Some improvements of the RA3AO transceiver. Scheme, description

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Some improvements of the RA3AO transceiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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PROTECTION OF THE RECEIVING PATH FROM OVERLOADS

When using separate antennas for RX and TX, a strong signal during transmission disrupts the operation of the receiving path. To prevent this, it is necessary to close the input of the receiver to the body in the TX mode (the changes that need to be introduced in Fig. 10 from [L] are shown in Fig. 1). An additional transistor and relay are located on the chassis under the DFT board.

Some improvements of the RA3AO transceiver
Ris.1

BALANCING THE MIXER IN THE RECEIVER

To facilitate balancing and adjusting the current through the mixer, additional resistors R1-R3 should be installed (see Fig. 2 in the article and Fig. 11 in the book). With the ratings of the parts indicated in the diagram, balancing to a minimum of noise turns out to be very sharp. It should be borne in mind that even a slight change in the current through the mixer by resistor R2 leads to a shift in the balancing point. You can control the noise with an avometer in the AC voltage measurement mode, connected to the "TLF" jacks. The current flowing through the mixer can be determined by the voltage drop across resistor R1.

Some improvements of the RA3AO transceiver
Ris.2

Additional elements are located at the top of the A2 board.

NOISE REDUCTION

If the transceiver is used in conjunction with a broadband amplifier and separate antennas for RX and TX, the noise during reception increases noticeably. As it turned out, the source of noise is the cascade on transistors VT3, VT4 in the output amplifier. It does not close when receiving and a significant current flows through it. Noise through the output stage and an external amplifier is radiated by the antenna and enters the receiver input. To prevent this, switches were introduced on transistors VT15, VT16 (see Fig. 30, a fragment is given in Fig. 3 in the article), opening transistors VT3, VT4 only during transmission.

Some improvements of the RA3AO transceiver
Ris.3

The newly introduced elements are installed on the A19 board by surface mounting.

BALANCING THE WIDE BAND POWER AMPLIFIER

To balance the power amplifier, a resistor R5 was additionally introduced (see Fig. 62 in [L] and Fig. 4 in the article). In this case, the input impedance of the PA for RF is preserved. After balancing, TV1 decreased noticeably, although this is not a cardinal solution to the problem.

Some improvements of the RA3AO transceiver
Ris.4

Literature

Drozdov V.V. Amateur KB transceivers. - M.: Radio and communication

Author: F.Sharapov (RA4PC), Leninogorsk, Tatarstan; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Civil radio communications.

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