Uncertainty due to low signal-to-noise ratio

Next, let's look at uncertainty due to low signal-to-noise ratio. The distortion
components we wish to measure are, we hope, low-level signals, and often
they are at or very close to the noise level of our spectrum analyzer. In such
cases, we often use the video filter to make these low-level signals more
discernable. Figure 6-7 shows the error in displayed signal level as a function
of displayed signal-to-noise for a typical spectrum analyzer. Not that the
error is only in one direction, so we could correct for it. However, we usually
do not. So for our dynamic range measurement, let's accept a 0.3 dB error
due to noise and offset the noise curve in our dynamic range chart by 5 dB
as shown in Figure 6-6. Where the distortion and noise curves intersect, the
maximum error possible would be less than 1.3 dB.

Figure 6-6. Dynamic range for 1.3 dB maximum error