QuantAsylum QA401, Руководство пользователя

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QuantAsylum QA401 User ’s Manual
Page
34
N+D (Noise + Distortion) is defined as the Power Measurement minus the power contained in the
fundamental. The fundamental is defined by the user, and can be either the frequency setting in the
Signal Generator 1, or it can be the highest peak, or it can be a user specified frequency. If the input is
disconnected, then a f
undamental won’t be present and the N+D will match the Power reading.
THD (Total Harmonic Distortion) is the ratio of the RMS signal power in the fundamental to the RMS sum
of the harmonics. The harmonic is selected to be 2x, 3x, 4x…n of the fundamental, u p to the
Measurement Stop setting. So, if you have a 1 KHz fundamental, and a 20 KHz Measurement Stop
setting, then the QA401 will measure up to and including the 20
th
harmonic. Similarly, if you have a 10
KHz fundamental and a 20 KHz Measurement Stop setting, then the THD will only reflect the 2
nd
harmonic.
THD+N (Total Harmonic Dist Noise) is the ratio of the RMS signal power to the Noise + Distortion
as defined above.
A more concise way to look at this is to consider the following definitions:
N
nyq
= This is the root-sum-square or all components, from the Nyquist Frequency down to (but
not including) DC
N = Noise. This is the non-signal and non-harmonic signals contained in the specified spectrum.
More specifically, this is the root-sum-square of the non-signal and non-harmonic signals.
D = Distortion. This is the sum of all harmonic peak values, not including the fundamental, up to
the Measurement Stop setting. Note that the QA100 will only consider up to 100 harmonics.
S = Signal. This is the peak level of the signal.
SNR = 20log(S/N
nyq
). This is the log ratio of the signal to noise. What is important here is that the
noise does NOT include the harmonics. Note that on the QA401, the SNR reading does not
automatically include all of the noise up until Nyquist. The SNR reading on the QA401 respects
the setting of the Measurement Stop frequency. To get a true SNR reading, you must ensure
that Measurement Stop is set to the Nyquist frequency.
SINAD = 20log(S/(N
nyq
+D))
THD = 20log(S/D)
THD+N = 20log(S/N+D))
QA401 Measurement Types
There are 4 basic measurements that can be done by the QA401. The measurements require the button
to be pressed for the measurement result to be displayed. This is done for two reasons. First, each
measurement takes time to perform. And second, showing a measurement on a presentation slide that