Operators Manual Owner manual

HDO6000 High Definition Oscilloscope

FFT

For a large class of signals, you can gain greater insight by looking at spectral representation rather than

time description. Signals encountered in the frequency response of amplifiers, oscillator phase noise and

those in mechanical vibration analysis, for example, are easier to observe in the frequency domain.

If sampling is done at a rate fast enough to faithfully approximate the original waveform (usually five

times the highest frequency component in the signal), the resulting discrete data series will uniquely

describe the analog signal. This is of particular value when dealing with transient signals because, unlike

FFT, conventional swept spectrum analyzers cannot handle them.

Because of its versatility, FFT analysis has become a popular analysis tool. However, some care must be

taken with it. In most instances, incorrect positioning of the signal within the display grid will significantly

alter the spectrum, producing effects such as leakage and aliasing that distort the spectrum.

An effective way to reduce these effects is to maximize the acquisition record length. Record length

directly conditions the effective sampling rate of the oscilloscope and therefore determines the frequency

resolution and span at which spectral analysis can be carried out.

Set Up FFT

1. Follow the usual steps to set up a math function, selecting FFT from the Frequency Analysis submenu.

2. Open the FFT right-hand dialog.

3. Choose to either:

l trunc(ate) - When the FFT transform size does not match the record length, truncate the record

and perform an FFT on the shorter record. This option increases the resolution bandwidth.

l zero-fill - When the source data for the FFT comes from a math operation that shortens the rec-

ord (as is commonly encountered in filtering operations like ERes), replace the missing data

points wityh data values whose amplitudes are interpolated to fit between the last data point

and the first data point in the record. This guarantees that there is not a first-order discontinuity

in the filled data. Since the data at the end of the record is filled data, it is advisable to select a

weighting window other than rectangular to minimize the effect of the fill on the resulting spec-

trum.

4. Check the Suppress DC box to make the DC bin go to zero. Otherwise, leave it unchecked.

5. Choose an Output type.

6. Optionally, choose a weighting Window. See the section below for more information about FFT

weighting windows.

7. Touch Algorithm and choose either:

922499 Rev B