Tektronix TriMode P7700 Series Technical Reference page 20

Theory of operation
8
The ground path for high frequency signal return currents is shown in the probe
architecture figure. (See Figure 3 on page 5.) The probe ground path is continuous
and along the following path:
Extends from the DUT ground vias connections at the probe tip input
Through the probe tip ground path
Through the TekFlex connector ground spring contacts
Along the main cable coaxial shields into the probe comp box
Through the probe amplifier circuit board ground plane inside the probe comp
box, and
Through the TekConnect interface to the host oscilloscope ground
A TriMode tip requires a short DUT ground reference for making low noise
single-ended probe measurements.
Although two ground via connections are available on the probe tip input, a
single ground via wire connection is usually adequate for making single-ended
measurements of both the A and B input signals or a common-mode measurement,
all of which are ground referenced.
If only one single ended signal will be connected to the probe, the user has the
choice of connecting the A input to the signal and the B input to ground, or
connecting the A input to the signal and the ground input of the probe to the
ground input of the DUT. In this situation, Tektronix recommends using A-B
mode with the B input connected to ground. Reasons for this recommendation
include that with the B input left disconnected, there is a possibility of an
interfering signal coupling into the input of the probe and distorting the measured
signal acquired on the A side.
A second reason for using A-B vs. A-ground is that it is often more convenient to
connect the differential inputs of the probe to a device and keep the wire lengths
short. The ground connections of the probe are set back from the tip and may not
be as convenient to connect to a DUT with tightly spaced test points.
The optional accessory P77BRWSR is a variable-spacing probe tip which operates
in Differential Input mode only. The P77BRWSR Browser probe tip does not have
a physical ground connection at the probe tip; but the differential measurement
process itself provides a high frequency virtual ground connection between the
A and B signal input pins. A low frequency ground connection is optionally
available at a square-pin socket on the browser probe tip housing.
The comp box receives power and control signals from the oscilloscope through
the TekConnect interface. The TekConnect interface is an intelligent probe
connection that includes many automated and manual control features. For
example, there is a probe S-parameter memory in the comp box that stores high
frequency characterization data for the probe signal path.
P7700 Series TriMode Probes Technical Reference