Noise Measurements; Measuring Noise Power Density - Rohde & Schwarz R&S FSP Series Operating Manual

R&S FSP

2.4 Noise Measurements

Noise measurements play an important role in spectrum analysis. Noise e.g. affects
the sensitivity of radio communication systems and their components.
Noise power is specified either as the total power in the transmission channel or as
the power referred to a bandwidth of 1 Hz. The sources of noise are, for example,
amplifier noise or noise generated by oscillators used for the frequency conversion
of useful signals in receivers or transmitters. The noise at the output of an amplifier
is determined by its noise figure and gain.
The noise of an oscillator is determined by phase noise near the oscillator frequency
and by thermal noise of the active elements far from the oscillator frequency. Phase
noise can mask weak signals near the oscillator frequency and make them impossi-
ble to detect.
2.4.1

Measuring noise power density

To measure noise power referred to a bandwidth of 1 Hz at a certain frequency, the
R&S FSP has an easy-to-use marker function. This marker function calculates the
noise power density from the measured marker level.
2.4.1.1 Measurement example – Measuring the intrinsic noise power density of the
R&S FSP at 1 GHz and calculating the R&S FSP's noise figure
1. Set the R&S FSP to its default state.
2. Set the center frequency to 1 GHz and the span to 1 MHz.
3. Switch on the marker and set the marker frequency to 1 GHz.
4. Switch on the noise marker function.
5. The measurement result is stabilized by averaging the trace
2.14
Press the PRESET key.
The R&S FSP is in its default state.
Press the FREQ key and enter 1 GHz.
Press the SPAN key and enter 1 MHz.
Press the MKR key and enter 1 GHz.
Press the MEAS key.
Press the NOISE MARKER softkey.
The R&S FSP displays the noise power at 1 GHz in dBm (1Hz).
Since noise is random, a sufficiently long measurement time has to be selected
to obtain stable measurement results. This can be achieved by averaging the
trace or by selecting a very small video bandwidth relative to the resolution
bandwidth.
Press the TRACE key.
Press the AVERAGE softkey.
The R&S FSP performs sliding averaging over 10 traces from consecutive
sweeps. The measurement result becomes more stable.
Getting Started
Noise Measurements
Operating Manual 1164.4556.12 - 06