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®
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®
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Operating Manual 1145.1084.12 – 30 364
The definition of the noise figure F
D
is based on a pure thermal noise signal (ΔN = 0):
F
D
= (P
Signal, 1
/ N
T, 1
) / (P
Signal, 2
/ P
Noise, 2
) = P
Noise, 2
/ (N
T, 1
* G
D
) where G
D
= P
Signal, 2
/ P
Signal, 1
ΔN is supposed to be amplified like the CW signal. Hence, the noise signal at the output of the DUT
equals to:
P
Noise, 2
= G
D
* (F
D
* N
T, 1
+ ΔN)
According to the equations above, it is possible to calculate both the gain G
D
and the noise figure F
D
of the
DUT, if the four power levels P
Signal, 2
, P
Signal, 1
, P
Noise, 1
, P
Noise, 2
can be measured with sufficient accuracy.
The measurement requires two steps:
Source noise calibration of the driving NWA port --> P
Signal, 1
, P
Noise, 1
. This step is required only
once for a given channel configuration.
Transmission measurement of the DUT --> P
Signal, 2
, P
Noise, 2
. The analyzer uses two different
detector settings to separate the CW signal and the noise signal contributions. The Average
detector yields the CW signal power; the RMS detector yields the sum of the CW signal and noise
power.
In principle, this short description also holds for mixer noise figure measurements. The major extensions
are the RF image band and the additional frequency ranges for calibration and for the measurement.
Conditions for the measurement
Option R&S ZVAB-K30, Noise Figure Measurement, provides the measurement including the
necessary (wide-band) IF filters. Internal source attenuators are recommended but may be
replaced by an external attenuator.
An additional option R&S ZVA-K31 in combination with option R&S ZVA-K4 provides the mixer
noise figure measurement. Due to the extended calibration procedure, mixer noise figure
measurements cannot be performed with external attenuators.
The measurement algorithm is based on the assumption that the DUT operates in its linear range
so that all input signal contributions are amplified by the same gain factor.
A set of power calibrations (source power calibration using an external power meter and
subsequent receiver power calibration) prior to the noise figure calibration and to the actual
measurement is essential for an accurate measurement of the noise power at the NWA receiver.
The Noise Figure Calibration is important to assess the internal noise figure of the NWA receiver
and generator.
The power calibration and noise figure calibration correspond to a transmission normalization of
the test setup. An additional system error correction (S-Parameter Calibration) using an AVG
detector calibrates the gain G
D
and improves the accuracy of the noise figure measurement.
The Noise Figure Setup Guide helps you perform all the necessary steps consistently and in the right
order.
Measuring the noise figure for setups with combined source paths (e.g. intermodulation distortion or
mixer delay measurements) may lead to inaccurate results.
The reason for this is that the noise added by the second generator (typically port 3) leads to an increased
total generator noise power, which is not taken into account for all possible measurement setups.
For these kinds of setups it is recommended to perform the noise figure measurements in a separate
channel, setting the unused combined source port to RF Off. Furthermore the RF Off Behavior system
setting should be set to RF Switch and Amplifiers (Slow, Lower Broadband Noise).
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