I'm having a pretty difficult time understanding how tuning works when using harmonics in spectrum analyzers. I'm reading about the topic from Keysight's AN 150.
[...] we first plot the LO frequency against the signal frequency axis. Multiplying the LO frequency by two yields the upper dashed line of Figure 7-3. As we did for fundamental mixing, we simply subtract the IF (5.1 GHz) from and add it to the LO second-harmonic curve to produce the 2− and 2+ tuning ranges. Since neither of these overlap the desired 1− tuning range, we can again argue that they do not really complicate the measurement process. In other words, signals in the 1− tuning range produce unique, unambiguous responses on our analyzer display.
I don't understand why it states that they don't overlap. I clearly see that for a pretty big range of the vertical axis, there will be two image frequencies: one corresponding to 1− (the desired one) and another one corresponding to 2−. In other words, for any signal frequency between 1 GHz (approximately) and 3.6 GHz, there will be two possible possible responses. So why does the text state that they don't overlap?
This is a much more basic doubt. I'm not sure I understand the basics of how internal harmonic mixing works. Is mixing with harmonics something that happens and is undesirable, or is it something deliberate that allows us to measure higher-frequency signals?