Wavecom W74PC V8.7.0 - Manual (Page 90)

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Analysis Tools WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V8.7.0
Analysis Tools
FFT
The real-time FFT analysis is a powerful spectrum analysis tool based on Fast Fourier Analysis (FFT), oper-
ating in real time with a 4096-point resolution. The signal is digitized, saved and its frequency spectrum
calculated and displayed.
The measurement is started by selecting a bandwidth. The display of the frequency spectrum has a linear
scale and covers a dynamic range of 60 dB. (The internal dynamic range is considerably higher due to the
16 bit A/D converter used, but the configured display resolution performs better in practical use.)
The frame refresh rate averages 10 to 20 pictures per second, allowing even fast signal changes to be dis-
played. The frame refresh rate is however dependent on the speed of the PC and its video card.
To start the FFT, click on the FFT button or use Analysis | Real-time FFT from one of the menus.
To activate the measurement cursors, click inside the FFT graphics window. The left and right cursors may
be moved around using the mouse. The cursor positions are displayed as absolute frequency values; the
center cursor automatically displays the center value.
To remove the cursors, click on the graphics window again.
When the cursors have been placed in the desired positions they may be moved right or left by moving
the center cursor. By doing this, the measurement of frequency spacing in MFSK and FDM systems is pos-
sible.
Adjustment of the Offset Frequency
An FFT spectrum calculation can be done from 0 Hz, up to the selected maximum frequency.
The offset frequency for decoding of VHF/UHF-DIR transmissions e.g., POCSAG, GOLAY/GSC, or ERMES
must be adjusted to the effective center frequency of the signal, e.g., 455 kHz.
A bandwidth of 24 kHz from 455 kHz to 479 kHz is sufficient for the measurement range of the FFT.
therefore the offset frequency must be offset to half of the bandwidth. The offset frequency adjustment is
calculated as the IF output frequency (455 kHz) minus half of the selected FFT bandwidth (24/2 kHz) =
Offset (443 kHz). The measurement range is now 443 kHz - 467 kHz.
For the 455 kHz receiver IF output of a shortwave receiver (e.g., HF-1000), the offset frequency is first
adjusted to 453.3 kHz to obtain the standard center frequency of 1,700 Hz. The HF-1000 BFO must now
be adjusted to 1,700 Hz.