Balanced Line
Understanding Balanced Lines
The key insight behind balanced transmission is symmetry. When two conductors are routed close together (twisted, parallel, or as a differential pair on a PCB), any external electric or magnetic field induces nearly identical voltages on both wires. A differential receiver subtracts the two signals, eliminating the common-mode interference while preserving the desired differential signal, which has opposite polarity on each conductor.
Balanced vs. Unbalanced Signal Decomposition
A Balanced Line is a two-conductor transmission line where both conductors carry equal-amplitude, opposite-phase signals referenced to a common ground plane. External electromagnetic interference couples...
Key specifications:
6 A | -500 MHz | -50 dB | -600 MHz | -60 dB | -300 MHz
Z0: = √(L/C) = √((R+jωL)/(G+jωC))
Balanced Line Types
| Type | Zdiff | Bandwidth | CMRR | Application |
|---|---|---|---|---|
| UTP Cat 6A | 100 Ω | DC-500 MHz | 40-50 dB | 10G Ethernet, PoE |
| STP Cat 7 | 100 Ω | DC-600 MHz | 50-60 dB | Industrial Ethernet, EMI zones |
| Twin-lead (300 Ω) | 300 Ω | DC-300 MHz | 20-30 dB | VHF/FM antenna feedline |
| PCB differential pair | 85-100 Ω | DC-28+ GHz | Layout dependent | PCIe, USB, HDMI, SerDes |
| Balanced microstrip | 100-200 Ω | DC-40 GHz | 30-40 dB | Differential RF ICs, filters |
Key Equations
Z0 = √(L/C) = √((R+jωL)/(G+jωC))
Propagation constant:
γ = α + jβ = √((R+jωL)(G+jωC))
Electrical length:
θ = βl = 2πl/λg radians
Comparison
| Aspect | Balanced Line Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | A Balanced Line is a two-conductor trans... | Application-dep. | Critical | Verify in sim |
| Operating range | External electromagnetic interference co... | Application-dep. | Critical | Verify in sim |
| Performance | Balanced lines include twisted pair, twi... | Application-dep. | Critical | Verify in sim |
| Integration | Understanding Balanced Lines The key ins... | Application-dep. | Critical | Verify in sim |
| Trade-off | When two conductors are routed close tog... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
What is the difference between balanced and unbalanced lines?
In an unbalanced line (coax), one conductor carries the signal and the shield is grounded. In a balanced line, both conductors carry the signal with equal amplitude but opposite phase. External interference couples equally to both wires and cancels at the differential receiver. This gives balanced lines 20-60 dB better noise immunity, at the cost of requiring a balun to interface with coaxial systems.
What is common-mode rejection and why does it matter?
CMRR measures how well a balanced system suppresses signals appearing identically on both conductors. External EMI from motors, switching supplies, or nearby transmitters couples equally to both wires. The differential receiver subtracts them, canceling the interference. Well-balanced twisted pair achieves 40-60 dB CMRR. This is why Ethernet, professional audio, and industrial control networks use balanced lines.
Why do RF systems typically use coax instead of balanced lines?
Coax is inherently shielded: the outer conductor blocks external fields and prevents radiation. Balanced lines have no inherent shielding and can radiate at RF frequencies. However, on PCBs with well-controlled differential traces, balanced lines offer superior noise rejection. Modern high-speed serial links (PCIe, USB, HDMI) use differential signaling for exactly this reason.