Agilent Technologies E4406A manual Baseband I/Q Measurement Views

Making Measurements

Using Baseband I/Q Inputs (Option B7C)

Baseband I/Q Measurement Views

Measurement result views made in Basic mode are available for baseband signals if they relate to the nature of the signal itself. Many measurements which relate to the characteristics baseband I and Q signals have when mixed and upconverted to signals in the RF spectrum can be made as well. However, measurements which relate to the characteristics of an upconverted signal that lie beyond the bandwidth available to the Baseband I/Q Input circuits can not be measured (the limits are ~5 MHz BW for individual I and Q signals, and ~10 MHz for composite I/Q signals).

Some views are appropriate for use with both RF and Baseband I/Q signals without any modification, while other views must be altered. Some examples of measurements with identical results views are QPSK EVM, Code Domain, and CCDF. For Spectrum measurements, identical views include the I and Q Waveform view and the I/Q Polar view. For Waveform measurements, identical views include the I/Q Waveform view, the Signal Envelope view, and the I/Q Polar view.

At RF frequencies, power measurements are conventionally displayed on a logarithmic vertical scale in dBm units, whereas measurements of baseband signals using Baseband I/Q inputs may be conveniently displayed as voltage using a linear vertical scale as well as a log scale.

Spectrum Views and 0 Hz Center Frequency

Some views must be altered to account for the fundamental difference between RF and baseband I/Q signals. For Spectrum measurements of I/Q signals this includes using a center frequency of 0 Hz for Spectrum views and the Spectrum Linear view. Occupied Bandwidth and Channel Power results are also displayed using a center frequency of 0 Hz.

The center frequency of baseband I/Q Spectrum displays is 0 Hz. Frequencies higher than 0 Hz are displayed as “positive” and those below 0 Hz are “negative”. The “negative” portion of a multi-channel baseband signal below 0 Hz corresponds to the portion of the signal that would lie below the carrier center frequency when it is upconverted, if no spectral inversion occurs. As 0 Hz is a fixed center frequency, the FREQUENCY Channel front-panel key has no active menu for baseband I/Q Spectrum measurements.

I only and Q only Spectrum views are conventional, displayed with 0 Hz at the left margin of the x-axis. When upconverted or multiplied, an I only or Q only signal could ultimately lie above or below the carrier center frequency, but in either case it will only occupy half the bandwidth. As 0 Hz is a fixed start frequency, the FREQUENCY Channel front panel key has no active menu keys. Use Span to change horizontal scale. The center frequency is displayed as half the current Span.

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