The decibel equivalent for any power ratio is
Equation 1.4
where 
p 1 /p 2 is a power ratio. 
In two circuits of equal impedance, the ratio of their powers may be determined as the square of the ratios of the voltages. This definition of a power ratio, in units of decibels, assumes equal impedances in the two circuits :
Equation 1.5
where 
v 1 / v 2 is the ratio of RMS voltages across two circuit elements of equal resistance, or in the case of identical signal shapes in the two circuits, such as sine waves, the ratio of voltage amplitudes across two circuits having equal complexvalued impedances, and 
the same formula and conditions apply to the determination of a power ratio from a ratio of currents. 
If the impedances in the two circuits differ , then the power ratio must be corrected for that difference.
Equation 1.6
where 
v 1 / v 2 is the ratio of RMS voltages across two circuit elements having resistances z 1 and z 2 respectively, or in the case of identical signal shapes in the two circuits, such as sine waves, the ratio of voltage amplitudes across two circuits having proportional complexvalued impedances. 
i 1 /i 2 is the ratio of RMS currents flowing through two circuit elements having resistances z 1 and z 2 respectively, or in the case of identical signal shapes in the two circuits, such as sine waves, the ratio of current amplitudes across two circuits having proportional complexvalued impedances. 
In some cases it is convenient to use decibel notation to express a ratio of the voltages (or currents) within two circuits without consideration of the level of power.
Equation 1.7
where 
v 1 / v 2 is the ratio of voltage amplitudes within the two circuits. Presumably, the two circuits carry signals with identical shapes, such as sine waves. 
i 1 /i 2 is the ratio of current amplitudes within the two circuits. Presumably, the two circuits carry signals with identical shapes, such as sine waves. 
The magnitude of a voltage, current, or power relative to a standard reference level may be expressed in absolute terms using decibel notation. For example, the notation dBmV refers to the following quantity:
Equation 1.8
where 
v 1 represents the voltage amplitude of a signal generally assumed to be sinusoidal. 
Units of nepers occasionally appear in transmissionline problems. One neper equals 8.685889638065 dB. Definitions of power, voltage, and current ratios in units of nepers use the natural (base e ) logarithm ln ().
Equation 1.9
where 
p 1 /p 2 is a power ratio, 
v 1 / v 2 is the ratio of voltage amplitudes within the two circuits. Presumably, the two circuits carry signals with identical shapes, such as sine waves. 

i 1 /i 2 is the ratio of current amplitudes within the two circuits. Presumably, the two circuits carry signals with identical shapes, such as sine waves. 

ln( x ) is the natural logarithm function (log base e of x ). 
POINT TO REMEMBER
Fundamentals
Transmission Line Parameters
Performance Regions
FrequencyDomain Modeling
Pcb (printedcircuit board) Traces
Differential Signaling
Generic BuildingCabling Standards
100Ohm Balanced TwistedPair Cabling
150Ohm STPA Cabling
Coaxial Cabling
FiberOptic Cabling
Clock Distribution
TimeDomain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. TwoPort Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )
Notes