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I am designing a simple HF receiver as a hobby project. The design is similar to the SoftRock Lite II, and the antenna input looks like this:

schematic

The SoftRock design doesn't really say how the inputs are connected (it just provides two holes on the PCB), but I will be connecting them to a BNC connector which could be isolated from the metal chassis, or not.

Obviously this transformer has some essential functions with the double secondary windings and is forming part of the filter, but let's assume I find other ways to meet those needs. The radio I'm designing will have three such antenna inputs, and it's important that I maximize isolation between each input. This is an SDR design, and each antenna, after going through a coherent mixer, goes to an ADC and antenna phasing happens in software.

My question is this: if my objective is maximum rejection of common-mode currents on the antenna feedline, what's the best way to accomplish that?

  • if I'm not mistaken, if I use a connector that is not isolated from the chassis, then common-mode currents should just flow around the chassis, so does the transformer add anything?
  • would a common-mode choke be better, or worthwhile?
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  • \$\begingroup\$ The transformer probably provides impedance matching to maximise power delivery from the antenna, and possibly voltage gain; as such it can have a big effect on the input noise figure. \$\endgroup\$
    – user16324
    Jan 7, 2015 at 11:45
  • \$\begingroup\$ Are the three inputs all using (potentially) the same input transformer? \$\endgroup\$
    – Andy aka
    Jan 7, 2015 at 12:58
  • \$\begingroup\$ @Andyaka I could, sure -- but I do need to maintain separate signals for each antenna, because the phasing happens in software. \$\endgroup\$
    – Phil Frost
    Jan 7, 2015 at 13:19

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The transformer is there to convert the input signal — regardless of whether it's balanced or unbalanced — into a true differential signal to drive the next stage. Note that the drawing is confusing: The two secondary windings need to be wired out-of-phase, not in-phase as a naive interpretation would imply.

This transformer also supplies the common-mode rejection that you are looking for. If you are using an unbalanced (e.g., BNC) antenna connection, you can simply connect the ANT RETURN signal to the shield of the connector and the chassis ground.

Note that the circuit ground (the one connected to the capacitor on the secondary side) should be connected to the chassis ground at only a single point, and preferably through a ferrite bead. This will keep any common-mode external RF currents out of the rest of the receiver circuitry.

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