Rohde & Schwarz 1321.0615.02, User Manual

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Modulation Analysis
R&S
®
Spectrum Rider FPH
238 User Manual 1321.1011.02 ─ 11
Label Description
THD Total harmonic distortion
Measures the ratio of the harmonics to the funda-
mental and harmonics. All harmonics inside AF
spectrum are considered up to the tenth har-
monic.
SNR Shows the ratio of the total power to the power of
noise in dB.
10.1.1 Demodulation Bandwidth
The demodulation bandwidth is not the 3 dB bandwidth but the useful bandwidth which
is distortion-free for phase and amplitude. See
Table 10-2 .
Therefore the following formulas apply:
●
AM: demodulation bandwidth ≥ 2 x modulating frequency
●
FM (CARSON's rule): demodulation bandwidth ≥ 2 x (frequency dev highest
modulating frequency)
If the center frequency of the analyzer is not set exactly to the signal frequency, the
demodulation bandwidth must be selected larger by the carrier offset, in addition to the
requirement described above.
In general, the demodulation bandwidth should be as narrow as possible to improve
the S/N ratio. The residual FM caused by noise floor and phase noise increases dra-
matically with the bandwidth, especially with FM.
10.1.2 Sample Rate, Measurement Time and Audio Lowpass Filter
Depending on the sample rate, the maximum demodulation bandwidths listed in the
Table 10-2 can be obtained during the measurement. The permissible value range of
the measurement time depends on the selected demodulation bandwidth and audio
lowpass filter.
Example: AM modulation with a modulating frequency of 1kHz
1. Calculate the demodulation bandwidth you require.
Demodulation Bandwidth = 2 x 1 kHz
2. Select the minimum demodulation bandwidth from table Table 10-2 .
Analog Demodulation
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