DB929B datasheet(39/51 Pages) CMLMICRO

39 / 51 page

4-Level FSK Modem Data Pump

Page 39 of 50

MX929B PRELIMINARY INFORMATION

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Fax: 336 744 5054

Doc. # 20480171.003

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA

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5.3

Clock Extraction and Level Measurement Systems

5.3.1

Supported Types of Systems

The MX929B is intended for use in systems where:

1. The Symbol Sync pattern is transmitted immediately on start-up of the transmitter, before the first Frame

Sync pattern (see Figure 23).

2. A Base Station may remain powered up indefinitely, transmitting concatenated Frames with or without

intervening Symbol Sync patterns (each Frame starting with the Frame Sync pattern and symbol timing

being maintained from one Frame to the next).

3. 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.

5.3.2

Clock and Level Acquisition Procedures with RF Carrier Detect

When the receiving modem is enabled or switched onto a channel, it needs to establish the received symbol

levels, clock 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 'Level Track'.

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. An 'RF received 8 symbol times' qualifying

function can be included in a radio's carrier detect circuitry to take this into account.

3. Write a SFS or SFP task to the Command Register with the AQSC and the AQLEV bits set to '1'.

4. When the modem interrupts to signal that it has recognized a Frame Sync pattern (or completed the SFP

task) then change the PLLBW bits to 'Medium'.

Once the receiving modem has achieved level and symbol timing synchronization with a particular channel -

as evidenced by recognition of a Frame Sync pattern - then subsequent concatenated Frames can be read by

simply issuing SFS or SFP tasks at appropriate times, keeping the ASQSC and AQLEV bits at zero, and the

PLLBW and LEVRES bits at their current 'Medium' and 'Level Track' settings, respectively.

Rx Signal from

FM discriminator

to Modem

Noise

Frame Sync

Rest of Frame

Set AQSC and AQLEV bits

to start Acquisition sequences

Level Measurement and Clock

Extraction Circuits

Increasing accuracy and lengthening response times

8-Symbol delay

Symbol Sync

Figure 23: Acquisition Sequence Timing

5.3.3

Clock and Level Acquisition Procedure without RF Carrier Detect

It is also possible to use the modem in a non-standard system where there is an indeterminate delay between

the RF transmitter turn on time and the transmission moment of the Symbol Sync pattern, or where a receive

carrier detect signal is not available to the controlling

PC, or where the transmitting terminal can send separate

unsynchronized Frames. In these cases, each Frame should be preceded by a Symbol Sync pattern, which

should be extended to about 100 symbols, and the procedure provided in Section 5.3.2.

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