R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Getting Started
2.5 Measurements on Modulated Signals
Measurements on Modulated Signals
If RF signals are used to transmit information, an RF carrier is modulated. Analog
modulation methods such as amplitude modulation, frequency modulation and
phase modulation have a long history and digital modulation methods are now used
for modern systems. Measuring the power and the spectrum of modulated signals is
an important task to assure transmission quality and to ensure the integrity of other
radio services. This task can be performed easily with a Signal Analyzer. Modern
Signal Analyzers also provide the test routines that are essential to simplify complex
measurements.
2.5.1 Measurements on AM Signals
The R&S FSQ detects the RF input signal and displays the magnitudes of its components
as a spectrum. AM modulated signals are also demodulated by this process.
The AF voltage can be displayed in the time domain if the modulation
sidebands are within the resolution bandwidth. In the frequency domain, the AM
sidebands can be resolved with a small bandwidth and can be measured separately.
This means that the modulation depth of a carrier modulated with a sinewave signal
can be measured. Since the dynamic range of a Signal Analyzer is very wide, even
extremely small modulation depths can be measured accurately. The R&S FSQ has
a test routine which measures the modulation depth in %.
2.5.1.1 Measurement Example 1 – Displaying the AF of an AM Signal in the Time
Domain
Test setup
Settings on the signal generator (e.g. R&S SMIQ)
Frequency: 100 MHz
Level: 0 dBm
Modulation: 50 % AM, 1 kHz AF
Measurement with the R&S FSQ
1. Set the R&S FSQ to the analyzer mode
➢ Press the SPECTRUM key.
The R&S FSQ is in the analyzer mode.
2. Set the center frequency to 100 MHz and the span to 0 kHz
➢ Press the FREQ key and enter 100 MHz.
➢ Press the SPAN key and enter 0 Hz.
Operating Manual 1313.9681.12 - 01 2.27