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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING New KB receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Two Czech shortwave radio amateurs designed an amateur communication KB receiver, solving the problem of tuning stability and eliminating interference on the mirror channel in a fundamentally new way. In this receiver, the frequency drift of the first local oscillator up to ± 150 kHz (!) Does not at all affect the stability of the reception of the radio station, regardless of the type of operation (AM, CW or SSB). The receiver range is 0,5-1-30 MHz. The block diagram of the receiver is shown in the figure. The signal voltage of the radio station from the antenna enters the input circuit 1 and from it to the resonant RF amplifier 2. The load circuit in the anode circuit of the RF amplifier lamp simultaneously acts as a low-pass filter with a cutoff frequency of 30 MHz (the highest that the receiver input circuit can be tuned to). Next, the amplified signal voltage enters the first mixer 3, at the output of which the first intermediate frequency equal to 40 ± 0,65 MHz is allocated. The bandpass filter 8 of the first mixer passes this frequency band.
The first local oscillator 4 operates in the range of 40,5-70 MHz without switching circuits. The voltage from its output is applied simultaneously to the first mixer 3 and the second mixer 5. In the second mixer, the frequency of the first local oscillator is mixed with one of the harmonics of the frequency of the auxiliary stabilized quartz (f = 1 MHz) local oscillator 6. Amplification and frequency multiplication of this local oscillator is carried out in the amplifying stage 7 , the anode circuit of which passes frequencies not higher than 33 MHz. As a result of mixing in the second mixer 5 the frequencies of the first local oscillator 4 and the harmonics of the second local oscillator 6 in the bandpass filter 9, which is loaded with the second mixer, a second (compensation) intermediate frequency is allocated, equal to 37,5 ± 0,15 MHz. The bandwidth of filter 9 is 300 kHz. Both intermediate frequencies are fed to the input of the third mixer 11, in the anode circuit of which a third (working) intermediate frequency is allocated, tunable within 2-3 MHz. The voltage of this frequency is amplified in the IF amplifier 12 and fed to the input of the fourth mixer 13, where, mixing with the voltage of the third local oscillator 14, it gives the fourth IF. The further stages of the receiver, indicated by one square with the number 15, are the same as for any superheterodyne, that is, the amplifier of the fourth IF-detector-low-frequency amplifier - S-meter. Having carefully examined the block diagram, we can distinguish two separate parts in it: the right one (nodes 12 to 15), which is a highly selective superheterodyne in the 2-3 MHz range, and the left one (nodes 1 to 11) - a converter according to a special scheme. How is high tuning stability achieved in such a receiver? To answer this question, consider what happens when the receiver is tuned to a radio station and while receiving this radio station. Assume that the receiver is to be tuned to a radio station operating at 14 MHz. To receive this radio station (just like all others), the first local oscillator 4 must generate such a frequency that, as a result of mixing the first and second IF, a third IF is allocated at the output of the third mixer in the range of 2-8 MHz (let's say 2 MHz). In this case, the first local oscillator is tuned to a frequency of 53,5 MHz. Then, on the load filter 8 of the first mixer 3, a frequency of 39,5 MHz (53,5 MHz - 14 MHz) will be allocated. This frequency will pass through filter 8 since its bandwidth extends from 39,35 to 40,65 MHz (40±0,65 MHz). And in the second mixer 5, the frequency of the first local oscillator will be mixed with the 16th harmonic of the second local oscillator b and a frequency of 37,5 MHz (53,5 MHz -16 MHz) will be obtained at the output of the mixer. The load filter 9 of the second mixer will miss this frequency, since its bandwidth lies within 37,5 ± 0,15 MHz. Thus, the input of the third mixer 11 will receive frequencies of 39,5 and 37,5 MHz, which will give us the desired third IF at the output of the mixer - 2 MHz. Now let's see what happens if the frequency of the first local oscillator decreases by 100 kHz against the nominal one (the case is almost unbelievable). Then the first IF will be equal to 39,4 MHz (53,4 MHz -14 MHz), and the second - 37,4 MHz (53,4 MHz -16 MHz). Both frequencies will pass through the appropriate filters. But at the output of the third mixer, the third IF will still be 2 MHz, since 39,4 MHz - 37,4 MHz - 2 MHz, the receiver setting will not change. Interference on the mirror channel and from spurious combination frequencies in such a receiver is not observed, since the value of the first IF is very large. Literature
Publication: N. Bolshakov, rf.atnn.ru
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