Specifications

5
Follow these guidelines:
(a) Ensure that mating connectors are clean and
not worn or damaged (see Reference 8 for further
information). If the measurement becomes unstable
when the cables and connectors are shaken lightly by
hand, try other cables or connectors.
(b) Use threaded connectors in the signal path
whenever possible. (For example BNC connectors are
very susceptible to stray signals.)
(c) Use double shielded cables (common flexible
braided coaxial cable is too porous to RF).
(d) Use shielded GPIB cables.
(e) Move the measurement setup to a screened room.
If a transmitter that has any frequency content within the
measurement bandwidth is nearby and any covers are off
of the DUT, move the measurement setup to a screened
room. Test for such signals with a spectrum analyzer with
a simple wire antenna on the input. Attenuate these stray
signals by 70 to 80 dB.
(f) Use shielding. This is especially important for
making measurements on an open PC breadboard. (See
Reference 5 for further information.)
(g) Use an analyzer with minimal electromagnetic
emissions. Devices may be susceptible to stray emissions
from some measurement instruments. Some modern
noise figure analyzers have electromagnetic emission
characteristics low enough to have negligible impact on
the measurement.
● HINT 2:
Minimize extraneous signals
A noise figure analyzer measures the noise power from
the noise source as affected by the DUT. It uses the power
ratio at two detected noise levels to measure the noise
figure of whatever is between the noise source and the
instrument’s detector. Any interference, airborne or
otherwise, is measured as noise power from the DUT
and can cause an error of any magnitude.
Figure 2-1 demonstrates the types of stray signals that
can get coupled into the signal path and affect the
measurement. Fluorescent lights, adjacent instruments,
computers, local TV and Radio stations, pocket pagers,
mobile phones and base stations are notorious for their
adverse effects on noise measurements. Random stray
signals can cause several tenths of a dB difference
between individual readings, and result in unstable
measurements (i.e. jitter that will not average to a
stable mean).
Figure 2-1
Interference
Noise
Source
Lights ( esp. fluorescent)
FM
TV
Computers
Double-shielded cables for IF (ordinary braid is
too porous)
Shielded GPIB Cables
Enclose all circuits
NF
Analyzer
RF comms basestation
Test connectors by shaking leads
DUT