Compact balun with out-of-band spurious suppression

A balun circuit comprising:

• an unbalanced port and a balanced port, wherein the balanced port comprises a first terminal, and a second terminal;
• a main line having a first end and a second end, the first end being coupled to the unbalanced port;
• a first sub-line electromagnetically coupled to the main line, the first sub-line having a first end and a second end, wherein the first end of the first sub-line is coupled to the first terminal of the balanced port;
• a second sub-line electromagnetically coupled to the main line, the second sub-line having a first end and a second end, wherein the first end of the second sub-line is coupled to the second terminal of the balanced port;
• a first series resonant circuit connected between the first terminal and the second terminal;
• a second series resonant circuit connected between the first end of the main line and ground;
• a third series resonant circuit connected between the second end of the main line and ground;
• and
• wherein each series resonant circuit has a resonance frequency that is tuned to suppress spurious signals outside of a passband of the balun circuit.