Controlling The Receiver; Changing Center Frequency; Setting The Mirror Frequency Key; Changing The Reference Level - Agilent Technologies 89410A Operator's Manual

Extending Analysis to 26.5 GHz with 20 MHz Information Bandwidth

Controlling the Receiver

As previously described, the system can be configured with or without the
70004A display. When the display is present, the center frequency and reference
level can be adjusted on the microwave receiver. While this may seem more
convenient, there are good reasons for using the features found in the example
program to control the receiver from the vector signal analyzer.

Changing Center Frequency

For example, when changing the center frequency, the example program labels
the traces on the vector signal analyzer to reflect the center frequency of the
measurement. This is useful since the x-axis annotation always shows a center
frequency of zero hertz whenever the receiver mode is set to CH1+jCH2.
To facilitate measurements over a range of frequencies, a frequency increment
function has been added to the example program. This function is accessed
from a sub-menu via the Freq Step key in the main menu. The sub-menu
contains keys for setting the desired frequency increment as well as stepping
the center frequency up and down with just one key stroke. These functions
take care of setting the 71910A center frequency, updating the annotation on
the 89410A, and mirroring the spectrum display if necessary.

Setting the Mirror Frequency Key

Another important function that the example program handles is the setting of
the mirror frequency key, used to flip or invert the spectrum of the signal about
zero hertz. This operation may be necessary depending on the center
frequency. In the microwave spectrum analyzer, center frequencies below 12.8
GHz will produce an output spectrum that is inverted. The frequency inversion
is due to the mixing scheme used in the instrument. When setting the center
frequency, the example program will check to see if the spectrum will be
inverted (based on center frequency) and will set the mirror frequency key to
compensate. The mirror frequency key actually causes the time data to be
conjugated; that is, the sign of the Q channel data is negated. Conjugating the
time data will not only mirror the spectrum, it will also correct the time data for
modulation analysis.

Changing the Reference Level

The example program is even more useful for changing the reference level
(input attenuation). When the input attenuation is adjusted in the microwave
receiver, the gain of the signal path may change. If it does, the absolute level
displayed on the vector signal analyzer will be wrong. When changing the
reference level, the example program will also change the gain calibration to
reflect any nominal changes in the gain of the signal path. This ensures that the
reference level can change without affecting the level of the result.
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