A spectrum analyzer will automatically add the attenuator setting to the measured absolute power.
The units you need to know are
- dB -- power relative to some reference value
- dBm -- power relative to 1mW
- dBc -- power relative to carrier power
The problem with using the RTL-SDR is that these devices are not designed for absolute power measurements, but rather for extracting digital data. As such, they do not have any compensation for their own frequency response, which is overlaid with the measurement data you have.
The main issue, however is that you are measuring in dBc, by normalizing your carrier power to 0dB and then looking at the harmonics. dBc give you a good estimate of the quality of your amplifier, but for regulatory purposes, only absolute power in dBm is relevant, which requires you to use a device that has been calibrated.
The attenuation setting on the spectrum analyzer needs to be chosen so the entire RF range is overload free, as overloads in the RF frontend would produce harmonics in the first mixer stage, before the selection filter.
Some analyzers include a high pass filter option, where higher harmonics are measured by replacing the attenuator with a high pass filter, allowing for better resolution while still remaining overload free; also, analyzers often provide a dedicated harmonics measurement that automatically searches the base frequency, adjusts the attenuator and reports back absolute and relative values with error margins.
No, it' doesn't. It depends on the characteristics of the load. If resitive, for example, then it presents the same impedance to all harmonics. Since the impedance of the electric transmission system generally goes up with frequency, a resistive load will actually reduce the harmonics more than the fundamental.
Best Answer
Yes. The switching pattern is the essential factor that determines the amplitude and phase of the harmonic components. The switching pattern determines the waveform of the current and/or voltage produced. The fourier analysis of the waveform provides the amplitudes and phase of the harmonic components. Usually only the amplitude is of interest, so the fourier analysis is simplified to give only the amplitude, but both the amplitude and phase can be obtained. You should be able to find extensive information on fourier analysis. Here is a link a PDF textbook chapter that seems to have the relevant information: LINK
Note that switching pattern is not the only factor. In many situations, the switching pattern determines the voltage waveform, but the voltage waveform must be applied to a complex loop impedance to determine the current waveform.