The system calibration includes
measuring the tuner at many states
that cover the whole Smith chart independently at every frequency. This
allows the selection of a good set of
s values at every measurement frequency, which is desirable because
good measurement accuracy and sensitivity requires the correct selection
of tuner impedance states. The reference planes of the tuning block must
go from the noise source plane to the
DUT input plane.
An in-situ system calibration can
be done after doing two network analyzer calibrations. The first is a two-port calibration at the DUT planes.
The second is a one-port calibration
at the noise source port. By subtracting the error terms of the two calibrations, the two-port S-parameters
from the noise source port to the
DUT input port can be determined.
If an optional load tuner is used,
then an additional one-port calibration at the noise receiver plane will
be used to get the S-parameters from
the DUT output port to the noise receiver port.
To save time in the event that something outside of the tuner changes (a
wafer probe, for example), the tuner
may be characterized separately. A
hybrid in-situ calibration can then
be done in the same manner to get a
fixed S-parameter block that does not
include the tuner.
After the system calibration is complete, the noise receiver is calibrated
one frequency at a time; the DUT
noise parameters are then measured
one frequency at a time.
3, 4 This is
done because the noise parameter extraction involves complex calculations
that are sensitive to small errors, so it
is important to select a good pattern
of source reflection coefficients to get
well-conditioned data.2 Making the
measurements one frequency at time
allows an ideal pattern to be selected.
One problem with the traditional
approach is that it is very time consuming, due to the large number of tuner
states (each requiring physical movement of the probe/carriage assembly),
and the large number of single-fre-quency noise measurements. It is common to sweep 400 or more frequency
points in S-parameter measurements,
but for measuring noise parameters,
that many frequencies would take days.
With long measurements, temperature
drift can cause significant errors. This
is exacerbated by the many lengths of
cables required for all the instrument
and component connections shown in
the measurement setup.
Since traditional noise parameter measurements are slow, they are
typically limited to a sparse set of frequencies. But this makes the scatter,
outliers and cyclical-frequency errors
difficult to interpret. A cyclical error is
common with imperfect network analyzer calibrations, where the system
errors will add at some frequencies
and cancel at others. This can cause
an aliasing effect, which can shift the
data values up or down. Smoothing
techniques can make the data look
better, but will not correct for this
type of data shift.
New Ultra-Fast Noise
The new ultra-fast noise parameter
measurement method (patent pending) typically speeds up measurements