UHF for MW receiver. Scheme, description

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UHF for MW receiver. Encyclopedia of radio electronics and electrical engineering

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Many 27 MHz receivers are built in a double frequency conversion (10,7 MHz, 465 kHz) scheme. This significantly complicates the receiver circuit. but it is necessary to obtain good selectivity in the image channel.

Simple receivers in the MW range are built according to a scheme with one frequency conversion with an IF of 465 kHz (imported - 455 kHz). However, in this case. even when using 3 ... 4 circuits tuned to the frequency of the working channel, it is not possible to suppress the mirror channel by more than 20 dB, which in this case is only 930 kHz away from the main one, which is clearly not enough under the conditions of modern loading of the MW range.

To solve this problem, you can use a two-stage UHF, the circuit of which is shown in the figure. Due to the original inclusion of circuits in the collectors VT1 and VT3, it provides suppression of the image channel (at a detuning of 930 kHz) by more than 40 dB, which is already comparable to double frequency conversion receivers. The total UHF gain is 30 ... 40 dB.

UHF for MW receiver

Let's consider the scheme in detail. The signal from the antenna is fed to the L1 circuit. C2, C3. Base VT1 is included in the circuit through a capacitive divider. To stabilize the mode, VT1 is covered by two OOS loops: for direct current - through R 1. C1, R2. R4, alternately - through R3. The so-called two-resonant oscillatory control L1 is included in the VT2 collector. C4. C5. Sat. A feature of this circuit is the presence of two closely spaced resonances: in series - along the circuit, the upper (according to the circuit) half of L2. C4. C5: and parallel. slightly lower in frequency - all L2, C4, C5, C6.

Since the stability of the VT3 mode for direct current is very important to ensure the stability of the rejection of the mirror channel. to ensure it, a deep OOS for direct current is used. the signal of which is taken from the resistor R7. amplified by a cascade on VT2 and fed through R6 to the VT3 base. These measures have led to the fact that the modes of transistors and UHF frequency response remain practically unchanged when the supply voltage changes from 6 to 10V.

The UHF output is loaded onto the mixer. Very good results were achieved when using the K 174XA26 microcircuit as a mixer and IF-LF paths.

Design

Coils L1. L2. L3 are wound on frames with a diameter of 4 mm with interlinear cores 50 HF, PEV-2 wire with a diameter of 0,36 mm and contain 16 turns each. Taps: at L1 from the fourth turn from the grounded end, at L2, L3 - from the middle.

UHF is mounted on a printed circuit board together with K 174XA26. Mounting requirements are common for high frequency devices. Coils L1 ... L3 are enclosed in screens. Capacitors C4. C10 - tuning, ceramic, type KT4-21 or similar.

Setting

For tuning, it is best to use any frequency response meter. having an appropriate range. The input circuit L1.C2, C3 has no features and is adjusted to the maximum reception. To set up a two-resonant circuit L2, C4. C5. C6 probe of the frequency response meter is connected to the base VT3. Rotating core L2. the maximum voltage is achieved at the operating frequency, then, by rotating the rotor C4, the maximum rejection is achieved at the frequency of the mirror channel. The last two operations should be repeated several times, achieving the best result.

When properly configured, the frequency rejection of the image channel can exceed 30 dB. Then, by switching the probe of the frequency response meter to the output of the capacitor C 13, which is right according to the scheme (that is, to the input of the mixer), the two-resonant circuit L3, C10, C11, C12 is set up similarly to the previous one. With the correct setting, the suppression of the image channel in the band 100 ... 150 kHz is at least 46 dB, and at the center frequency it reaches 60 dB. The value of C 13 is selected taking into account the input impedance of the mixer, based on a compromise between the required UHF gain and the suppression of the image channel. With a very small capacitance of C13, the gain drops, but the rejection of the mirror channel improves. With an increase in capacity C13 - vice versa.

Since the image channel rejection band is quite narrow, a given UHF receiver must be designed to operate on one or more adjacent channels.

Author: I. Kovalchuk (EU1XX), Minsk; Publication: N. Bolshakov, rf.atnn.ru

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