High receiver sensitivity - simple methods. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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High receiver sensitivity - simple methods. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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High sensitivity, 0.25-0.15 μV, with a minimum number of amplification stages, allows you to get the proposed mixer. Its conversion steepness is much higher than that of any other mixers. The dynamic range is not high, at about 40 dB, but it is quite suitable for use in radio microphone receivers, broadcast receivers, portable stations, alarm devices, and more. The mixer diagram is shown on pic.1.

(click to enlarge)

The input signal and the local oscillator signal are fed into the base circuit of the transistor VT1. Due to this, the local oscillator does not require much power. The L1C4 circuit is tuned to the operating frequency and is connected to the VT1 base through a large capacitor. Although the transistor is switched on according to the OE circuit for input signals, the stage does not have a strong shunting effect, since it works for these signals not as a conventional amplifying stage, but as a mixer. Due to this, the circuit is completely included in the base circuit and at the same time has a fairly sharp setting. At the intermediate frequency, VT1 turns out to be connected according to the common base circuit due to the large capacitance C3. The output impedance of the stage also turns out to be high, which allows you to include the IF circuit in the collector circuit directly. To ensure good filtering of the local oscillator voltage, the load is made in the form of a P. filter if the IF stage follows. The mixer works better with low IFs of 0.455-1.6 MHz, although it also gave good results for 10.7 MHz.

An example of including L2 and a 10.7 MHz filter is shown in Figure 2. As VT1, the transistor KT-368A, KT399 was used. When choosing ratings, it should be taken into account that with a decrease in capacitance C5, the steepness increases, but the circuit may self-excite.

Before the mixer, UHF should be turned on with a small gain. One option is shown in pic.3.

(click to enlarge)

Here, for simplicity, the transistor operates in the barrier mode. This is part of a single-channel portable radio circuit that does not require pairwise selection of quartz resonators, due to the absence of a ceramic filter. The IF frequency is determined by the tuning of the circuits within 600-900 kHz. In addition, the choice of the fundamental frequency of the resonator is also not critical, since the amplitude of even and odd harmonics is sufficient for the operation of the mixer. The required harmonic is extracted by the base circuit at the same time as the input signal. The local oscillator signal is fed through the reverse biased junction of the VD1 diode. A K174XA6 chip was used as a detector.

Author: Shatun Alexander Nikolaevich, Kharkov region, Dergachi; Publication: N. Bolshakov, rf.atnn.ru

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