Lambda SR1, Руководство по эксплуатации, Страница: 139 / 317

SR1 Operation 139
© 2014 Stanford Research Systems
Norm alized Frequency Response of Elliptical Filter
If desired, the original chirp signal can be acquired first, without passing it through the DUT, and saved
as an offline trace. Then after the spectrum through the DUT is acquired the graph's "Ratio" function can
be used to normalize the output to the input producing a true frequency response plot such as the one
seen above right.
The same process of acquiring a reference input plot and then normalizing the output to it can be used
to measure the phase response of devices in a single FFT shot, however, the process is so much
simpler using the dual-channel FFT analyzer (FFT2) that we'll postpone the discussion of phase
response until then.
Because the synchronous chirp source has a periodic time-dependent amplitude profile, it may
fool the input autoranging control into switching ranges too frequently. Depending on the FFT
bandwidth (which determines the chirp length) it may be useful to turn off input autoranging
when using the synchronous chirp signal.
For the same reason, be sure to select the "Uniform" window when using the synchronous
chirp. Window functions attenuate the beginning and the end of the timre record for the FFT
analyzers which when using the chirp means that certain frequencies will be attenuated more
than others. This destroys the "flat" nature of the synchronous chirp signal.
Using the FFT1 Analyzer With the Time Domain Detector
The FFT1 analyzer can be used together with the Time Domain Detector to create a powerful tool for
analyzing noise and distortion spectra in realtime. The Time Domain Detector outputs a digital signal
which represents the input signal with the fundamental notched out and any bandwidth limiting and
weighting filters applied. This signal can be routed directly to the input of the FFT analyzer. To
implement this setup, select the type of the A0 analyzer as "Time Domain Detector". Set the TDD input
to analog Hi-Bandwidth and the TDD function to THD+N Ratio. Now set the type of the A1 analyzer to
FFT1 and set its input to "Other Analyzer." The spectrum displayed is the spectrum of the input signal
with the fundamental removed by the TDD's notch filter and the remaining noise and distortion amplified
by the TDD's post-filter gain.