About I+jQ Measurements (Input Channels)

An I+jQ logical VSA input channel measures data from two instrument channels where one channel supplies the real component of the data (I, or In-phase) and the other channel supplies the imaginary component (Q, or Quadrature-phase). The VSA supports up to 8 instrument channels for an Analyzer Configuration. Since each I+jQ logical input channel requires two instrument channels, there are four possible I+jQ selections: Single, Dual, Triple, or Quad I+jQ.

To view the default instrument channel assignment, select one of the I+jQ channel configurations, and then select Input > Channels > Custom.

For connections to specific measurement hardware, see Available Measurement Hardware for a link to your specific hardware.

There are several applications for the I+jQ input mode, including the following:

Measuring the baseband components of a transmitted signal or signals, prior to converting to RF.
Transmitter/receiver measurements where the I and Q components are input directly into the VSA for analysis.
RF measurements using an external quadrature downconverter connecting the I/Q outputs to 2-channel baseband measurement hardware for analysis.
See Setting up a Digital Demodulation Measurement to learn how to use I+jQ for Digital Demodulation Measurements.

RF measurements can be made using an external quadrature downconverter by connecting the I/Q outputs to 2 channel baseband measurement hardware. "External" refers to measurement hardware that is not controlled by the VSA--for example, an Keysight 71910A Wideband Receiver with I and Q outputs. The measurement and configuration settings used by the external quadrature downconverter are input to the VSA using External Frequency Calibration.

Range and Coupling

Use either the "All" Logical Instrument selection when adjusting range and coupling or select the Couple Paired Channels checkbox to synchronize all the range and coupling settings for active channels. "All" will couple all channels together (for example, for Dual I+jQ (C1+jC2, C3+jC4), all four channels will be coupled with together). Couple Paired Channels couples I and Q channels for each VSA channel (for example, for Dual I+jQ (C1+jC2, C3+jC4), VSA Channels 1 and 2 are coupled together and are independent of VSA Channels 3 and 4, which are also coupled together).

I+jQ Measurements and Recordings

Signals that are recorded in I+jQ mode should be played back in I+jQ mode. While they can be played back as 2 or 4 separate channels, the time corrections will not, in general, be consistent. 2-channel Baseband recordings, however, can be played back in I+jQ mode and time corrections will be accurate.