Spectrum analysis.
That's not. It has roughly 101 10kHz wide windows that works perfect if you set around 1MHz span. In case of lower span those windows overlap and you don't get any better resolution and with higher span there will be gaps, the whole things becomes like a comb filter (using more point from PC may help but can become very slow). That means that you can easily miss a signal although its there. So maybe its OK for very rudimentary spectral analyses if you know its limitations but its better not to think about it as something that can be used for spectrum analyzes.
Generally it can have issues (my, just like most, has) above 25-260 MHz if the temperature is too high or if its too low (a recently developed feature of mine that after cold start around 1 min there are zig-zags) of if battery becomes low.
The push-able rotating button becomes sticky quite quickly and needs rotation from the edge.
The LCD is also rather small.
All in all its a good thing and it definitely worth its price but one has to learn accepting its limitations.
There is also a bigger display version NanoVNA-F that is also probably better worked out, however bigger, heavier and pricier as well. As the operational principle is the same, most limitations apply there as well.
What happens is that in harmonic mode the DUT's response to the fundamental frequency bleeds in, and what you are measuring isn't the impedance at the frequency shown, with S11 errors of 3dB in the above plots. It will show perfect calibration but show you wrong impedances once you measure something other than the cal kit.
While this direction is more pronounced due to the stronger fundamental, the opposite can also cause surprises. Thus the clock signal is a good idea to keep it simple but I look forward to the next version that you are in and hope clean spectral signal will be used there.
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