Measuring Time Data (Vector)

Vector measurements let you view and analyze time-domain data. Time data provides a time-domain view of the input data before FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. processing. The following points will help you interpret time data:

• FFT measurements: Time data may look similar to an oscilloscope display in some cases but you will see a distorted signal, particularly at high frequencies. The distortion occurs because the waveform is made up of the discrete samples from a time record which is optimized for FFT measurements. FFT measurements require a lower sample rate than would be required for an optimum time domain display.

• Baseband vs. Zoom: the VSA must be in baseband mode (Time Data set to Baseband) to show data similar to an oscilloscope. In this case, the measurement starts at 0 Hz and the center frequency is not be set explicitly.

  You can enter zoom mode either by setting the center frequency explicitly or by selecting Time Data: Zoom (from the MeasSetup > Frequency menu). In zoom mode, the time data is displayed relative to the center (local oscillator) frequency rather than relative to DC. Therefore, zoom time data has a different appearance because baseband data is real while zoom data is complex.

• Averaging: Time and time exponential averaging are the only types of averaging which affect time data displays. Other average types only display the most recent time record.

• Transient signals: In baseband mode wth maximum span, some signals, particularly square waves and transients, may appear to have excess ringing or overshoot because of the abrupt frequency cut-off of the anti-alias filters.