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dBm (600 Ω) = dBm (50 Ω) – 10.79
Signal Imperfections
For sine waves, common signal imperfections are easiest to describe and observe in the frequency domain, using a
spectrum analyzer. Any output signal component with a frequency different from the fundamental (or "carrier") is
considered to be distortion. Those imperfections can be categorized as harmonic distortion, non-harmonic spurious,
or phase noise, and they are specified in decibels relative to the carrier level, or "dBc."
Harmonic Distortion
Harmonic components occur at integer multiples of the fundamental frequency and are usually created by non-
linear components in the signal path. At low signal amplitudes, another possible source of harmonic distortion is the
Sync signal, which is a square wave with many strong harmonic components that can couple into the main signal.
Although Sync is highly isolated from the instrument's main signal outputs, coupling can occur in external cabling.
For best results, use high-quality coaxial cables with double or triple shields. If Sync is not required, leave it
unconnected or off.
Non-Harmonic Spurious
One source of non-harmonic spurious components (called "spurs") is the digital-to-analog converter (DAC) that
converts the digital waveform values into voltage. Non-linearity in this DAC gives rise to harmonics that can be
higher than the Nyquist frequency and will therefore be aliased to a lower frequency. For example, the fifth harmonic
of 30 MHz (150 MHz) could create a spur at 100 MHz.
Another source of non-harmonic spurs is the coupling of unrelated signal sources (such as the embedded
controller's clocks) into the output signal. These spurs usually have constant amplitude and are most troublesome at
signal amplitudes below 100 mVpp. For optimal signal purity at low amplitudes, keep the instrument's output level
relatively high and use an external attenuator.
Phase Noise
Phase noise results from small, instantaneous changes in the output frequency ("jitter"). On a spectrum analyzer, it
appears as a rise in the apparent noise floor near the frequency of the output signal. The phase noise specification
represents the amplitudes of the noise in 1 Hz bands located 1 kHz, 10 kHz, and 100 kHz away from a 30-MHz sine
wave. Be aware that spectrum analyzers also have phase noise, so the levels you read may include analyzer phase
noise.
Quantization Noise
Finite resolution in the waveform DAC causes voltage quantization errors. Assuming the errors are uniformly
distributed over a range of ±0.5 least-significant bit, the equivalent noise level for standard waveforms is
approximately -95 dBc. At this level, other sources of noise in the instrument dominate. Quantization noise can be of
concern, though, in arbitrary waveforms that do not use the whole range of DAC codes (-32767 to +32767). Scale
arbitrary waveforms to use the entire range, if possible.
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Keysight EDU33210 Series User's Guide
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