Glossary Terms and Definitions Beginning with the Letter  v

Examples: V_CC is a positive-voltage supply and the collector terminal of bipolar transistors is connected to the V_CC supply or to a load which connects to V_CC. V_SS connects to the source terminal of a FET, etc.

V+ and V- are also common ways to refer to a supply voltage.

The term originated in the 1970s along with "SSI" (small-scale integration), "LSI" (large-scale), and several others, defined by the number of transistors or gates per IC. It was all a bit silly since improving technology obviously makes numerical definitions meaningless over time. And it varies by industry -- a VLSI analog part is quite different from a VLSI digital logic part or a VLSI memory part.

Eventually, the pundits began trying terms like "ULSI" (ultra-large-scale). Engineers, meanwhile, ignored it all and spent their time building better devices instead of making up new words for them.

The terms LSI and VLSI are now usually used as general terms, referring to a product or technology that subjectively has more devices than typical products in the category. Maxim/Dallas Semiconductor has observed a technical trend in analog and mixed signal toward increasing complexity. Many of our parts include complex control, such as the MAXQ microcontroller core, with many times more devices than most analog parts.

Using a common plumbing analogy, voltage is similar to water pressure and current is analogous to flow (e.g. liters per minute).

In equations, the symbol E is often used (as in: E = IR). V is the symbol for the unit of measure, Volt.

Electrical power is measured in watts (W): The voltage times the current measured each instant. In a direct current system or for resistive loads, the wattage and VA measurements will be identical. But for reactive loads, the voltage and current are out of phase and the volt-ampere spec will be greater than the wattage.

For determining power, watts are appropriate. For determining capacity for the driving circuits (circuit breakers, wiring, and uninterruptible power supplies, for instance), VA is appropriate.

In an ideal system, 100% of the energy is transmitted. This requires an exact match between the source impedance, the characteristic impedance of the transmission line and all its connectors, and the load's impedance. The signal's AC voltage will be the same from end to end since it runs through without interference.

In real systems, mismatched impedances cause some of the power to be reflected back toward the source (like an echo). Reflections cause destructive interference, leading to peaks and valleys in the voltage at various times and distances along the line.

VSWR measures these voltage variances. It is the ratio of the highest voltage anywhere along the transmission line to the lowest. Since the voltage doesn't vary in an ideal system, its VSWR is 1:1. When reflections occur, the voltages vary and VSWR is higher -- 1.2:1 or 2:1, for instance.

Mathematically:

VSWR is the voltage ratio of the signal on the transmission line:

VSWR = |V(max)| / |V(min)|

where V(max) is the maximum voltage of the signal along the line, and V(min) is the minimum voltage along the line.

It can also be derived from the impedances:

VSWR = (1+)/(1-)

where (gamma) is the voltage reflection coefficient near the load, derived from the load impeadance (ZL) and the source impedance (Zo):

= (ZL-Zo)/(ZL+Zo)

If the load and transmission line are matched, = 0, and VSWR = 1:1.