LeCroy SDA, Руководство оператора

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SDA-OM-E Rev H
width. The table of Window Frequency-Domain Parameters lists the ENBW values for the
implemented windows.
•
Leakage - In the power spectrum of a sine wave with an integral number of periods in the
(rectangular) time window (that is, the source frequency equals one of the bin frequencies),
the spectrum contains a sharp component whose value accurately reflects the source
waveform's amplitude. For intermediate input frequencies this spectral component has a
lower and broader peak.
The broadening of the base of the peak, stretching out into many neighboring bins, is termed
leakage . It is due to the relatively high side lobes of the filter associated with each frequency
bin.
The filter side lobes and the resulting leakage are reduced when one of the available window
functions is applied. The best reduction is provided by the Blackman–Harris and Flattop
windows. However, this reduction is offset by a broadening of the main lobe of the filter.
•
Number of Points - The FFT is computed over the number of points (Transform Size) whose
upper bounds are the source number of points, and by the maximum number of points
selected in the menu. The FFT generates spectra of N/2 output points.
•
Nyquist Frequency - The Nyquist frequency is equal to one half of the effective sampling
frequency (after the decimation): Delta f x N/2.
•
Picket Fence Effect - If a sine wave has a whole number of periods in the time domain
record, the power spectrum obtained with a rectangular window will have a sharp peak,
corresponding exactly to the frequency and amplitude of the sine wave. Otherwise the
spectrum peak with a rectangular window will be lower and broader.
The highest point in the power spectrum can be 3.92 dB lower (1.57 times) when the source
frequency is halfway between two discrete bin frequencies. This variation of the spectrum
magnitude is called the
picket fence effect (the loss is called the scallop loss).
All window functions compensate for this loss to some extent, but the best compensation is
obtained with the Flattop window.
•
Power Spectrum - The power spectrum (V
2
) is the square of the magnitude spectrum.
The power spectrum is displayed on the dBm scale, with 0 dBm corresponding to:
V
ref
2
= (0.316 V
peak
)
2
,
Where V
ref
is the peak value of the sinusoidal voltage, which is equivalent to 1 mW into 50 Ω .
•
Power Density Spectrum - The power density spectrum (V
2
/Hz) is the power spectrum
divided by the equivalent noise bandwidth of the filter, in hertz. The power density spectrum
is displayed on the dBm scale, with 0 dBm corresponding to (V
ref
2
/Hz).
•
Sampling Frequency - The time-domain records are acquired at sampling frequencies
dependent on the selected time base. Before the FFT computation, the time-domain record
may be decimated. If the selected maximum number of points is lower than the source
number of points, the effective sampling frequency is reduced. The effective sampling
frequency equals twice the Nyquist frequency.
•
Scallop Loss - This is loss associated with the picket fence effect.