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Chapter 4 165
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.