RF

Balanced Signal / Balanced Port

Balanced signal is a concept that does not apply to a single signal line. It is a concept that apply to a pair of signal lines. Balanced Signal means one original signal and one mirror image of the original signal running along a pair of signal lines as illustrated below. The conventional type of signal i.e the signal running along a single signal line is called Single-Ended signal (unbalanced signal).

Why Balanced Signal ?

Whenever any new technology comes out (especially in engineering field), we would ask why we need the new technology. You might have the same question for Balanced signal. Why we need, why we use the balanced signal ?

There are several well-known advantage of Balanced Signal.

First advantage is that with the balalanced signal combined it can enjoy the effect of doubling the amplitude of the received signal as illustrated below. As you know, the larger amplitude you have at the reciever side, the easier you can detect the signal.  Due to this property, most of RF path operating at very low power or most of high speed digital line with very low driving voltage uses balanced signal.

The second advantage is that balanced signal shows high resistance to external noise. Let's suppose a noise is injected to a balanced signal. Since the same amount (same polarity, same amplitude) of noise (like (B)) is added to both the oroginal signal and mirror image of the signal, the shape of the noised signal becomes as (C). Then, when the mirror imaged signal gets flipped over at the reciever, the direction of noise signal become opposite between the two signal as shown in (D). If you sum up these two copies of the signal, you would get the amplified signal with the noise removed as shown in (E).

Another advantage is that the balanced signal is highly resistant to the noise applied to the ground line (ground plane). By the nature of ground line, the noise on the ground line is interpreted as the noise with the same polarity and same amplitude on signal line as shown in (A).  Any noise with the same polarity and same amplitude applied to both lines on the balanced signal lines get removed at the reciever when the mirror imaged signal get flipped and summed up with the original copy as shown in (B) and (C).

Any Drawbacks ?

There are no technology that has advantagies only (of course, there is no technology that has drawbacks only). So balanced signal has its own drawbacks. The most common/serious drawback is that it is very vulnerable to phase offset between the two copies of the signal. Actually this kind of phase offset can break the main principle of balanced signal. Let's suppose some phase offset was introduced while the signal is traveling along the line as shown in (C). When this pair of signal is reiceved by the reciever and the mirror imaged signal gets flipped over, the noise cannot cancel out since it is position is different and the amplitude of the main signal gets smaller than the perfectly balaced signal.

Due to this vulnerability to phase offset, it requires very strict matching criteria if it is RF component or trace line and it requires very strict design rule if it is digital circuit.