Chapter 5
Signal Generation Fundamentals
NI PXIe-5450 User Manual
5-12
ni.com
Frequency Domain Fundamentals
SFDR
Spurious–free dynamic range (SFDR) is the usable dynamic range before
spurious noise interferes with or distorts the fundamental signal. For
specification purposes, the amplitude of the fundamental signal is usually
–1 dBFS. SFDR is the measure of the difference in amplitude between the
fundamental signal and the largest harmonically or nonharmonically
related spur from DC to the full Nyquist bandwidth (half the sampling rate).
A spur is any frequency bin on a spectrum analyzer, or from a Fourier
transform, of the analog signal. SFDR is expressed in dBc.
The following figure illustrates how SFDR is measured.
THD
The total harmonic distortion (THD) of a signal is the ratio of the sum of
the powers of the first five harmonics above the measured fundamental
frequency to the power of the fundamental frequency. THD is usually
expressed in dB or dBc. Measurements for calculating the THD are made
at the output of a device under specified conditions.
Amplitude
Frequency
RMS Fundamental Signal
Largest Spur
Spurious Free Dynamic Range
f
N
f
S
/2
f
S
