What is Trace Noise in a VNA?

Trace noise is the stability of the trace that is presented on the display. It limits the maximum resolution of the vector network analyzer, and it is considered an error that when presented on the trace can present as a jitter, creating points that are not from the signal, but from the noise. Most modern oscilloscopes come with features to help smooth this jitter and create traces that are easier to read.

The trace noise of a VNA measurement is related to the signal to noise ratio of the measurement or the distance between the reading and the noise floor at a particular IF Bandwidth. If the Dynamic range of the VNA is given as 140 dB at maximum output level and 10 Hz IF Bandwidth and the user makes a measurement at those settings at -130 dB then the signal to noise ratio is 10 dB. 

If the measurement is made at a more convenient 10 kHz IF Bandwidth for faster sweep speed then the dynamic range is reduced to 110 dB ( 30*Log(10K/10) ) . Now if a measurement is made at -100 dB then the signal to noise ratio is again 10 dB. 

That noise is 10^(-10/20) or a ratio of 0.316 RMS in linear terms. So a normalized unit vector sees a 0.316 vector added to it with random phase. Worst case that's 0.684 to 1.316 so the trace noise is -3.3 to +2.4 dB RMS for this example. 

This is why the Dynamic Range specification of a Vector Network Analyzer is so important.