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In following RF receiver circuit (30 MHz), two wires of a dipole antenna are connected to SMA connectors and the signal goes to an amplifier. I don't understand what is purpose of this balun transformer in circuit. If anybody can explain? enter image description here

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    \$\begingroup\$ What is the source of this circuit i.e. where did it come from? \$\endgroup\$
    – Andy aka
    Apr 29, 2020 at 13:22
  • \$\begingroup\$ It is a working circuit to receive signals transmitted at 30 MHz. \$\endgroup\$
    – adeel asif
    Apr 29, 2020 at 13:23
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    \$\begingroup\$ That isn't what I asked for. \$\endgroup\$
    – Andy aka
    Apr 29, 2020 at 13:26
  • \$\begingroup\$ I don't understand ...Are you asking who gave me this circuit? \$\endgroup\$
    – adeel asif
    Apr 29, 2020 at 13:27
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    \$\begingroup\$ Then speak with your internal people because there isn't enough information contained in your question to make a judgement from my point of view. \$\endgroup\$
    – Andy aka
    Apr 29, 2020 at 13:30

3 Answers 3

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Without much information about the signal connected to the input it is hard to say. However, one of the main applications of baluns is the rejection of common mode signals. In you schematic, since both connectors are expecting to receive the "same" RF signal, it seems that the balun is being used to attenuate common mode signals , e.g. noise. The common mode signals which are in phase in both lines, will cancel each other out through the opposite magnetic field generated by the balun.

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The purpose of the transformer is as follows:

  1. To create a ground reference for the balanced signal being fed to the input of the differential amplifier. This would also improve the balance in the system.

  2. To create a DC path to ground for static charges on the dipole elements.

  3. To nullify common mode currents in the feeder. This offers redundancy to a prime function of the differential amplifier.

The schematic has been redrawn to make it easier to comprehend.

enter image description here

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Possibility: -

If you consider that with the balun in place, either input individually can now properly drive the input amplifier. This might mean: -

  • The system might be alternatively used with a monopole antenna and then the balun will provide an anti-phase signal to the other amplifier input and might help improve performance.

  • The implication of the above is that this might also be used as a test fixture allowing a single ended input (such as from a signal generator) to be used on either input.

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