Specifications

7
If time constraints limit the number of averages during
DUT measurement (e.g. in manufacturing), use more
averages during calibration. This will make the
correction for all subsequent DUT measurements more
accurate.
Modern noise figure analyzers have the ability to do point
averaging or trace averaging. Point averaging makes all
measurements for the first frequency point, calculates and
displays their average, then moves to the next frequency
point. The process is repeated until each frequency
has been measured and averaged the number of times
specified in the measurement setup. Trace averaging
measures only once at each frequency point through one
entire sweep of the frequency range. It then begins the
second sweep, averaging each individual measurement
at each frequency with the previous average for that
frequency, as it sweeps. It repeats this process until the
number of sweeps or “traces” averaged together equals the
number of averages specified in the measurement setup.
Both types of averaging give the same answer. Point
averaging is faster overall since the analyzer’s tuner has
to retune fewer times for all measurements to complete.
Trace averaging displays a rough measurement over the
entire frequency range faster. This enables a user to see
any obvious problems with the measurement (e.g. an
extraneous signal) sooner.
Use trace averaging first. Watch a few sweeps across
the display and look for indications of RF interference
such as a spike in the response at a single frequency, or
even a small step in the response.
● HINT 4:
Use averaging to minimize display jitter
Noise measurement inherently displays variability or
jitter because of the random nature of the noise being
measured. Averaging many readings can minimize
displayed jitter and bring the measurement closer to
the true mean of the noise’s gaussian distribution.
Modern noise figure analyzers enable the user to select
the number of readings that will be averaged for each
measurement. This will reduce jitter in the measurement
by the square root of N, where N is the number of
measurements in the average. The table below shows
some examples of the effect of averaging on jitter. For
example, jitter may be reduced by almost 70% by averaging
approximately 10 readings.
Increasing the averaging will add time to make the
measurement. This would affect the device test time and
total test throughput in manufacturing. There is a trade-off
between the speed of the measurement and the level of
jitter.
Decreasing the bandwidth of the measurement increases
proportionately the number of readings necessary to
obtain the same level of jitter reduction. For example,
half the bandwidth requires twice as many readings of
noise figure averaged together to obtain the same jitter
reduction; one fourth requires four time as many readings,
and so on. These additional readings do not necessarily
extend the time for the measurement proportionately.
N SQRT % jitter
(N) reduction
110
4250
16 4 75
64 8 87.5
256 16 93.75