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2.5 Considerations
for Microwave
Measurements
Lake Shore offers microwave configurations with ground-signal-ground (GSG) probe
geometry optimized for substrates patterned with coplanar waveguide structures.
Both signal and ground traces of the microwave structures must be patterned on the
top layer of the substrate to facilitate top side probing. Measurements can be
performed on both passive and active devices to characterize performance metrics
such as S-parameters, noise figure, or load-pull parameters.
Proper use of GSG microwave probes is more complex than of DC/RF probes. Proper
probe alignment of the GSG points with respect to the test substrate is required, as is
the proper probe planarization with respect to the plane of measurement. Also,
calibration may be desired to separate the frequency dependent losses of the
measurement setup from the actual device under test.
The remaining sections in this chapter describe details of the microwave probe
measurement setup, as well as concepts and techniques that are important for
making good microwave measurements in the probe station.
2.5.1 Microwave Cables
and Connectors
Microwave cables form transmission lines that carry high frequency signals from the
signal connection point outside the vacuum chamber to probe points near the cooled
sample. The type and quality of microwave cables and their associated connectors
determine the frequency range and overall performance of microwave
measurements in the probe station. Properly installed, the cables provide a low loss,
broad band electrical path with minimal crosstalk. Also, they need to be compatible
with the cryogenic temperatures and vacuum for the Model CRX-6.5K.
The geometry of the Model CRX-6.5K requires that the cables extend 229 mm (9 in)
into the vacuum chamber with a single 90 degree bend. The total length of the
microwave cable is approximately 279 mm (11 in). Probe station layout inherently
contributes some signal loss. Lake Shore recommends the following for best
performance:
1. Calibrate the measurement setup as described in section 2.5.4.
2. Re-tighten the connectors to manufacturer’s specified torque after repeated
thermal cycling.
3. Keep external cables as short and direct as possible.
Microwave probes must be specified with the same frequency and connector type as
the probe cable. TABLE 2-3 summarizes the two microwave probe frequency ranges
and associated connectors.
For reference, FIGURE 2-29 to FIGURE 2-30 show plug and socket, head-on views of
the three standard types of microwave connectors. Note that both types look very
similar. Side by side, differences in the connectors can be seen primarily in the head-
on view in the thickness of outer conductor and spacing between the inner and outer
conductor.
The connector types are physically similar and can sometimes, but not always, be
interchanged; care should be taken not to damage probes or cables by attempting to
mate them improperly.
Highest rated
frequency
Connector
Mates with
40 GHz
K-type (2.92 mm)
Standard SMA connectors
67 GHz
V-type (1.85 mm)
2.4 mm connectors
TABLE 2-3
Microwave probe frequency ranges and associated connectors
