Iq Analysis Concepts; Purpose; Complex Spectrum Measurement - Keysight X Series Measurement Manual

Concepts

IQ Analysis Concepts

IQ Analysis Concepts

Purpose

IQ Analysis (Basic) mode is used to capture complex time domain data from wide
bandwidth RF signals. This mode preserves the instantaneous vector relationships of
time, frequency, phase and amplitude contained within the selected digitizer span or
analysis BW, at the analyzer's center frequency, for output as IQ data. This IQ data
can then be utilized internally or output over LAN, USB or GPIB for use with
external analysis tools. Each measurement description specifies the types of data
available remotely for that measurement.
Within the IQ Analyzer mode, basic frequency domain, time domain and IQ
measurements are available as initial signal and data verification tools in preparation
for deriving the IQ data output. This is accomplished using the Complex Spectrum
and IQ Waveform measurements. Although Complex Spectrum and IQ Waveform
are defined as measurements in the IQ Analyzer (Basic) mode, they act primarily as
tools to verify the signals and data as stated above.

Complex Spectrum measurement

Purpose
This measurement is FFT (Fast Fourier Transform) based. The FFT-specific
parameters are located in the Advanced menu. The Complex Spectrum
measurement provides a display in the upper window of power versus frequency
with current (yellow trace) and average (blue trace) data. In addition, an IQ
waveform of voltage versus time is provided in the lower window. One advantage of
having an I/Q view available while in the spectrum measurement is that it allows you
to view complex components of the same signal without changing settings or
measurements.
Measurement method
The measurement uses digital signal processing to sample the input signal and
convert it to the frequency domain. With the instrument tuned to a fixed center
frequency, samples are digitized at a high rate, converted to I and Q components with
DSP hardware, and then converted to the frequency domain with FFT software.
Troubleshooting hints
Changes made by the user to advanced spectrum settings, particularly to ADC range
settings, can inadvertently result in spectrum measurements that are invalid and
cause error messages to appear. Care needs to be taken when using advanced
features.
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