Common Analysis and Display Functions
R&S
®
FSW
490
User Manual 1173.9411.02 ─ 19
7.4.1.8
Measuring Phase Noise
Phase noise is unintentional modulation of a carrier; it creates frequencies next to the
carrier frequency. A phase noise measurement consists of noise density measure-
ments at defined offsets from the carrier; the results are given in relation to the carrier
level (dBc). The phase noise marker function measures the noise power at the delta
markers referred to 1 Hz bandwidth. Marker 1 is used as the reference for the phase
noise measurement. By default, the current frequency and level of marker 1 are used
as the fixed reference marker. However, a peak search can be started to use the cur-
rent signal peak as the reference point, or a reference point can be defined manually.
Since the reference point is fixed, the reference level or the center frequency can be
set so that the carrier is outside the displayed frequency range after phase noise mea-
surement is started. Or a notch filter can be switched on to suppress the carrier.
Alternatively, the reference point can be determined automatically by a peak search
after each sweep. This function can be used to track a drifting source during a phase
noise measurement. The delta marker 2, which shows the phase noise measurement
result, keeps the delta frequency value. Therefore the phase noise measurement leads
to reliable results in a certain offset although the source is drifting. Only if the marker 2
reaches the border of the span, the delta marker value is adjusted to be within the
span. In these cases, select a larger span.
The result of the phase noise measurement is the difference in level between the refer-
ence point and the noise power density. It is indicated as the function result of the
phase noise marker in the Marker Table.
The sample detector is automatically used and the video bandwidth set to 0.1 times the
resolution bandwidth (RBW). The two settings are taken into account in the correction
values used for the noise power measurement. To obtain stable results, two pixels on
the right and the left of the delta marker position are taken for the measurement. The
procedure for determining the noise power is identical to the method used for the noise
power measurement (see
chapter 7.4.1.7, "Measuring Noise Density"
7.4.1.9
Measuring Characteristic Bandwidths (n dB Down Marker)
When characterizing the shape of a signal, the bandwidth at a specified offset from its
peak level is often of interest. The offset is specified as a relative decrease in ampli-
tude of n dB. In order to measure this bandwidth, you could use several markers and
delta markers and determine the bandwidth manually. However, using the n dB down
marker function makes the task very simple and quick.
The n dB down marker function uses the current value of marker 1 as the reference
point. It activates two temporary markers T1 and T2 located on the signal, whose level
is n dB below the level of the reference point. Marker T1 is placed to the left and
marker T2 to the right of the reference marker. The default setting for n is 3 dB, but it
can be changed.
If a positive offset is entered, the markers T1 and T2 are placed below the active refer-
ence point. If a negative value is entered (for example for notch filter measurements),
the markers T1 and T2 are placed above the active reference point.
Marker Usage
