Keysight Technologies E8740A-070, Startup Manual

Page: 37 / 51

Keysight E8740A-070 Start-up Guide 37
Calibration and Amplitude Correction
IQTools provides a procedure where it uses a generated multitone signal to
measure the relative amplitude response of the system. It utilizes this information
to pre-distort the generated waveform so that the resulting waveform at the
output of the system is corrected and has a flat response. The correction is only as
good as the accuracy of the measured multitone signal, hence a calibrated
measurement system is essential for the correction method to work.
One possible method to calibrate the system is to use a harmonic mixer together
with Keysight’s spectrum analyzers’ external mixer port to measure the
upconverted multitone spectrum. There are however limitations with the external
mixer port as it is suitable mainly for measuring relatively narrowband signal. The
external mixer port operates at an IF center frequency of 322.5 MHz and since
there is no pre-selector at the RF input of the smart mixer, any signal with twice
the bandwidth of the IF frequency would suffer image frequency aliasing.
However, Keysight’s smart mixer (M1970E) has a dual conversion path mode. The
input signal is converted to first IF of 1.5175 GHz in the smart mixer internally
before another down-conversion to 322.5 MHz, thus allowing an image-free
bandwidth of 3 GHz for analysis.
In the following section, we will discuss the procedure to calibrate amplitude
correction in IQTools using the Keysight smart mixer. For bandwidths larger than
3 GHz, a block conversion using downconverter’s and the RF port of the signal
analyzer may be used. A brief discussion on using this method will be included in
“Calibration and Amplitude Correction” on page 37. IQTools also facilitates
calibration using VSA89600 software and Keysight digital scopes but these
methods will not be discussed in this guide.
Calibrating system using M1971E Smart Mixer
In this example, we will calibrate the signal generation system from 78 to 80 GHz.
As in “Generating a 79 GHz FMCW Radar Signal Manually with IQTools” on
page 16, the output frequency from the N5183B source is set at 37.5 GHz which
will result in a carrier frequency of 75 GHz due to the x2 harmonic factor of the
N9029ACST upconverter.