510 Keysight N9927-90001 User’s Guide
AM/FM Metrics (SA Only and Option 355)
Using AM/FM Metrics
Table 25-1 Measurement Details
Measurement details
Modulation Rate The modulation rate quantifies how fast the modulation is and equates to the
frequency of the baseband signal that modulates the carrier.
SINAD (Signal to Noise
and Distortion Ratio)
Another commonly used distortion measure is signal to noise and distortion
ratio (SINAD). SINAD, in nature, is the reciprocal of the modulation distortion
provided by the “distortion/total Vrms,” but the SINAD is usually expressed in
a logarithmic term as follows:
Where:
S = Signal Power
D =
N = Noise
SINAD = [(S + D + N) / (D + N)] OR
Refer to “SINAD and THD Calculations” on page 516.
THD (Total Harmonic
Distortion)
The total harmonic distortion (THD) measurement further isolates the total
harmonic distortion component from the noise component in the total
unwanted signal. It helps the user to troubleshoot the root causes of
modulation distortion.
S = Signal Power
D =
N = Noise
THD = [(D + N) / (S + D + N)]
RF spectrum of the
modulated signal
This is the most traditional spectrum analyzer measurement viewing the
modulated carrier signal power in frequency domain. Prior to being
modulated, the signal power of a sinusoidal carrier concentrates at the carrier
frequency. By contrast, modulation causes sidebands indicating the power
redistribution over frequencies. The pattern of the sidebands depends upon
the modulation format.
Demod waveform This measurement retrieves the baseband signal from the modulated signal
via the demodulation process, and displays the baseband signal in a pattern of
modulation depth/deviation versus time. Since the modulation
depth/deviation is directly proportional to the instantaneous amplitude of the
baseband signal, the measurement result helps to intuitively evaluate the
quality of the baseband signal.
AM modulation depth AM modulation depth = (Peak-to-Peak / 2)
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