in order to preserve the allocated sideband, a high-frequency converter is installed in the signal circuit (Fig. 2b).
High-frequency phase shifters according to fig. 2, but it is convenient to perform on digital microcircuits simultaneously with dividing the local oscillator frequency by 4, but the frequency range of digital phase shifters is limited to tens of megahertz. The range of phase shifters made on discrete LCR elements is not much wider, since at high frequencies the influence of parasitic inductances and capacitances of mounting and other circuit elements begins to strongly affect. In any case, it is not possible to perform a phase shifter on discrete elements without tuning elements. The general trend in the transition to high frequencies is the use of circuits with distributed parameters, in particular, long lines. The RF phase shifter can also be performed on a line with an electrical length of a/4. In practice, it is more convenient to take a line with a length of only λ / 8 and direct the RF signals from the input and from the local oscillator towards each other, as shown in Fig. 3.
The relative phase shift of the signals at the inputs of the mixers will be just π/2. which is required. But at the same time, mixers are needed, in which both the signal and the local oscillator are fed to the same input, i.e. conventional balanced mixers are not suitable here. But the mixer on counter-parallel diodes, proposed by the author more than 20 years ago, is the best fit! In it, the local oscillator frequency is half the signal frequency and the conversion occurs according to the law F \u2d 2fl, - fc or F \u16d Ic - XNUMXfl. The line length at the LO frequency will be only λ/XNUMX, but since the LO phase, as well as the frequency, doubles during conversion, quadrature signals are still formed at the outputs of the mixers. In the practical implementation of a quadrature counterpropagating-wave mixer, it is advisable (but not necessary) to use the traveling wave mode in the line. To this end, the input impedances of mixers with output impedances of signal sources connected in parallel must be equal to the characteristic impedance of the line. Input and output capacitances must be compensated by connecting inductors in parallel or otherwise. The line can be made in the form of a piece of coaxial cable, in the form of a printed microstrip line, or on lumped elements. As an example of the practical implementation of the mixer in fig. 4 shows a practical diagram of the input part of an experimental heterodyne receiver at a frequency of 46 MHz. The input circuit is formed by the elements L1C1, and the UFC is assembled according to the source follower circuit on a field-effect transistor VT1. The buffer stage of the local oscillator on the transistor VT3 is made exactly according to the same scheme. The local oscillator of the receiver is made according to the scheme of a capacitive three-ton circuit on a bipolar transistor VT2 using a quartz resonator at a frequency of 23 MHz. A tuning resistor R6 is installed in the power supply circuit of the local oscillator, which allows you to select the level of the local oscillator signal on the mixer diodes in order to obtain the maximum transfer coefficient.
Through separating capacitances C3 and C8, RF signals are fed to the ends of the line with mixers connected to them on diodes VD1-VD4. The line itself, due to the not too high frequency, is made in the form of a U-shaped link of a low-pass filter on lumped elements L2C9C10. The cutoff frequency of the link lies much higher than the frequency of the signal, so it introduces only a phase shift, not attenuation of the RF signals. The output capacitances of the source followers and the input capacitances of the mixers are taken into account when setting up the corresponding adjustment of the capacitances of the link by trimmer capacitors C9 and C10. Capacitors C11 and C12 filter out the high frequency components at the mixer outputs and limit the audio bandwidth. Coil L1 contains 7 turns of PEL 0,5 wire and is made on a frame with a diameter of 5 mm with a magnetite trimmer. the line coil L2 is wound on a high-frequency ring with an outer diameter of 9 mm (a cheek of the magnetic circuit SB-9) and contains 8 turns of PEL wire 0,25. Inductor L3 is needed only to close the mixer circuit for direct current, its inductance is not critical. Setting up the device comes down to setting the input circuit and setting the heterodyne voltage level to the maximum signal at the output and adjusting the phase shift in the channels. For this purpose, the I and Q signals are supplied after appropriate amplification (the author used a dual K157UD2 op-amp). to the X and Y inputs of the oscilloscope. By setting the same gain for the channels, by adjusting the capacitors C9 and C10, they achieve the correct circle on the screen. The described device provided a noise-limited sensitivity of several microvolts (the task of obtaining maximum sensitivity was not set) and the accuracy of the phase shift of the signals at the outputs was better than a few degrees, in any case, the shape of the figure on the oscilloscope screen was indistinguishable from a circle in the entire range of beat frequencies from direct current up to several kilohertz. Author: V.Polyakov, Moscow
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