Improvement of selectivity of receivers. Scheme, description

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Improvement of selectivity of receivers. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Simple receivers with an intermediate frequency (IF) of 465 kHz have, as a rule, low selectivity. There is no or very little out-of-band suppression in such receivers.

The proposed scheme allows to improve the suppression of the lower sideband by 23...24 dB and can be built into almost any receiver with 465 kHz IF. The circuit contains two mixers on anti-parallel diodes and phase shifters, described by V. Polyakov [1].

(click to enlarge)

Setting the local oscillator is produced by the L3 core and the selection of capacitance C3. The generation frequency should be set to about 232 kHz. At the same time, noise is heard in the phones of the included receiver, the level of which decreases when the core is screwed in or unscrewed. By screwing in the core, you should find the position at which the noise begins to decrease, while the generation frequency corresponds to the lower slope of the frequency response of the IF. This can be done more accurately if there is a GKCH (frequency response meter). By connecting an oscilloscope or an RF voltmeter to the L4 tap, adjust the L5, C6 circuit to resonance (232 kHz). Then apply a signal from the GSS to the input of the IF, corresponding to the lower sideband (460 ... 464 kHz), and with the resistor R5 achieve the minimum signal at the receiver output.

Details

The ratio of the number of turns of coils L1 and L2 is 4:1...2:1. For standard IF circuits with ferrite cups, L2 has 15 ... 30 turns. For L3, L4 and L5 cores SB-1 are used. L3 contains 100 turns, L5 - 200 turns with a tap from the middle, L4 - 30 turns. Wire - with a diameter of 0,12 ... 0,15 mm. L6 and L7 - chokes D0,1 500 μH. L8 is wound in two wires and has 400 ... 500 turns. The wire is 0,1 mm in diameter, the core is ShZxb from a small-sized ULF transformer. L9 - 300 turns of wire 0,1mm on the ring K 16x8x4 2000 IM. Capacitors C2, C3, C4 - KSO type.

In the manufactured receiver, the non-working sideband suppression was obtained by at least 26 dB. In the IF path, a piezofilter FP1P-0,15 was used.

Literature

1. Polyakov V.T. Direct conversion transceivers. - M.: DOSAAF, 1984.

Author: Yu. Ziryukin (EU3AS), Brest region, Bereza; Publication: N. Bolshakov, rf.atnn.ru

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