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Chapter 7 Tutorial
Signal Imperfections
4
7
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.
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 33210A’s phase noise specification represents the
amplitude of the noise in a 1 Hz bandwidth, 10 kHz away from a 10-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
33210A’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 will exhibit proportionally higher relative
quantization errors.
33210A users guide.book Page 319 Wednesday, July 16, 2008 11:16 AM
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