User's Manual

SECTION 4: FACTORY TEST PROCEDURE
M64450G25-FCCRpt.doc Page 38
The mobile radio’s microcontroller, upon detecting a step response in the RSSI (caused by the base
station's transmitter coming up to power), waits a period of time equal to the programmed value of the
mobile'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 base station's transmitter causes a step increase in the mobile'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.
The figure below is a plot of the down-link 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. Note there is a very short period of quiet time (no
modulation) followed by approximately 12 milliseconds of modem synchronization time (sync time).