Channel Match; Iq Gain, Delay Match - Agilent Technologies 89410A Operator's Manual

Extending Analysis to 26.5 GHz with 20 MHz Information Bandwidth

Channel Match

Channel match is also important for preserving the dynamic range of the
measurement. System errors that cause mismatch include gain imbalance,
delay mismatch and frequency response differences between the I and Q
signals.
A frequency response mismatch can occur in both the IF module and the vector
signal analyzer input channels. Only the center 20 MHz of the IF module's 100
MHz bandwidth is used (I and Q each have 50 MHz bandwidth), so the
mismatch there is relatively small. The mismatches in the vector signal analyzer
are compensated by the instrument's built-in calibration routines.

IQ Gain, Delay Match

The example program only attempts to match gain and delay. The program
determines the gain and delay parameters by measuring the I and Q signals
generated when the 300 MHz calibrator is connected. To simplify the
measurement, the vector signal analyzer is taken out of the channel combine
mode (CH1 + jCH2) and put into two-channel mode.
The center frequency of the spectrum analyzer is adjusted to 291 MHz. With a
300 MHz signal and a center frequency of 291 MHz, the I and Q outputs are both
9 MHz sine waves. In a perfect system, these 9 MHz sine waves would have a
phase difference of 90 degrees and would have identical amplitudes. In a real
system, the amplitudes will probably be different, and the phase difference will
be something other than 90 degrees.
As shown in the following illustration, the phase error has two components—
error due to quadrature error and error due to IQ delay mismatch. Quadrature
error in the IF module produces a phase error that is independent of the
relationship between the calibrator frequency and the center frequency of the
measurement. Delay mismatch produces a frequency-dependent phase error
term that is zero when the center frequency and the calibrator frequency are
the same (that is, the difference frequency is zero and I and Q have no AC
component).
To determine the delay and quadrature error, the center frequency is adjusted
in 2 MHz steps from 291 MHz to 309 MHz, and the phase differences between I
and Q are recorded. Then, using a least-mean-square algorithm, a line is fit to
the phase error verses frequency data.
The slope of the line corresponds to the delay mismatch, and the offset to the
quadrature error. The absolute signal level and gain match are measured at the
same time as the phase. At each center frequency, the magnitude squared of the
2 2
signal (I + Q ) is computed, as is the ratio between I and Q.
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