Agilent Technologies E4406A What are Baseband I/Q Signals?, Why Make Measurements at Baseband?

Making Measurements

Using Baseband I/Q Inputs (Option B7C)

What are Baseband I/Q Signals?

In transmitters, the term baseband I/Q refers to signals that are the fundamental products of individual I/Q modulators, before the I and Q component signals are combined, and before up conversion to IF or RF frequencies.

In receivers, baseband I/Q analysis may be used to test the I and Q products of I/Q demodulators, after an RF signal has been down converted and demodulated.

Why Make Measurements at Baseband?

Baseband I/Q measurements are a valuable means of making qualitative analyses of the following operating characteristics:

•I/Q signal layer access for performing format-specific demodulation measurements (e.g. CDMA, GSM, W-CDMA):

•Modulation Accuracy – i.e. I/Q plane metrics

—Rho

—Error Vector Magnitude; RMS, peak, 95%

—carrier feed-through

—frequency error

—magnitude and phase errors

•Code-domain analysis (including code-specific metrics)

•CCDF of I2+Q2

•Single Sideband (SSB) metrics for assessing output quality

•Basic analysis of I and Q signals in isolation including: DC content, RMS, P-P levels, CCDF of each channel

Comparisons of measurements made at baseband and RF frequencies produced by the same device are especially revealing. Once signal integrity is verified at baseband, impairments can be traced to specific stages of up conversion, amplification, or filtering by RF analysis. Likewise, impairments to signal quality that are apparent at RF frequencies may be traceable to baseband using baseband analysis.

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