[sdiy] Equation for converting V PP to dBm?

[John Speth]

> I've been researching how to convert a Peak to Peak Voltage into dB
and I've
> come up with 4 slightly different equations which all give different
answers.
> I've also had a few different people tell me that different ones were
the
> 'right' one - so I thought I'd ask here.

>From Wikipedia: "The decibel (dB) is a logarithmic unit that indicates
the ratio of a physical quantity (usually power or intensity) relative
to a specified or implied reference level."

The important words are "ratio" and "relative".  Vpp isn't directly
convertible to dB without a reference value.  You or I could say "m"
stands for 1 millivolt and doing so we established a reference level.
It follows then that dBm = 20 * log(Vpp / 1 mV).  (FYI, dBm usually is a
power expression relative to 1 mW.)

dB is usually used for specifying circuit gain in our world - that is dB
= 20 * log(Vout / Vin).  The formula for power transmission is different
but rooted in the same math.

The equations you quoted are used in various different measurement
situations but #4 is used to indicate gain and is most appropriate for
the 2164.  I think the other equations are equivalent assuming a sine
wave signal (I think).

JJS