Transceiver input circuit. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Transceiver input circuit. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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The authors of all transceiver designs have to deal with the problem of antenna switching between the receiving and transmitting paths. The use of electromagnetic relays for these purposes reduces the reliability of the device as a whole and limits the upper speed limit for half-duplex telegraph operation. High reliability and speed have electronic switching circuits. Many tube output stage designs use a circuit similar to that shown in Figure 1. C1, L1, C2 form a P-circuit, the coupling capacitor Ccv has a capacitance of 1 ... 5 pF, the L2C3 circuit has a high quality factor and is used as a narrow-band preselector during reception. Diodes VD1, VD2 are used to protect the UHF from the powerful signal of its transmitter.

Transceiver input circuit
Ris.1

The circuit is simple and reliable in operation, however, as the experience of its operation on my radio station has shown, often in the ranges of 15 and 40 meters, when trying to fine-tune the L2C3 preselector circuit to the frequency of the received station, the air is clogged with all sorts of interfering signals and interference. This happens due to the conversion of powerful signals from broadcasting stations nearby in frequency ranges of 16 and 41 m, respectively, on the non-linear resistance of the diodes VD1, VD2. You can get rid of this disadvantage by simply reducing Cc, but this leads to a loss of sensitivity. A fairly effective measure of struggle is to apply a blocking bias to the diodes in the RX mode, but this does not completely solve the problem. I managed to completely get rid of this drawback by assembling the input circuit according to the scheme of Fig. 2.

Transceiver input circuit
Ris.2

Range switching circuits are not shown in the figure. The transistor is controlled by applying a voltage of the appropriate polarity to the gate. Resistor R1 limits the forward biased gate current to a safe level, its value is not critical. With a negative gate voltage, the resistance of the VT1 channel is very high, and it does not worsen the quality factor of the L2C3 preselector circuit.

An important advantage of a field-effect transistor is the linearity of the channel resistance at low drain-source voltages, therefore, no frequency conversions occur on it. In transmit mode, a positive bias reduces the channel impedance to a few ohms, which shunts the UHF RX input and protects it from the transmitter signal. Control voltages may be different. For this type of transistor, the control voltage of the RX mode can be within (-10 ... -15V), the TX mode - 0 ... 15V.

Author: V. Vishnevsky (UB5AJS), Sumy. Publication: N. Bolshakov, rf.atnn.ru

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