Specifications

13
Figure 8-1
● HINT 8:
Choose the optimal measurement bandwidth
Select a measurement bandwidth no larger than the
pass band of the DUT. Modern noise figure analyzers
provide a selection of various measurement bandwidths
to enable measurements that are more relevant to current
practical applications (e.g. individual wireless GSM
channels). 4 MHz bandwidths were common in past
generations of noise figure instruments; modern analyzers
can measure down to at least 100 kHz bandwidth.
If the measurement bandwidth is similar to the DUT’s
operational pass band, then any instabilities between
the center frequency of the analyzer’s final IF and the
DUT’s operational bandwidth will lead to an error in the
displayed gain. The analyzer may recognize the noise
power outside of the DUT’s pass band during calibration
and inside during measurement. The extent of this effect
depends on the shape of the DUT’s pass band overlapping
with the shape of the analyzer’s final IF pass band. This
issue is less significant if the analyzer’s pass band is
narrower than the DUT’s pass band.
Past generations of noise figure instruments required
additional time to obtain the same level of jitter reduction
in a narrower bandwidth. For the same jitter reduction,
half the bandwidth required twice the time, one fourth
required four times as long, etc. In modern full-featured
noise figure analyzers, this time penalty is mostly
eliminated due to digital signal processing (DSP)
techniques that measure a group of adjacent frequencies
simultaneously.
Frequency
Measurement
Bandwidth Setting
Noise power detected during
calibration but not during
DUT measurement
understates DUT's noise figure
Noise
Power
Lowest possible actual
measurement bandwidth due
to downward frequency drift
Analyzer's frequency instability
due to drift during time required
for all measurements to be made
and averaged
Highest possible actual
measurement bandwidth due
to upward frequency drift