Agilent Technologies E4406A manual What are Baseband I/Q Inputs?

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.

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