Figure 5 . Harmonics above the 26th alias again and reflect back from left to right. The ADCs vary slightly in linearity, gain, offset, frequency response, and timing. Offset differences between the ADCs result in spectral components at 1 GHz and 2 GHz. High-speed scopes have 8 bits of resolution as opposed to the 12 bits or more in FFT analyzers.
Fewer bits result in more quantization errors.
The noise power of an ideal ADC decreases by 6.02 dB for each additional bit of resolution. With fewer bits than FFT analyzers and vector signal analyzers, your DSO's quantization noise can add nonlinearities to a signal. The DSO's noise floor is higher, by 20 to 40 dB, and the nonlinearities of the ADC process are correspondingly higher than in an FFT analyzer. Nonlinearities contaminate the FFT by allowing the quantization noise to mix with your signal, generating spurious spectral components instead of random noise. The samples from an equivalent-time record represent many independent acquisitions.