User`s guide

Evaluating Your Measurement Results 14
Agilent E5505A User’s Guide 339
.
To list the marked spurs
A list of spurs can be displayed by accessing the Spurs List function in the
View menu.
Forest of spurs
A so called forest of spurs is a group of closely spaced spurs on the phase noise
plot. A forest of spurs is often caused by improper shielding that allows stray
RF energy to be picked up by the DUT wiring, etc. A breadboarded or
prototype circuit should be well shielded from external RF fields when phase
noise measurements are being made.
Table 54 shows actions to take to eliminate spurs.
Table 53 Spurs on the graph
Offset Frequency Number of Averages Upward Change for Marking Spurs (dB)
< 100 kHz
< 430
≥ 417
≥ 812
≥ 30 6
> 100 kHz Any 4
Table 54 Actions to eliminate spurs
Spur Sources Description Recommended Action
Internal Potential spur sources within the
measurement system include the phase noise
system, the DUT, and the reference source.
Typical system spurs are –120 dBc, and they
occur at the power line and system vibration
frequencies in the range of from 25 Hz to 1
kHz, and above 10 MHz.
If you do not have a plot of the system’s noise and spur
characteristics, perform the system Noise Floor Test. If
you suspect that the DUT or the reference source may
be the spur source, check each source using a
spectrum analyzer or measuring receiver (such as an
Agilent 8902A). Also, if additional sources are available,
try exchanging each of the sources and repeating the
measurement.
External Spur sources external to the system may be
either mechanical or electrical. When using
the Phase Lock Loop measurement technique,
the system’s susceptibility to external spur
sources increases with increases in the Peak
Tuning Range set by the VCO source.
Shorten coaxial cables as much as possible
(particularly the Tune Voltage Output cable). Make sure
all cable connections are tight. It may be possible to
identify an external spur source using a spectrum
analyzer with a pick-up coil or an antenna connected to
it.