On 3/12/2020 4:13 AM, John Kaufmann via Topband wrote:
I think you are confusing voltage and power. For incoherent sources like amplifier noise, the voltages of multiple incoherent sources add in a root-sum-squared (RSS) fashion. The voltage of the sum of eight incoherent sources is square root of eight times a single noise source, assuming equal combining ratios. However, because power is proportional to the square of voltage, then the*power* of the combined sum is the sum of the individual noise powers.
As long as they are measured at the same point in any given circuit, dB computations for power and for voltage yield the same result provided that the nature of the signals, their frequencies, and phase relationships are taken into account. It's important to remember that the fundamental definition of dB is the log of a POWER ratio; it gets tied to voltage when circuit impedance is defined. And that PHASE has meaning only at a single frequency.
The above computations can get messy when impedances vary with frequency, and as the phase relationship between coherent signals vary with frequency, position, and time offsets.
73, Jim K9YC _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
