R&S FSW Series, Руководство пользователя

Страница: 313 / 1450

Measurements and Results
R&S
®
FSW
313 User Manual 1173.9411.02 ─ 43
range. The center frequency is set to this frequency and the reference level is adjusted
accordingly.
For measurements in zero span, the center frequency remains unchanged.
The Harmonic Distortion measurement then performs zero span sweeps at the center
frequency and at each harmonic, i.e. at frequencies that are a multiple of the center
frequency.
As a result, the zero span sweeps on all harmonics are shown, as well as the RMS
values and the total harmonic distortion (THD).
An application note discussing harmonics measurement is available from the Rohde &
Schwarz website:
1EF78: Measurement of Harmonics using Spectrum Analyzers
7.10.2 Harmonic Distortion Basics
Measuring the harmonics of a signal is a frequent problem which can be solved best
using a signal analyzer. In general, every signal contains harmonics. Harmonics are
generated by nonlinear characteristics, which add frequencies to a pure sinewave.
They can often be reduced by low pass filters. Since the signal analyzer itself has a
nonlinear characteristic, for example in its first mixer, measures must be taken to
ensure that harmonics produced in the signal analyzer do not cause spurious results. If
necessary, the fundamental wave must be attenuated selectively with respect to the
other harmonics with a high pass filter. Harmonics are particularly critical regarding
high-power transmitters such as transceivers because large harmonics can interfere
with other radio services.
Harmonic distortion can be determined as the level of the individual components, or as
the root mean square of all components together, the total harmonic distortion (THD).
The THD is set in relation to the power of the fundamental frequency (= center fre-
quency).
Obtainable dynamic range
When harmonics are being measured, the obtainable dynamic range depends on the
second harmonic intercept of the signal analyzer. The second harmonic intercept is the
virtual input level at the RF input mixer at which the level of the 2nd harmonic becomes
equal to the level of the fundamental wave. In practice, however, applying a level of
this magnitude would damage the mixer. Nevertheless the available dynamic range for
measuring the harmonic distance of a DUT can be calculated relatively easily using the
second harmonic intercept.
As shown in Figure 7-52 , the level of the 2 nd harmonic drops by 20 dB if the level of the
fundamental wave is reduced by 10 dB.
Harmonic Distortion Measurement