Rohde & Schwarz R&S SMU200A, Service Manual

Page: 59 / 294

R&S SMU200A Test Procedures
1007.9845.82 1.33
E-12
Frequency Modulation (Option R&S B20/B22)
Test Methodes
Test assembly
See section "
Standard Test Assembly for Analog Modulations
", page
1.3 .
Test Method "Spectrum
Measurement"
•
The FM/PhiM deviation is determined by a spectrum measurement.
All relevant spectral components of the modulation spectrum are
measured and modulation index m is computed via Bessel
functions.
To suppress AM components, left and right spectral lines are
averaged. From the ratio of the averaged spectral components a
recalculation is done to get the modulation index m. Since the ratio
of the spectral lines is used, the result is corrrect, if a sufficient
number of lines is used. Missing lines at the spectrum borders do
not deteriorate the result.
•
Restrictions: This method is suitable only with sine shaped
modulation signal. Measuring large deviations at low modulation
frequencies tends to get time consuming, since many spectrum
lines are to be measured and recalculation may be ambiguous. It is
recommended to use this method up to modulation index 100.
Settings on spectrum analyzer:
AMPT REF LEVEL test level
FREQ CENTER test frequency
Set SPAN to fmod/4, maximal 80 kHz and resolution bandwidth to
fmod/20, maximal 20 kHz.
First measure the carrier amplitude as zero order of Bessel function,
then determine the n orders up to int(m + 1) by measuring the
spectral lines CF + (n * fmod) and CF - (n * fmod). Calculate the
average of the amplitude of both lines for each order. Now the
measured modulation index m can be calculated by numerical search
for the error minimum.
The PhiM deviation equals the modulation index m, FM deviation is
calculated as m * fmod.