User's Manual

F32700N25.doc Page 23
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The base station's microcontroller, upon detecting a step response in the RSSI (caused
by the fixed radio's transmitter coming up to power), waits a period of time equal to the
programmed value of the base station's carrier detect delay time. The microcontroller
then instructs the modem to search for the modem synchronization preamble. When
the base station instructs the modem to look for sync tones, the modem's CS line
transitions low. This can be seen in the above plot. Approximately 10 milliseconds after
the fixed radio's transmitter causes a step increase in the base station's RSSI, the CS
signal goes low momentarily. As can be seen, the sync tones are stable by this time
and the modem quickly establishes synchronization.
Downlink Hardware Timing Verification
Figure 2-3 displays a plot of the downlink timing characteristics. Channel 1 is
connected to RSSI, channel 2 is connected to recovered audio, and channel 3 is
connected to the modem CS pin. The scope is in the high-resolution acquisition mode.
There is a very short period of quiet time (no modulation) followed by
approximately 12 milliseconds of modem synchronization time (sync time).
Figure 2-3: Downlink Timing Characteristics Plot