5.5 Signal search

The 'signal search' function enables the receiver to search a signal within the frequency tracking width. A signal search is triggered either manually or automatically if the signal disappears for a certain time. The beacon receiver monitors the 'threshold fault' to detect if the signal is present.

When a signal search is initiated, the receiver steps over the frequency range from f-ftrw to f+ftrw in steps of 0.8 msbw . The receiver detects at which frequency the highest level is achieved and finally tunes the receiver to this frequency. The frequency offset of the frequency tracking is adjusted for this, the nominal frequency remains unchanged. The duration of the search cycle depends on the bandwidth an tracking range settings, the maximum expected value (bw=6kHz, range = +/- 150kHz) is 1 minute.

The behavior of the frequency track facility is controlled by the following operational parameters:

• Signal search enable --- ssen Setting this parameter to ON enables the automatic signal search function. If the input signal falls below the alarm threshold for a certain time, a signal search is initiated.
• Signal search delay --- sdly This parameter defines the time, the receiver waits after the signal was lost until a search scan is started. The valid range of this parameter is 0 .. 600 seconds.
• Alarm threshold --- thrh The threshold value which primarily is used to generate a 'signal missing' fault is very important for the signal search function. If the measured input level falls below this threshold for at least sdly seconds, a search cycle is triggered. You must sxet the threshold to a reasonable value in order to let the receiver detect that the signal is missing. .ts Frequency Tracking width ftrw This also parameter defines the frequency range which is covered by the search procedure. A search cycle always starts at f-ftrw and ends at f+ftrw .
• Force search --- sfrc If a 1 is written to this parameter, the receiver immediately starts searching, regardles of the ssen setting. When operated at with a web browser, clicking to 'SEARCH NOW' executed this command. Reading back the sfrc parameter always returns an empty string.

If the receiver is tuned to a new frequency while the signal search function is enabled, the frequency offset is set to 0 and the receiver operates at this frequency for sdly seconds. If the received signal stays below the threshold during this time, the receiver starts searching.

After a search cycle has been finished, the receiver is tuned to that frequency which gave the highest level during the search. If after a short delay for signal settling the receive level still is below the threshold, the next search cycle starts immediately.

The signal search operates in steps of approx. 80% of the measurement bandwidth. This is to make the search procedure as fast as possible. The signal frequency therefore is not determined very accurately. If you have enabled the frequency tracking function at the same time, the receiver performs a frequency track cycle directly after the signal search. If frequency tracking is switched off, the receiver tuning is left at the frequency offset the search alorithm has found.