Similar Line for Third-order Distortion

We can construct a similar line for third-order distortion. For example,
a data sheet might say third-order distortion is 85 dBc for a level of 30 dBm
at this mixer. Again, this is our starting point, and we would plot the point
shown in Figure 6-2. If we now drop the level at the mixer to 40 dBm, what
happens? Referring again to Figure 6-1, we see that both third-harmonic
distortion and third-order intermodulation distortion fall by 3 dB for very
dB that the fundamental tone or tones fall. Again it is the difference that
is important. If the level at the mixer changes from 30 to 40 dBm, the
difference between fundamental tone or tones and internally generated
distortion changes by 20 dB. So the internal distortion is 105 dBc. These
two points fall on a line having a slope of 2, giving us the third-order
performance for any level at the mixer.

Figure 6-2. Dynamic range versus distortion and noise