Specifications

6
Alternately, insert a well matched attenuator
(pad) between the noise source and the DUT to
attenuate multiple reflections. As an example, with a
10 dB attenuator, the re-reflections are attenuated by
20 dB. The advantage of an attenuator vs. an isolator is
broadband response. The disadvantage is that the noise
source’s ENR values will be reduced by the attenuator’s
insertion loss (10 dB in this example).
If the DUT has a high output reflection coefficient
(S22) and low or no insertion gain (S21), then, in some
cases, place a low noise pre-amp between
the DUT and the measurement instrument to
reduce the total measurement uncertainty. The
pre-amp should have an input reflection coefficient (S11)
as low as possible. The effective bandwidth of a pre-amp
with very low S11 is usually narrow. More than one may
be required for the entire frequency range of interest. To
identify an appropriate pre-amp, see Agilent Technologies
Application Note 57-2 (Reference 5).
● HINT 3:
Minimize mismatch uncertainties
Mismatch at connection planes will create multiple
reflections of the noise signal in the measurement and
calibration paths (as shown in Figure 3-1). Mismatch
uncertainties at these planes will combine vectorially
and will contribute to the total measurement uncertainty.
One method to reduce the mismatch uncertainty is to
place an isolator in the RF path between the noise
source and the DUT. This isolator can prevent multiple
re-reflections from reaching the DUT and can suppress
the build-up of error vectors. Isolators, however, operate
over restricted frequency ranges. Several may be needed
for the frequency range of interest. Isolators also add to
path losses. As a result compensation is required. Full-
featured noise figure analyzers have a loss compensation
feature to account for the insertion losses of any isolators.
Figure 3-1
Measuring
System
Noise
Source
DUT
Calibration
Measurement
Mismatch
Uncertainty
ρ
1
ρ
2
ρ
4
ρ
3
ρ
= reflection coefficient at a reference plane