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Chapter 7 Tutorial
Signal Imperfections
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).
For example, at 15 MHz, the DAC produces harmonics at 30 MHz and 45
MHz. These harmonics, which are 20 MHz and 5 MHz from the function
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generator's 50 MHz sampling frequency, will appear as spurs at 20 MHz
and 5 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.
Phase Noise Phase noise results from small, instantaneous changes in
the output frequency ("jitter"). It is seen as an elevation of the apparent
noise floor near the fundamental frequency and increases at 6 dBc / octave
with the carrier frequency. The 33220A's phase noise specification
represents the amplitude of the noise in a 1 Hz bandwidth, 10 kHz away
from a 20-MHz carrier.
Quantization Errors Finite DAC resolution (14 bits) leads to voltage
quantization errors. Assuming the errors are uniformly distributed over
a range of ±0.5 least-significant bit (LSB), the equivalent noise level is
-86 dBc for a sine wave that uses the full DAC range (16,384 levels).
Similarly, finite-length waveform memory leads to phase quantization
errors. Treating these errors as low-level phase modulation and assuming
a uniform distribution over a range of ±0.5 LSB, the equivalent noise
level is -76 dBc for a sine wave that is 16K samples long. All of the
33220's standard waveforms use the entire DAC range and are 16K
samples in length. Any arbitrary waveforms that use less than the
entire DAC range, or that are specified with fewer than 16,384 points,
will exhibit proportionally higher relative quantization errors.
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