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Agilent PN 894400-2
Measuring Phase Noise with the
Agilent 89400 Series Vector Signal
Analyzers
Product Note
The characterization of phase noise is increasingly 89410A dc-10 MHz vector signal analyzer
important in modern communications systems. The 89410A has an input bandwidth of 0 to 10 MHz,
In the past, it has been a very difficult and time- and intrinsically lower phase noise than other RF
consuming measurement. The Agilent Technologies spectrum analyzers due to its baseband implemen-
89400 Series vector signal analyzers (VSAs) greatly tation. Practically, the instrument's input noise
facilitate phase noise characterization measure- makes up the fundamental phase noise measure-
ments for designers of systems with all but the most ment's dynamic range limitation, and will be dis-
demanding requirements. The power of these vector cussed later.
signal analyzers is in their ability to make very
fast direct phase noise measurements in any The user needs to downconvert the signal-under-
domain. For example, transmitter designers are test into the 0 to 10 MHz input bandwidth. The
often interested in the actual spectral density phase noise bandwidth of interest dictates how
around the carrier or inte-grated band power in close the carrier can be to either dc or 10 MHz. If
adjacent channels, whereas users interested in the possible, the instrument should be phase locked to
recovery of digitally modulated information may the signal-under-test, although it is not necessary.
be most concerned with peak or RMS phase devia-
tion of the recovered vectors. The 89400 Series VSAs
89440A dc-1.8 GHz vector signal analyzer
quickly and easily produce results in any of these
While most of the following discussion applies to
domains. They are also capable of mathematically
all the 89400 Series VSAs, some differences apply.
locking to unlocked or drifting carriers, allowing
The 89440A can directly measure input signals up
fast and accurate averaged measurements even
to 1.8 GHz. However, since the RF section is essen-
under these conditions. This is a powerful feature,
tially a heterodyned receiver, its local oscillator
not available in other spectrum analyzers, that
phase noise will reduce the phase noise measure-
allows the recovery of close-in phase noise infor-
ment's dynamic range below the raw capability of
mation even for drifting carriers.
the 89410A itself. Refer to the Agilent 89400 Series
VSA data sheet for specifications on phase noise
For many situations, measurements which previ-
performance of each specific analyzer.
ously had taken minutes or tens of minutes to com-
plete can be performed in seconds. Expect meas-
urement speed improvements of 10X to 100X.
Direct L(f) measurements If the signal-under-test is unlocked and drifting too
L(f), the single-sided phase noise density in units much to make a satisfactory averaged measurement,
of dBc/Hz, is a common phase noise measurement. an alternate method is to use an 89400 Series VSA
In the 89400 Series VSAs, L(f) is facilitated by with demodulation and auto-carrier functions. See
trace math and/or an Instrument BASIC program "Phase Perturbation Measurements" later in this note.
to normalize the noise density to carrier power,
and as with all measurements the instrument per- Adjacent-channel power measurement
forms very fast averaging for very accurate noise This is an important variation of the phase noise
measurements. In fact, the 89400 Series VSAs pro- measurement. The 89400 Series VSAs have an
duce complete spectral results for 401 frequency explicit "C/N" feature which accomplishes the adja-
points with averaging in less time than traditional cent-channel power measurement in cases where
spectrum analyzers take to measure noise density the carrier is distinguishable as a single tone.
for a single frequency point using noise markers.
In cases where the carrier is heavily modulated or
Using the instrument's built-in trace math, define a defined over a frequency band, trace math solves
function F1 = PSD/K1, where K1=10 ((carrier power in dBm)/10). the problem. Again let F1 = PSD/K1, where
(This is easily automated using an I-BASIC program). K1 = 10((carrier power in dBm)/10), but this time carrier power
This function is the desired result L(f), assuming the is measured using band-power markers. The entire
noise density is dominated by phase noise rather measurement can be executed with a single key-
than AM or input noise. The "f" in L(f) is generally stroke using an I-BASIC program. Figure 2 shows
understood to be the frequency offset from the car- an example of this measurement.
rier frequency. Figure 1 shows an example of this
measurement; although the annotation shows units
of dBm/Hz, the actual units are dBc/Hz due to the
normalization to carrier power.
Figure 1. L(f) measurement
2
Phase perturbation measurements frequency during the measurement, then 5% of each
Phase perturbation measurements include phase end of the spectrum drifts out of the measurement
jitter, time jitter, phase deviation, peak-phase devi- span and will not be correctly demodulated. This
ation, and RMS-phase deviation. They all refer to would correspond to the upper 10% of the demodu-
the phase or time deviation from ideal of the infor- lated frequency span being invalid.
mation, due to phase noise or phase distortions in
the system. The 89400 Series VSAs greatly facili- A typical measurement situation is illustrated in
tate these measurements with demodulation and Figure 3. The top trace shows the phase-demodulated
time-domain features. power spectral density (PSD), while the bottom trace
shows the time waveform of the demodulated infor-
Select PM Demodulation for the instrument mode. mation (in this case, noise). Considerable informa-
If the signal-under-test is not phase locked to the tion can be obtained directly from this measurement:
VSA, simply turn Auto-Carrier on ("Phase and
Frequency" PM auto type). This built-in function