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Noise Uncertainty

Question asked by PeterM on May 31, 2007
Latest reply on Jun 1, 2007 by PeterM
I have recently started using an 8753C and I am trying to understand the accuracy limits of my measurements.  I am a bit uncertain as to how the noise can be included in the uncertainty calculations.

There are two types of noise, Low-Level (Noise Floor) and High-Level (Phase Noise effects etc.), and the 8753C specifications provide values for each, e.g.

Low-Level Noise: -100 dBm
High-Level Noise: 0.004 dB

The 8753C specifications also has some nice equations for the uncertainty of transmission and reflection measurements, but these ignore both noise sources.

Looking on the web-site I have found, in the PNA help, similar equations that do include the noise sources:

Noise^2 = (Nt x S11)^2 + Nf^2

Where Nt is the trace (high-level) noise and Nf is the noise floor.

The difficulty that I have is this:

In all the specifications that I can find the noise floor figure is a dBm value.  Does it make sense to include and absolute power (units dBm or mW) value in a calculation in which all other values are unit-less?

I'd be grateful for any thoughts,