221ConceptsBaseband I/Q Inputs (Option BBA) Measurement ConceptsBaseband I/Q Inputs (Option BBA) Measurement ConceptsThe N9020A Option BBA Baseband I/Q Inputs provides the ability to analyzebaseband I/Q signal characteristics of mobile and base station transmitters. Thisoption may be used only in conjunction with the following modes:• IQ Analyzer (Basic)• 802.16 OFDMA (WiMAX/WiBro)• cdma2000• GSM/EDGE• TD-SCDMAWhat are Baseband I/Q inputs?Option BBA consists of a Baseband Input module, four input connectors, and acalibration output connector. The connectors are at the left side of the front panel.The two ports labeled “I” and “Q” are the “unbalanced” inputs.An unbalanced or “single-ended” baseband measurement of an I or Q signal is madeusing a probe connected to the I or Q connector. A simultaneous I/Q unbalancedsingle-ended measurement may be made using two probes connected to the I and Qinput connectors.If “balanced” signals are available, they may be used to make a more accuratemeasurement. Balanced signals are signals present in two separate conductors, aresymmetrical about ground, and are opposite in polarity, or out of phase by 180degrees.Measurements using balanced signals can have a higher signal to noise ratioresulting in improved accuracy. Noise coupled into each conductor equally in a“common mode” to both signals may be separated from the signal. The measure ofthis separation is “common-mode rejection”.To make a balanced measurement, the two connectors labeled “I” and “Q” are usedin conjunction with the I and Q inputs. The terms “I-bar” and “Q-bar” may beapplied 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 abalanced measurement.Balanced baseband measurements are made using the I and connectors for I onlysignal measurements, while the Q and connectors are used for a Q only signalmeasurement. Balanced measurements of I/Q require differential probe connectionsto all four input connectors. For details of probe selection and use, refer to“Selecting input probes for baseband measurements” on page 223.What are Baseband I/Q signals?In transmitters, the term baseband I/Q refers to signals that are the fundamentalproducts of individual I/Q modulators, before the I and Q component signals arecombined, and before upconversion to IF or RF frequencies.
