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152 Chapter 4
Making Measurements
Using Baseband I/Q Inputs (Option B7C)
What are Baseband I/Q Inputs?
Option B7C consists of a Baseband Input module, four 50 Ω BNC
connectors, and internal cabling. The four BNC connectors are grouped
into pairs at the upper left corner of the front panel. The upper two
connectors labeled “I” and “Q” are the “unbalanced” inputs.
In practice, an unbalanced or “single ended” baseband measurement of
an I or Q signal is made using a probe connected to the I or Q connector.
A simultaneous I/Q unbalanced single ended measurement may be
made using two probes connected to the I and Q input connectors.
If “balanced” signals are available, they may be used to make a more
accurate measurement. Balanced signals are signals present in two
separate conductors, are symmetrical about ground, and are opposite in
polarity, or out of phase by 180 degrees.
Measurements using balanced signals can have a higher signal to noise
ratio, improving accuracy. Noise coupled into each conductor equally in
a “common mode” to both signals may be separated from the signal. The
measure of this separation is “common-mode rejection.”
To make a balanced measurement, the lower two connectors labeled “I
”
and “Q
” are used in conjunction with the I and Q inputs. The terms
“I-bar” and “Q-bar” may be applied to the signals, as well as the inputs
themselves. Probes (customer provided) must be used to input balanced
baseband I/Q signals. This may be referred to as a balanced
measurement.
Balanced baseband measurements are made using the I and I
connectors for I only signal measurements, while the Q and Q
connectors are used for a Q only signal measurement. Balanced
measurements of I/Q require differential probe connections to all four
input connectors. For details of probe selection and use, refer to
“Selecting Input Probes for Baseband Measurements” on page 156.