Making measurements on balanced transmission lines

Stray electrical fields are everywhere. They infiltrate signal transmitting and receiving devices as well as the transmission lines that connect them, taking the form of noise and unwanted signals. The effect is to raise the signal to noise ratio, obscuring desired analog and digital information. There are mitigation techniques, but to be effective they must be used in conjunction with one another. An obvious answer is shielding, which may take the form of a grounded conductive layer such as the metallic braided shielding in coaxial cable, or a grounded metal enclosure for electrical equipment.

Another mitigation technique is balanced signal transmission, which takes advantage of common-mode rejection. To reduce outside interference and improve signal-to-noise ratio, balanced signal transmission must fulfill certain conditions. The two conductors, both of the same type, must have equal impedance to ground, to other circuits and along their lengths. They must also connect to a differential amplifier at the receiver. Moreover, within the transmitter the two-wire circuit must be balanced.

Differential signaling takes place when, at the source, the signals on the two conductors are equal in amplitude but opposite in polarity with respect to ground and to each other. At the receiver, only the difference between the signals is amplified, so any common-mode voltage is absent at the output of the amplifier.

Interference and noise picked up along the transmission line are, for the most part, common mode, so they are rejected. However, these voltages can become problematic where they capacitively couple to parallel lines that have unequal capacitance with respect to the offending sources. Early telephone engineers building long transmission lines discovered they could realize balanced transmission by means of crossover links placed at intervals of several hundred yards.

In today’s telecommunications environment, the technique takes the form of twisted pairs. At higher frequencies and with greater ambient electronic noise, this technique is becoming ever more relevant.

Category 5e (enhanced) replaced Category 5 cable in 2001. It is unshielded, depending on a good twist rate rather than on metallic shielding to reject noise in residential Ethernet applications. Cat 5e cable consists of four individually twisted pairs in a loose-fitting jacket that can be removed at terminations without nicking the individual conductors. Stranded conductors for patch cords and solid conductors for premises installations are available, and both have been tested for acceptable attenuation at 100 MHz, making them suitable for LAN installations.

The moderate cost of Cat 5e means it is a good choice for telephone zone wiring. With four color-coded pairs, there is the potential to terminate additional lines down the road. As with the newer Cat 6a (augmented), which has a greater twist rate for higher-frequency common-mode rejection, the terminations are a limiting factor where close twisting cannot be sustained.

There is a point to note about making measurements on balanced lines: Measurements won’t be accurate if the balanced lines are connected directly to the input of an ordinary instrument, because the inputs are unbalanced, i.e. grounded. Specifically, this kind of balanced-to-unbalanced connection increases the chances of ground-loop current flow that can introduce significant levels of electrical noise into the signal path.

The way to avoid this difficulty is to employ a balun. A balun is usually made up of a special kind of transformer. Its two outputs will be equal and opposite. These are the connections that go to the balanced line. The balun input is unbalanced and is matched to the impedance of the test instrument, usually 50 Ω. Note that the balun output impedance doesn’t necessarily match its input impedance. Baluns used for making measurements on twisted pair such as Cat5 cable generally have a 100-Ω output impedance to match the 100-Ω Cat5 characteristic impedance.

Use of a balun in measurements introduces some additional complexities. Baluns have their own parasitic circuit elements which may need to be accounted for in measurement calculations. Also, the most important performance criterion is how close the balanced outputs are to having equal power and 180° phase, called balance. Phase balance is the measure of how closely the inverted output is to 180° out of phase with the non-inverted output, usually given in degrees. Besides the balun structure, mismatches in the lengths of the output lines affect the balance. Typical phase balance for standard microwave baluns is ±15° max and ±10° typical.

There are several other performance factors that apply when baluns are employed. They include amplitude balance, which is ±1 dB typical. If two identical signals with identical phase are injected into the balanced ports of the balun, the amount of attenuation this signal experiences from the balanced to unbalanced port is called common mode rejection ratio (CMRR) and is expressed in dB. Common baluns will have 15-20 dB of CMRR.