Direct conversion receiver. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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Schematic diagram of the receiver for operation in the range of 80 m is shown in fig. 1. The signal from the antenna through the coupling capacitor C1 is fed to the input circuit L1C2C3C4 and then to the mixer, made on two back-to-back silicon diodes V1, V2. The mixer load is an L3C10C11 U-shaped low-pass filter with a cutoff frequency of 3 kHz. The local oscillator voltage is applied to the mixer through the first filter capacitor C10.

Ris.1

The local oscillator of the receiver is assembled according to the scheme with capacitive feedback on the transistor V5. The local oscillator circuit coil is included in the collector circuit. The local oscillator and the input circuit are tuned in range simultaneously by a dual block of variable capacitors C3, C6, and the local oscillator tuning frequency (1,75-1,9 MHz) is twice as low as the input circuit tuning frequency.

The bass amplifier is made according to a scheme with a direct connection between the cascades on transistors V3, V4. The load of the amplifier is high-resistance phones with a DC resistance of 4 kOhm, for example, TA-4.

The receiver can be powered by any 12 V source, the current consumption is about 4 mA.

The receiver coils L1 and L2 are wound on frames with a diameter of 6 mm and are adjusted with 600NN ferrite cores with a diameter of 2,7 and a length of 10-12 mm (widely used unified frames from the coils of broadcasting receivers can be used). Winding - turn to turn. L1 contains 14 turns of PELSHO 0,15 wire, L2 - 32 turns of PELSHO 0.1 wire. The taps for both coils are from the 4th turn, counting from the grounded terminal.

The L3 filter coil with an inductance of -100 mH is wound on a K18X8X5 magnetic circuit made of 2000NN ferrite and contains 250 turns of PELSHO 0,1-0,15 wire. You can use the K10X7X5 magnetic circuit from the same ferrite, increasing the number of turns to 300, or K18X8X5 from 1500NM or ZOOONM ferrite (in this case, the winding should consist of 290 and 200 turns, respectively). In extreme cases, in the absence of ferrite magnetic circuits, the filter coil can be replaced with a resistor with a resistance of 1-1,3 kOhm. The selectivity and sensitivity of the receiver will deteriorate somewhat. The block of variable capacitors was used from the "Speedol" receiver. You can use another block, but always with an air dielectric. To facilitate tuning to the SSB station, it is desirable to equip the unit with at least the simplest vernier.

In the local oscillator of the receiver, transistors KT315 and KT312 with any letter index work well. For a low-frequency amplifier, almost any low-frequency pnp transistors are suitable. It is desirable, however, that V3 be low-noise (P27A, P28, MP39B), and the current transfer coefficient of both transistors should be at least 50-60. Capacitors C2, C4. C5, C7-CSO or ceramic. The rest of the parts can be of any type.

The receiver chassis consists of a front panel measuring 180x80 mm and two side bars 110 mm long and 20 mm high, screwed on the sides of the front panel in its lower part. All these details are made of duralumin. A circuit board measuring 180x55 mm made of foil-coated getinax is attached to the slats. The arrangement of parts is shown in fig. 2. The sketch of the printed circuit board is not given, since the location of the conductors depends on the dimensions of the parts used. Printed montage is not required. If the board is made of non-foil material, several ground rails should be run along the board.

Ris.2

The larger the area of ​​the "earth" conductors, the better the shielding of parts from external and internal pickups.

Setting up the receiver begins with checking the modes of transistors for direct current. The voltage at the collector of the transistor V3 should be 7-9 V. If it differs from the specified, select the resistor R3.

The voltage at the emitter of transistor V5 should be 6-8V. It is regulated by the selection of the resistance of the resistor R1.

Then you should make sure that there is generation by closing the terminals of the coil L2. The noise level in the telephones should decrease somewhat due to the reduction of mixer noise.

Having connected the antenna, they receive any station and select the position of the L2 coil tap (within ± 1-2 turns) according to the highest reception volume.

The sensitivity of the receiver depends on the thoroughness of this operation. The tuning range is set by the core of the L2 coil using the GSS or by listening to the signals of amateur stations. Lastly, the input circuit is tuned by rotating the core of the coil L1 at the highest reception volume. The connection with the antenna is established by the capacitor C1 so that most stations are heard at medium volume. This eliminates the need for a dedicated volume control on the receiver. A properly tuned receiver has a gain, measured as the ratio of audio voltage on telephones to high frequency voltage at the antenna terminal, of about 15. This value includes the transfer coefficients of the input circuit, the mixer and the gain of the bass amplifier. The intrinsic noise voltage of the receiver, reduced to the antenna terminal, does not exceed 1 μV. A telegraph signal of 1,5-2 μV is already well understood in telephones. The air noise when using an antenna only a few meters long is far superior to the receiver's own noise. However, to obtain sufficient reception volume, it is desirable that the length of the antenna be at least 15-20 m.

The selectivity of the receiver in the adjacent channel is determined by the low-pass filter L3C10C11 and is 35 dB at a detuning of ±10 kHz. The best selectivity can give a two-section filter. The real selectivity of the receiver was also measured. An interfering AM signal with a modulation factor of 30%, a detuning of ±50 kHz and an amplitude of 0,1 V produces the same voltage at the receiver output as a useful signal with an amplitude of 10 μV.

There are side reception channels, as in any direct conversion receiver, at the frequencies of the signal harmonics, i.e. 7; 10,5; 14 MHz, etc. However, they are attenuated by at least 50 dB. You can improve the suppression by increasing the quality factor of the input circuit or by applying a two-circuit input filter.

Author: V. Polyakov (RA3AAE), Moscow; Publication: N. Bolshakov, rf.atnn.ru

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