FSK code check is started in automatic mode by selecting the Analysis | Code Check button or from VHF/UHF-DIR | Analysis | FSK Code Check. This function has a high degree of accuracy when determining baud rate, frequency shift and center frequency of HF FSK systems.
As an additional help FSK code check also offers an option to start the analysis manually after user entry of baud rate, frequency shift and center frequency. To utilize this option the Auto button must be deselected. The software will then start data acquisition using the pre-selected parameters. If a pre-selected parameter has been changed data acquisition is atomatically restarted.
The selection of baud rate, frequency shift or center frequency is done by clicking on the status bar fields or using the Demodulator menu.
If the FSK code check cannot identify a mode, the code check should be repeated. Data acquisition is continuously performed as a background task. Heavy fading or other disturbances during data acquisition may prevent the identification of a mode.
You can also restart data acquisition by pressing the Resync button.
Because this kind of signal is not continuous, there is a need for a Noise Gate. The status of the noise gate is displayed in the status bar; it examines the input signal and tries to determine if a valid signal is available which can be processed, or if the input consists only of noise.
Steps:
Ø After starting the FSK code check the frequency shift, center frequency and the baud rate are determined. The values of these parameters are displayed in the appropriate fields after the measurement has completed.
Ø The software then proceeds with code and system analysis. The incoming bit stream is tested and compared against known modes.
Ø The name of each identified system is displayed in the output window. An identified system may be immediately started by double-clicking the system in the output window - for instance double-clicking ACARS in the figure above will interrupt FSK code checking and immediately start ACARS.
Ø The Hits-Checks will show you how often during checking a mode was successfully determined. If multiple modes are listed this will indicate which mode should be tried.
Ø If a mode is uniquely identified, the software will switch to the identified mode and decoding is initiated with the measured values of mode, baud rate, shift, and center frequency.
Ø If two or more different systems are identified or if too many transmission errors occur, an automatic switch to a mode will not take place.
Noise Gate Mode…
The Noise Gate Mode can be selected by Options | Noise Gate Mode… or by double-clicking the Noise Gate Mode field in the Status Bar.
Ø In Enabled mode, a Noise Gate is used. This is necessary as this kind of signal is not continuous. The noise gate examines the input signal and tries to determine if a valid signal is available which can be analyzed, or if the input consists only of noise.
Ø In Disabled mode the Noise Gate is switched off.