Notes for using FFT analyzer

Firstly you may check if there is amplitude dependency or not when using the damping.
If there is any amplitude dependency, you have to measure by damping factor as the frequency response function is lack of symrmetry.

The synchronized sin sweep is the best for the input signal of exciter.
It is desirable for FFT analyzer to have auto range function which adjusts automatically its voltage range, in/output voltage range and reflects the gain of frequency response function. It is better to have a function which changes the level of output signal according to the gain of frequency response function.

The signal with the following characteristics is recommended when it take time to measure sin sweep.

 

(1)Swept sine signal

Characteristics

It is high sine wave swept signal. The crest factor (maximum/effective value) is very low as 1.4. While the degree of concentrating the signal is very high.

The leakage errors will not occur as it obtains synchronization with FFT frame analyzing and the rectangular window as window function can be used.  

There is no effect of addition due to the same signal all the time, however, recently it is frequently used as the accurate frequency response function can be measured in a short time.

It is apparently a limited swept signal in a narrow band on the device which the signal synchronization, the frequency band limitation, the resolution change can be done by frequency zooming.

The following figure is for the band limited swept signal.

(2)Multiple sine signals

Characterics

• They consist of the multi sine waves which have sine waves as equal level at any resolution points. The crest factor is around 3.  The degree of concentrating the signal is moderate.

• The leakage errors will not occur as it obtains synchronization with FFT frame analyzing and the rectangular window as window function can be used.

• There is no effect of addition due to the same signal all the time, however, recently it is frequently used as the accurate frequency response function can be measured in a short time.

• When the logarithmic resolution is obtained by arranging sine waves at logarithmic intervals, the logarithmic resolution can be measured by selecting and displaying the logarithmic resolution frequency on the FFT side.

The following figure is for the band limited multiple sine signals.

(3)Synthesis of impulse

Characteristics

It is the signal which synthesizes the 1st wave of sin wave to adjust to the peak position. The crest factor is around 30.

It is easy to correspond to the zooming analysis comparing with the normal impulse.

Rightly, the beginning and the end is “zero”. The leakage errors will not occur as it obtains synchronization with FFT frame analyzing and the rectangular window as window function can be used.

It can be used for the input signal for the exciter in the damping rate method.

(4)Random signal

Charactericss

Normally the M series signal is used and the crest factor is around 3. The degree of concentrating the signal is low.

As it has all frequency signals, it can be used for the zooming analysis.

The hanning window is normally used as it does not obtain synchronization with FFT frame analyzing. However, in case the loss factor is measured, the rectangular window is preferable. When using the hanning window, it is careful for the leakage errors but also the measurement lower limit of the loss factor.

Since this has also the band limitation, it is desirable to range the band limitation within the zoom bandwidth when performing zoom analysis.

The addition is very effective, thus it is necessary to add enough as there are many missing frequencies time to time.

For the measurement of the device with nonlinear elements, the burst random is used.