Miniaturised half-wave balun

A minaturised half-wave balun having a given operating frequency and comprising:

• a single-ended I/O port comprising a first signal carrying terminal for connection to a source impedance;
• a differential I/O port comprising second and third signal carrying terminals for connection to a load impedance;
• at least one transmission line comprising a first transmission line section and a second transmission line section of equal length and characteristic impedance, and wherein the length of said at least one transmission line is substantially less than one half of the wavelength of an RF signal at said operating frequency;
• a first loading shunt capacitor connected to a first circuit node at a first end of said first transmission line section, said first loading shunt capacitor having a capacitance CA1;
• a second loading shunt capacitor connected to a second circuit node at a first end of said second transmission line section, wherein the capacitance of said first loading shunt capacitor CA1 is substantially equal to the capacitance of said second loading shunt capacitor CA2, said second ends of said first and said second transmission line sections being connected together at a third circuit node;
• and
• a shunt capacitive element connected at said third circuit node, wherein the capacitance CB of said shunt capacitive element is substantially related to CA1 and CA2 by the equation:
• C
• A
• ⁢
• ⁢
• 1
• =
• C
• A
• ⁢
• ⁢
• 2
• =
• C
• B
• 2
• ;
• wherein said first signal carrying terminal is coupled to said first transmission line section,
• wherein said second signal carrying terminal is connected to said first transmission line section at said first circuit node,
• wherein said third signal carrying terminal is connected to said second transmission line section at said second circuit node, and
• wherein the capacitance of said shunt capacitive element is chosen so that the common mode impedance of said differential I/O port at a selected frequency is substantially zero Ohms.

A miniaturised half-wave balun having a given operating frequency and comprising:

• a single-ended I/O port comprising a first signal carrying terminal for connection to a source impedance;
• a differential I/O port comprising second and third signal carrying terminals for connection to a load impedance;
• at least one transmission line comprising a first transmission line section and a second transmission line section of equal length and characteristic impedance, and wherein the length of said at least one transmission line is substantially less than one half of the wavelength of an RF signal at said operating frequency;
• a first loading shunt capacitor connected to a first circuit node at a first end of said first transmission line section;
• a second loading shunt capacitor connected to a second circuit node at a first end of said second transmission line section, said second ends of said first and said second transmission line sections being connected together at a third circuit node;
• and
• a shunt capacitive element connected at said third circuit node, wherein the capacitance of said shunt capacitive element is chosen so that the common mode impedance of said differential I/O port at a selected frequency is substantially zero Ohms;
• wherein said first signal carrying terminal is coupled to said first transmission line section, wherein the second signal carrying terminal is connected to said first transmission line section at a point along the first transmission line section between the first circuit node and the third circuit node and at a distance e from the first circuit node, and
• wherein the third signal carrying terminal is connected to said second transmission line section at a point along the second transmission line section between the second circuit node and the third circuit node and at a distance e from the second circuit node;
• and
• wherein a differential mode component ZDL of the load impedance is matched to the source impedance ZS approximately according to the equation:
• Z
• DL
• =
• [
• 2
• ⁢
• (
• L
• -
• e
• )
• L
• ]
• 2
• ⁢
• Z
• S
• where L is the electrical length of the first transmission line section and the second transmission line section.