Dynamic Range: How it works

With a constant LO level, the mixer output is linearly related to the input
signal level. For all practical purposes, this is true as long as the input signal
is more than 15 to 20 dB below the level of the LO. There are also terms
involving harmonics of the input signal:

These terms tell us that dynamic range due to internal distortion is a
function of the input signal level at the input mixer. Let's see how this works,
using as our definition of dynamic range, the difference in dB between the
fundamental tone and the internally generated distortion.

Distortion is often described by its order. The order can be determined by
noting the coefficient associated with the signal frequency or the exponent
associated with the signal amplitude. Thus second-harmonic distortion is
second order and third harmonic distortion is third order. The order also
indicates the change in internally generated distortion relative to the change
in the fundamental tone that created it.

Now let us add a second input signal:

Figure 6-1. Changing the level of fundamental tones at the mixer