3-22
Making Time Domain Measurements
Transforming CW Time Measurements into the Frequency Domain
Transforming CW Time Measurements into the Frequency Domain The analyzer can display the amplitude and phase of CW signals versus time. For example,
use this mode for measurements such as amplifier gain as a function of warmup time (i.e.
drift). The analyzer can display the measured parameter (e.g. amplifier gain) for periods of
up to 24 hours and then output the data to a digital plotter for hardcopy results.
These "strip chart" plots are actually measurements as a function of time (time is the
independent variable), and the horizontal display axis is scaled in time units. Transforms
of these measurements result in frequency domain data. Such transforms are called
forward transforms because the trans fo rm from time to frequency is a forward Fourier
transform, and can be used to measure the spectral content of a CW signal. For example,
when transformed into the frequency domain, a pure CW signal measured over time
appears as a single frequency spike. The transform into the frequency domain yields a
display that looks similar to a spectrum analyzer display of signal amplitude versus
frequency.
Forward Transform Measurements
Figure 3-17 shows an example of a measurement using the Fourier transform in the
forward direction, from the time domain to the frequency domain.
Figure 3-17 Amplifier Gain Measurement
Interpreting the Forward Transform Vertical Axis
With the log magnitude format selected, the vertical axis displays dB. This format
simulates a spectrum analyzer display of power versus frequency.