Fluke PM-3394B, PM-3380B, PM-3390B Frequency domain transformations, Relative FFT, Absolute FFT

USING THE COMBISCOPE INSTRUMENTS 3 - 49

Scaling can be adjusted with the "CURSORS TRACK and delta" knobs via the MATHPLUS - PARAM menu option.

PROGRAM EXAMPLE:

CALL Send(0, 8, "CALCulate:INTegral:STATe ON", 1) ’Integral CALC1 on

CALL	Send(0, 8, "CALCulate2:DERivative:POINts 35", 1)’35 differential points
CALL	Send(0, 8, "CALCulate2:DERivative:STATe ON", 1) ’Differential CALC2 on

3.9.4Frequency domain transformations

The result of an FFT (Fast Fourier Transformation) calculation is displayed as a trace of amplitude values (vertically) versus frequency values (horizontally). The vertical result can be expressed as a relative or an absolute amplitude value. The CALCulate:TRANsform:FREQuency:TYPE command selects between the RELative and ABSolute result. The DISPlay:WINDow:TEXT<n>:DATA? query allows you to read the calculated amplitude and frequency value.

RELATIVE FFT:

A relative FFT calculation consists of a frequency (Hz) and an amplitude in (dB), relative to the frequency component with the largest amplitude.

ABSOLUTE FFT:

An absolute FFT calculation consists of a frequency (Hz) and an amplitude in dBm (dB with respect to 1 milliwatt), dBμV (dB with respect to 1 microvolt), or Vrms (Volt RMS) as selected via the front panel CURSORS - READOUT softkey menu.

The following FFT window functions can be selected using the CALCulate:TRANsform:FREQuency:WINDow command:

•The FFT RECTangular function transforms a repetitive time amplitude trace into its power spectrum.

•The FFT HAMMing and HANNing functions reduce the side lobes by applying a Hamming respectively Hanning window to the input signal. This improves the visibility of the minor frequency components if the limited area is not accurately selected.

The resulting FFT trace is a MIN/MAX (envelope) trace, which means that each trace point is determined twice (one for the MINimum envelope and one for the MAXimum envelope). The FFT trace points are scaled between +4 and -4 divisions on the screen. So, the samples values that are returned as response to a TRACe? query are shifted 4 divisions upwards. The values of the resulting FFT trace points are between -0 dB and -80 dB. This results in the following relation between screen position and sample value: