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Measurement uncertainty
There are several key contributions to overall noise fi gure measurement
uncertainty. When selecting a noise fi gure solution, it is important to choose the
method that minimizes the main contributor to overall noise fi gure uncertainty.
Some of these contributions can be found on instrument data sheets—for
example, instrument uncertainty, excess noise ratio (ENR) uncertainty, and
jitter—while others depend on the interaction between the test system and
the DUT. For example, there are two sources of error due to imperfect system
source match (a deviation from the ideal 50 ohms). The fi rst is mismatch error,
which results from non-ideal power transfer between the test system and the
DUT. The second source of error is from the interaction between the noise
generated within the DUT and the source match (Γ
s
) seen by the DUT. The
following fi gure compares noise fi gure measurement uncertainty between the
Y-factor method and the cold-source method (as implemented on the PNA-X).
The example amplifi er has a noise fi gure of 3 dB, gain of 15 dB, input and output
match of 10 dB, and moderate noise parameters (F
min
= 2.8 dB, Γ
opt
= 0.27 + j0,
R
n
= 37.4). For the Y-factor method, the uncertainty is calculated in two different
ways: one with the noise source connected directly to the DUT, and one with an
electrical network simulating the switches and cables from an automated-test-
equipment (ATE) setup placed between the noise source and the DUT (with loss
correction). The PNA-X example includes the ATE network.
With the Y-factor method, the main sources of error are due to mismatch
between the noise source and DUT, as well as the interaction between the noise
generated by the DUT and the system. The simulated ATE network (inserted
between the noise source and DUT) causes the errors to increase. For the
PNA-X’s source-corrected cold-source method, the largest source of error is
the ENR uncertainty of the noise source, which affects the measurement of the
PNA-X’s internal noise receivers during calibration.
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Total uncertainty
ENR uncertainty
Mismatch
DUT noise/Г
S
interaction
S-parameter
Jitter
Uncertainty contributors
Uncertainty breakdown
dB
Y-factor with ATE network
Y-factor with noise source
connected to DUT
PNA-X with ATE network
Notes:
Noise source = 346C
97% confi dence
Find out more www.agilent.com/fi nd/nf
Figure 1. Breakdown of the major
contributors to noise fi gure
measurement uncertainty for the
Y-factor and cold-source (with
source correction) techniques
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