Agilent Technologies 33120A, User Manual

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At higher output frequencies, additional
DAC
errors produce
non-harmonic spurious outputs. These are signals “folded back” or
aliased to a frequency within the signal bandwidth. A “perfect”
DAC
will
also produce a wideband noise floor due to amplitude quantization.
The noise floor for a 12-bit
DAC
will be near the -74 dBc level; this
corresponds to a noise density of -148 dBc/Hz for sine wave outputs
from the 33120A.
Amplitude Quantization
ˆ
– ( 20 x log
10
( N x 4096 ) + 1.8 ) dBc
where “N” is the fraction of available DAC codes used to describe
the signal waveshape (0
ˆ
N
ˆ
1).
Another type of waveform error visible in the frequency domain is
phase truncation error . This error results from time quantization of the
output waveform. Whenever a waveshape is described by a finite number
of horizontal points (length), it has been sampled in time (or quantized)
causing a phase truncation error. Spurious signals caused by phase
truncation introduce jitter into the output waveform. This may be
regarded as time (and phase) displacement of output zero crossings.
Phase truncation causes phase modulation of the output signal which
results in spurious harmonics (see the equation below). For lower output
frequencies, the phase accumulator periodically
does not advance
RAM
addresses, causing the
DAC
to deliver the same voltage as recorded on
the previous clock cycle. Therefore, the phase “slips” back by 360
•
/ points
before continuing to move forward again. When
RAM
address increments
are the same on each cycle of the output, phase truncation error (and
jitter) are essentially zero. All standard waveshapes in the 33120A are
generated with at least 16,000 waveform points which results in
spurious signals below the wide-band noise floor of the
DAC
.
Phase Truncation Harmonics
ˆ
– 20 x log
10
(P) dBc
where “P” is the number of waveform points in RAM.
7
Chapter 7 Tutorial
Signal Imperfections
277