Encyclopedia of radio electronics and electrical engineering / radio reception

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Increasingly, in amateur technology, there are designs of equipment that are fully or partially built not on a discrete element base, but on integrated circuits that functionally replace one or more nodes. In this article, the author proposes a version of an amateur superheterodyne receiver for a range of 160 meters, in which frequency conversion is performed by the SA612A microcircuit, which is an active double balanced mixer.

With all the simplicity of the circuit, this is a full-fledged radio receiver with one frequency conversion. The electrical circuit diagram of the proposed receiver is shown in fig. 1. Detailed information about the SA612A chip can be found in "Radio", 2004, No. 4, p. 48, 49.

(click to enlarge)

The device works like this. The radio frequency signal is selected by the bandpass filter L1C2C3C4L2 and through the coupling capacitor C6 is fed to the input of the mixer DA1 (pin 1). The local oscillator signal is applied to pins 6 and 7 of the DA1 chip. Tuning the frequency of the local oscillator is carried out by changing the control voltage on the varicap VD1.

An intermediate frequency signal equal to 500 kHz, defined as the difference between the frequencies of the input signal and the local oscillator signal, is separated by an electromechanical filter Z1. The electromechanical filter FEM-035-500V-3.1 was used as a filter of the main selection.

A demodulator and a 2 kHz reference frequency generator circuit are assembled on the DA500 chip. The audio frequency signal through the simplest low-pass filter on the C19R6 elements is fed to the audio frequency amplifier (DA4 chip).

UZCH is covered by the AGC system. The current key VT1 is used as a control element of the AGC system. Although it is designed to work in switching circuits, it works well in such a linear circuit, having the undeniable advantage of high channel resistance with zero gate bias. That is, in the absence of a signal, it practically does not shunt the UZCH input. The UZCH gain is about 46 dB. The receiver's output can be heard through headphones or a speaker. The volume of the signal is regulated by resistor R9.

The sensitivity of such a receiving path is at least 3 μV with a signal-to-noise ratio of 12 dB, the parameters for the adjacent receive channel and in the passband are determined by the characteristics of the applied EMF. The appearance of the radio receiver assembly is shown in fig. 2.

The input bandpass filter coils L1 and L2 are made on SB9 armored cores and contain 30 turns of PEV 0,15 wire each. The tap for the L1 coil is made from the 6th turn (counting from the grounded end), for L2 - from the 15th turn. Coil L3 is wound on a polystyrene frame with a diameter of 8 mm with a tuning core made of carbonyl iron and contains 40 turns of PEV-0,15 wire.

Instead of a small-sized PEM in a radio receiver, you can apply a conventional EMF to the upper or lower sideband. When using an EMF with a lower lateral GPA, it is necessary to rebuild so that its frequency is 1300-1500 kHz. An EMF that does not have taps is connected as shown in Fig. 3.

Instead of a varicap of the specified type, any low-voltage varicap with a nominal capacitance of 20 ... 30 pF can be used. In the absence of KP501, KR1KT1064 or KR1KT1014 is quite suitable as VT1. The latter can be installed sideways by straightening the leads on one side.

First, tune the receiver's GPA. By setting the tuning knob R1 to the lower (according to the diagram) position, by adjusting the core of the coil L3, the value of the generated frequency of 2300 kHz is achieved. The frequency can be controlled by connecting a frequency meter to pin 7 of the DA1 microcircuit through a capacitor with a capacity of 68 ... 120 pF. After that, the R1 slider is moved to the upper position and the value of the generated frequency is again controlled. It must be at least 2500 kHz. If the GPA tuning range turns out to be less than necessary, it will be necessary to reduce the capacitance of the capacitor C8. Then repeat the test again. If the tuning range of the GPA turns out to be much larger than necessary, the capacitance C8 should be increased. Bandpass filters are easily adjusted for maximum reception volume.

Author: Alexey Temerev (UR5VUL), Svetlovodsk, Ukraine

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