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7
Tutorial
Signal Imperfections
For sine waveforms, signal imperfections are easiest to describe and observe in
the frequency domain using a spectrum analyzer. Any component of the output
signal which has a different frequency than the fundamental (or "carrier") is
considered to be spurious. The signal imperfections can be categorized as
harmonic, non-harmonic, or phase noise and are specified in "decibels relative to
the carrier level" or "dBc".
Harmonic Imperfections
Harmonic components always appear at multiples of the fundamental frequency
and are created by non-linearities in the waveform DAC and other elements of the
signal path. At low amplitudes, another possible source of harmonic distortion is
due to the current flowing through the cable connected to the function generator's
Sync output connector. This current can cause a small square-wave voltage drop
across the resistance of the cable's shield and some of this voltage can be imposed
on the main signal. If this is a concern for your application, you should remove the
cable or disable the Sync output connector. If your application requires that you
use the Sync output connector, you can minimize the effect by terminating the
cable in a high impedance load (rather than into a 50Ω load).
Non-Harmonic Imperfections
The biggest source of non-harmonic spurious components (called "spurs") is the
waveform DAC. Nonlinearity in the DAC leads to harmonics that are aliased, or
"folded back", into the passband of the function generator. These spurs are most
significant when there is a simple fractional relationship between the signal
frequency and the function generator's sampling frequency (50 MHz).
Another source of non-harmonic spurs is the coupling of unrelated signal sources
(such as the microprocessor clock) into the output signal. These spurs usually have
a constant amplitude (≤ -75 dBm or 112 µVpp) regardless of the signal's amplitude
and are most troublesome at signal amplitudes below 100 mVpp. To obtain low
amplitudes with minimum spurious content, keep the function generator's output
level relatively high and use an external attenuator if possible.
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Keysight 33210A User's Guide
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