Concepts and features
R&S
®
ZNA
290User Manual 1178.6462.02 ─ 29
S
in
and S
Out
denote the signal power at the input and output of the DUT, respectively.
N
in
and N
out
represent the noise present at the input of the DUT and at the output of the
DUT, respectively. The gain G
D
of the DUT is defined by the ratio of the output signal
power to the input signal power:
The resulting formula for the noise factor F
D
shows that the calculation depends on
precise knowledge of:
●
the noise that is present at the input of the DUT,
●
the noise present at the output of the DUT
●
the gain of the DUT
These quantities must be measured during the different steps of a noise figure mea-
surement. In a post-processing step, these individual measurements are then used to
calculate the noise figure of the DUT.
Typically, the noise factor F
D
is displayed in decibel which is then called the noise fig-
ure NF
D
= 10 log F
D
. Noise figure and noise factor are used interchangeably through-
out this manual.
4.7.11.2 Algorithmic implementation
Cold surce method
Historically, the noise figure of DUTs was measured using the Y-factor method with a
noise figure analyzer or with a spectrum analyzer. Because these measurement instru-
ments had no internal sources, they used an external broadband noise source
(switched on and off) to measure the noise and the gain of the DUT.
Nowadays, with the use of modern network analyzers, there is no need for an addi-
tional noise source and the Y-factor method. Instead, the so-called cold source method
is utilized, which uses broadly available and highly accurate power meters for calibra-
tion. The noise emitting from the DUT is measured with switched off internal VNA
source and a matched termination at the DUT input (typically a VNA port). The gain of
the DUT is measured with an active source and the calibrated VNA receivers.
Equivalent circuit
The internal calculations are performed in the noise temperature representation.
Compared to calculations using noise factors (or noise figures), this method can easily
account for different physical temperatures of the involved components. Furthermore,
with noise temperatures there is no need to apply corrections factors, if the compo-
Optional extensions and accessories
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