The IF section provides filtering and variable gain. The filtering can be used to
prevent unwanted signals from reaching subsequent blocks in the system and
causing distortion. The example program sets the filters to their widest
bandwidth to obtain the best accuracy. This ensures the minimum amount of
frequency response error (magnitude and phase) and ensures the highest
accuracy for the IQ demodulation. The gain of the IF section is set so that the
IQ demodulator operates over a signal level range where it is most linear. The
IF gain resolution is 1 dB. The 89410A input range is set to be compatible with
the the full-scale output of the IQ demodulator.
Mirrored Spectrums
The vector signal analyzer is responsible for reducing the measurement
bandwidth to 20 MHz and below. When the microwave receiver is used at
frequencies below 12.8 GHz, the spectrum obtained using I+jQ is mirrored
about the center frequency. There are two ways to compensate for this
mirroring. The first is to simply swap the I and Q outputs. While this works, it’s
inconvenient and makes calibration more difficult.
A simpler way to compensate for the mirroring is to conjugate the complex
signal. In other words, I–jQ instead of I+jQ. The vector signal analyzer has a
spectral mirror key which conjugates the CH1+jCH2 data. This corrects both
time and frequency domain results without affecting the I and Q calibration.
IBASIC Example Program
As a measurement system, there is an obvious need for software to link the two
instruments together. This software should provide a user interface to the
system, as well as provide for system calibration.
An Instrument BASIC example program is available which provides these
functions. The primary purpose of the example program is calibration.
However, it also provides for simple control of center frequency and reference
level.
Once the system is calibrated and the center frequency and reference level are
properly adjusted, the program is paused or terminated to provide access to all
the measurement and analysis features of the vector signal analyzer.
The example program is written in Instrument BASIC and is included on the
Instrument BASIC Example Programs disk (this disk comes with Instrument
BASIC, option 1C2 ). It can be run on the 89410A, or on an external controller.
You can also obtain the example program from your local Agilent sales
representative.
Extending Analysis to 26.5 GHz with 20 MHz Information Bandwidth
16 - 6
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