Chapter 5 301
Concepts
Analog Modulation Concepts
Modulation SINAD Measurement Concepts
Purpose
Modulation SINAD (SIgnal to Noise And Distortion) measures the amount of
Modulation SINAD contained in the modulated signal by determining the ratio of
fundamental power to harmonic and noise power. Modulation SINAD is reciprocal
of modulation distortion provided by Modulation Distortion measurement. This is
another way to quantify the quality of the modulation process
Measurement Technique
Modulation SINAD is defined as:
Equation 5-4
where: P
total
= the power of the total signal,
P
signal
= the power of the wanted modulating signal, and
P
total
− P
signal
= the total unwanted signals which include harmonic distortion
and noise.
See Figure 5-11 on page 299. First, the received signal is demodulated and filtered
to remove DC, then the filtered signal is transformed by an FFT into frequency
domain. Next, total power in the total filter band is measured as P
total
, the peak
power of the modulated signal is computed as P
signal
, the square root of the ratio of
P
total
to P
total
− P
signal
is calculated. The result is signal’s Modulation SINAD. It
can be expressed as dB or %.
dB
ModulationSINAD
20
P
total
P
total
P
signal
–
-----------------------------------log×=
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