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Mathematics

R&S

RTO

193User Manual 1316.0827.02 ─ 06

Conventional oscilloscopes calculate one FFT per capture. The R&S RTO can calculate

multiple FFTs per capture by dividing one capture into several frames. Thus, the RTO

can visualize how the frequency content of a signal changes over time which helps to

detect intermittent or sporadic signal details. Furthermore, the R&S RTO allows consec-

utive frames to overlap. This is especially useful in conjunction with window functions

since it enables a gap-free frequency analysis of the signal.

The overlapping factor can be set freely. The higher the overlap factor, the more frames

are used. This leads to more individual results and improves detection of transient signal

effects. However, it also extends the duration of the calculation. The size of the frame

depends on the number of input signal values (record length), the overlap factor, and the

FFT size (number of samples used for FFT calculation).

Frame

length

Overlap area

Record

length

Window functions

Each frame is multiplied with a specific window function after sampling in the time domain.

Windowing helps minimize the discontinuities at the end of the measured signal interval

and thus reduces the effect of spectral leakage, increasing the frequency resolution.

There are a number of window functions that can be used in FFT analysis. Each of the

window functions has specific characteristics, including some advantages and some

trade-offs. These characteristics need to be considered carefully to find the optimum

solution for the measurement task.

For details, see "Window type" on page 201.

Gating functions

You can restrict the time base of the input signal for which FFT analysis is to be performed.

There are various methods to do so:

●

Define absolute start and stop times for the time base extract

●

Define relative start and stop values that define a percentage of the original time base

●

Couple the time base extract for FFT to an active zoom area.

The gate area can be indicated in the signal diagram, if desired.

FFT Analysis