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I wonder what kind of the input power signal is in the S11 parameter. Is the sine or cosine I know correct?

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Well, a S11 number is a phase and amplitude for a single frequency, so yes, w.r.t. that frequency the excitation is a harmonic oscillation (i.e. a cosine with any arbitrary phase, which includes the sine). These are the only signals that have only one frequency, so your question answers itself!

However, there's nothing wrong with testing multiple frequencies at once, when looking at a linear system (like the ones you can describe with S-parameters). In that case, the signal is composed of harmonic signals – but that, says Fourier, can be done to any sufficiently well-behaved signal, anyways.

Note that a VNA might still choose to only emit a clean-as-possible single tone – if any of the system parameters is non-linear, you'd be in deeper trouble if you had multiple tones to intermodulate.

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  • \$\begingroup\$ Is there any product marketed as a VNA that uses a stimulus other than a single tone sinusoid? If an instrument uses a non sinusoidal stimulus I might call it a TDR,or something else, and it might be able to do equivalent measurements, but I wouldn't call it a VNA. \$\endgroup\$
    – The Photon
    Commented Aug 10, 2021 at 14:48
  • \$\begingroup\$ Don't know about products, but I've seen multiple SDR projects approach things like that. Also, not the craziest thing to do if you're able to produce a frequency comb to begin with (but I don't think these things are called VNA in the optical domain). But yes, as said, there's very good reason to not go multi-tone (or even wideband) for sensing. (Unless you're building a passive intermodulation tester + VNA, which is a thing, too.) \$\endgroup\$ Commented Aug 10, 2021 at 15:38
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The only difference between a sine and cosine is the phase, or time reference. When your VNA generates a single frequency, you can describe it as sine or cosine, or -sine or -cosine, the signal is the same but for the reference.

The in the S11 measurement, the incident signal at the reference plane is taken as the reference for the reflected signal, so the S11 measurement is the same however you describe the incident signal.

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