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[Technical article, rfforum Editorial Team]
A balun converts between a balanced (differential) port and an unbalanced (single-ended, usually coax or microstrip) port. Three families show up constantly in RF front ends: Marchand, magnetic transformer, and lattice (lattice / all-pass) baluns. They fail in different ways.
1. Magnetic transformer balun
A bifilar or transmission-line transformer on a ferrite (or air) core. Strengths: very wide bandwidth from low MHz up to a few GHz if the winding is short, excellent amplitude balance when wound carefully, and galvanic isolation if you need it. Weaknesses: ferrite loss and saturation at high power or DC bias through the winding, and the upper frequency limited by winding length and interwinding capacitance. Common in HF/VHF and in 50:200 Ω or 50:75 Ω matching.
2. Marchand balun
Two coupled-line sections, each about λ/4, arranged so that the unbalanced port drives one strip and the balanced outputs are taken from the coupled strips with the right phase. Strengths: planar (PCB or IC), good for microwave frequencies, no ferrite. Weaknesses: fractional bandwidth typically 30–50 % for a single section (multi-section and compensated layouts do better), needs tight, controlled coupling, and the "ground" references of the balanced ports must be laid out carefully or common-mode pops up. Very common in CMOS/RFIC mixers and in PCB double-balanced diode mixers.
3. Lattice (and related LC) balun
Lumped lattice or "lattice-equivalent" networks of L and C that provide 180° and impedance transformation over a designed band. Strengths: compact at UHF, easy to tune with discrete parts. Weaknesses: narrowband compared with a good transformer, sensitive to component tolerance, and balance degrades fast off-center. Useful when you need a balun inside a few hundred MHz of bandwidth and board area is tight.
4. What to check on the bench
- Amplitude balance: |S21| − |S31| in dB across the band (aim < 0.5 dB for mixers).
- Phase balance: ang(S21) − ang(S31) should be 180° ± a few degrees.
- Common-mode rejection: drive the balanced port in common mode (or use a 3-port measurement) and see how much appears at the unbalanced port.
- Looking into the unbalanced port with the balanced side terminated in the design differential impedance should give a good match; terminating each balanced leg to Z0/2 through a good ground is the usual fixture trick.
Discussion: for a 2–4 GHz double-balanced mixer on PCB, do you prefer a Marchand printed balun or a tiny wirewound transformer, and what balance numbers do you actually get after assembly?
— rfforum 编辑部 / Editorial Team
A balun converts between a balanced (differential) port and an unbalanced (single-ended, usually coax or microstrip) port. Three families show up constantly in RF front ends: Marchand, magnetic transformer, and lattice (lattice / all-pass) baluns. They fail in different ways.
1. Magnetic transformer balun
A bifilar or transmission-line transformer on a ferrite (or air) core. Strengths: very wide bandwidth from low MHz up to a few GHz if the winding is short, excellent amplitude balance when wound carefully, and galvanic isolation if you need it. Weaknesses: ferrite loss and saturation at high power or DC bias through the winding, and the upper frequency limited by winding length and interwinding capacitance. Common in HF/VHF and in 50:200 Ω or 50:75 Ω matching.
2. Marchand balun
Two coupled-line sections, each about λ/4, arranged so that the unbalanced port drives one strip and the balanced outputs are taken from the coupled strips with the right phase. Strengths: planar (PCB or IC), good for microwave frequencies, no ferrite. Weaknesses: fractional bandwidth typically 30–50 % for a single section (multi-section and compensated layouts do better), needs tight, controlled coupling, and the "ground" references of the balanced ports must be laid out carefully or common-mode pops up. Very common in CMOS/RFIC mixers and in PCB double-balanced diode mixers.
3. Lattice (and related LC) balun
Lumped lattice or "lattice-equivalent" networks of L and C that provide 180° and impedance transformation over a designed band. Strengths: compact at UHF, easy to tune with discrete parts. Weaknesses: narrowband compared with a good transformer, sensitive to component tolerance, and balance degrades fast off-center. Useful when you need a balun inside a few hundred MHz of bandwidth and board area is tight.
4. What to check on the bench
- Amplitude balance: |S21| − |S31| in dB across the band (aim < 0.5 dB for mixers).
- Phase balance: ang(S21) − ang(S31) should be 180° ± a few degrees.
- Common-mode rejection: drive the balanced port in common mode (or use a 3-port measurement) and see how much appears at the unbalanced port.
- Looking into the unbalanced port with the balanced side terminated in the design differential impedance should give a good match; terminating each balanced leg to Z0/2 through a good ground is the usual fixture trick.
Discussion: for a 2–4 GHz double-balanced mixer on PCB, do you prefer a Marchand printed balun or a tiny wirewound transformer, and what balance numbers do you actually get after assembly?
— rfforum 编辑部 / Editorial Team
