VT DSO-2A20 Manual Rev. 1.1
Virtins Technology
used in the spectrum analyzer. FFT size of the spectrum analyzer should be equal to or
greater than the Record Length of the oscilloscope. Stop the oscilloscope first before
stopping the signal generator.
(4) Signal generator excites a single frequency or a narrow frequency band one at a time
across the frequency range of interest and the response is analyzed one at time and
assembled together in the end.
A linear frequency swept sine signal can be used as the stimulus and peak hold function
of the spectrum analyzer can be used to assemble the result. No synchronization between
the signal generator and oscilloscope is required. Just start the signal generator first and
then start the oscilloscope. You can set the number of frames to be peak-held by right
clicking anywhere within the spectrum analyzer window, selecting [Spectrum Analyzer
Processing]> “Inter-Frame Processing”> “Peak Hold”, and choosing a number in the
“Frames” field. Hanning Window is recommended in the spectrum analyzer. If the
assembled curve shows some unresolved “gaps”, then lengthen the sweep time of the
signal generator and / or reduce the FFT size of the spectrum analyzer to ensure that each
resolved frequency band in the spectrum analyzer is excited and analyzed as the sweep
goes.
Frequency stepped sine can also be used as the stimulus. Device Test Plan software
module is required in order to perform automated test to step through the frequency band
of interest.
Notes:
(1)
To measure the frequency response of a DUT accurately, the overall frequency response
of the setup itself should be flat enough in the frequency range of interest. The bandwidth
of a VT DSO under 16-bit mode is about 0.443 of the sampling frequency selected. Thus
select a sampling rate such that the bandwidth of the oscilloscope is wider than the
frequency range of interest of the frequency response measurement. The wide-band noise
generated by the signal generator can be considered to span from 0 Hz to ½ of the
sampling frequency of the signal generator. The frequency range of the stimulus should
be equal to or a bit wider than the frequency range of interest of the measurement. If the
frequency range of the stimulus is too wide, then the excitation energy in the frequency
range of interest will be reduced, resulting in reduced signal-to-noise ratio in the
measurement. Thus select a sampling rate of the signal generator such that the frequency
range of the generated noise is just enough to cover the frequency of interest of the
measurement. However, if a sine wave is used instead, the highest sampling rate of the
signal generator is recommended. Any signal generated by the signal generator is further
limited by the analog bandwidth of the signal generator.
(2)
It is possible to compensate the frequency response of the setup if only magnitude
frequency response is to be measured. Connect the output of the signal generator to the
input of the oscilloscope using the same cabling to be used with the DUT, and then
measure the frequency response using the same measurement setting to be used with the
DUT. After the magnitude frequency response of the setup itself is obtained, right click
anywhere within the spectrum analyzer window and select [Spectrum Analyzer Generate
Frequency Compensation File (Flat)], then provide a file name and save it. Right click
anywhere within the spectrum analyzer window again and select [Spectrum Analyzer
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