Measuring Signal-To-Noise Of A Modulated Carrier - Keysight X Series Measurement Manual

Measuring Signal-to-Noise of a Modulated Carrier

Signal-to-noise (or carrier-to-noise) is a ratio used in many communication systems
as indication of the noise performance in the system. Typically, the more signals
added to the system or an increase in the complexity of the modulation scheme can
add to the noise level. This can reduce the signal-to-noise ratio and impact the
quality of the demodulated signal. For example, a reduced signal-to-noise in digital
systems may cause an increase in EVM (error vector magnitude).
With modern complex digital modulation schemes, measuring the modulated carrier
requires capturing all of its power accurately. This procedure uses the Band Power
Marker with a RMS average detector to correctly measure the carrier's power within
a user adjustable region. A Noise Marker (normalized to a 1 Hz noise power
bandwidth) with an adjustable noise region is also employed to allow the user to
select and accurately measure just the system noise of interest. An important key to
making accurate Band Power Marker and Noise Power measurements is to insure
that the Average Type under the Meas Setup key is set to "Auto".
In this example a 4 carrier W-CDMA digitally modulated carrier is used as the
fundamental signal and the internal noise of the analyzer is measured as the system
noise.
Step
1 Set up the signal sources.
2 Instrument setup.
3 Set the analyzer to the
Spectrum Analyzer mode.
4 Preset the analyzer.
Action
a. Setup a 4 carrier W-CDMA signal.
b. Set the source frequency to 1.96
GHz.
c. Set the source amplitude to –10 dBm.
• Connect the source RF OUTPUT to the
analyzer RF INPUT as shown.
• Press Mode, Spectrum Analyzer
• Press Mode Preset.
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Measuring Signal-to-Noise of a Modulated Carrier
Notes
This enables the spectrum
analyzer measurements
Measuring Noise