FX919A Datasheet, PDF (35/44 Pages) CML Microcircuits – 4-Level FSK Modem Data Pump

4-Level FSK Modem Data Pump
FX919A
1.6.3 Clock Extraction and Level Measurement Systems
The FX919A is intended for use in systems where:
- The Symbol Sync pattern is transmitted immediately on start-up of the transmitter, before the first
Frame Sync pattern (see Figure 17).
- A terminal may remain powered up indefinitely, transmitting concatenated Frames with or without
intervening Symbol Sync patterns (each Frame having a Frame Synch pattern and symbol timing being
maintained from one Frame to the next).
- A receiving modem may be switched onto a channel before the distant transmitter has started up, or
may be switched onto a channel where the transmitting station is already sending concatenated
Frames.
Whenever the receiving modem is enabled or switched onto a channel it needs to establish the received
symbol levels and timing and look for a Frame Sync pattern in the incoming signal. This is best done by the
following procedure.
1. Ensure that the Control Register's PLLBW bits are set to 'Wide' and the LEVRES bits to 'Slow Peak
Detect'.
2. Wait until a received carrier has been present for 8 symbol times. This 8-symbol delay gives time for
the received signal to propagate through the modem's RRC filter and can usefully be included in the
radio's carrier detect circuitry.
3. Write a SFS or SFSH task to the Command Register with the AQSC and AQLEV bits set to '1'.
4. When the modem interrupts to signal that it has recognised a Frame Sync pattern (or completed the
SFSH task) then change the PLLBW bits to 'Medium'.
Once the receiving modem has achieved level and symbol timing synchronisation with a particular channel - as
evidenced by recognition of a Frame Sync pattern - then subsequent concatentated Frames can be read by
simply issuing SFS or SFSH tasks at appropriate times, keeping the ASQSC and AQLEV bits at zero, and the
PLLLBW and LEVRES bits at their current 'Medium' and 'Slow Peak Detect' settings.
noise
Received signal
from FM discriminator
to Modem :
Set AQSC and AQLEV bits
to start acquisition sequences :
Level Measurement and
Clock Extraction circuits :
Symbol Sync
Frame Sync
rest of frame
8-symbol delay
Increasing accuracy and lengthening response times
Figure 17 Acquisition Sequence Timing (Transmitter Power-Up)
© 1996 Consumer Microcircuits Limited
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D/919A/4