Quadrature - Agilent Technologies 89410A Operator's Manual

Extending Analysis to 26.5 GHz with 20 MHz Information Bandwidth

Quadrature

The final calibration is for quadrature error. Although a value for quadrature
error was obtained in the channel match calibration, the number cannot be
used by itself to set the quadrature adjustment DAC in the IF module as the
mapping from DAC setting to phase is not known.
The DAC setting for the quadrature is determined much like the DAC setting
for the residual DC. First, the phase difference between the channel one and
channel two signals is measured and the current DAC setting recorded. A
second measurement of the phase difference is made after the DAC has been
adjusted by a small amount.
The two DAC values and the two phase difference values can then be used to
compute how far to adjust the DAC to compensate for the quadrature error
observed in the delay calibration. The example program performs this
calibration with the center frequency adjusted to 309 MHz. Although the
frequency is not critical, the center frequency should be different than the
calibrator frequency to avoid potential problems with errors introduced by
residual DC in I and Q.
The example program was designed to work with a 300 MHz calibrator. Any
frequency can be used, provided the calibrator frequency variable in the
program reflects that frequency. This might be desirable if better absolute
amplitude accuracy is desired at some frequency other than 300 MHz. Other
parameters, such as delay, are not greatly affected by the center frequency of
the microwave spectrum analyzer.
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