FX929B Datasheet, PDF (24/48 Pages) CML Microcircuits – Flexible Operating Modes

4-Level FSK Modem Data Pump
FX929B
If the received symbol rate and Xtal frequency are both within ±20ppm of nominal then selection of the 'Narrow
Bandwidth' setting will give better performance, especially through fades or noise bursts which might otherwise
pull the PLL away from its optimum timing, but in this case it is recommended that the PLLBW bits are only set
to 'Narrow Bandwidth' after the modem has been running in 'Medium Bandwidth' mode for about 200 symbol
times.
The 'Hold' setting disables the feedback loop of the PLL, which continues to run at a rate determined only by
the actual Xtal frequency and the setting of the Control Register CKDIV bits.
1.5.5.4 Mode Register
The contents of this 8-bit write only register control the basic operating modes of the modem:
Mode Register B7: IRQNEN - IRQN Output Enable
When this bit is set to '1', the IRQN chip output pin is pulled low (to Vss) whenever the IRQ bit of the Status
Register is a '1'.
Mode Register B6: INVSYM - Invert Symbols
This bit controls the polarity of the transmitted and received symbol voltages.
B6 Symbol
0
'+3'
'-3'
Signal at TXOP
Above V_BIAS
Below V_BIAS
Signal at RXFB
Below V_BIAS
Above V_BIAS
1
'+3'
'-3'
Below V_BIAS
Above V_BIAS
Above V_BIAS
Below V_BIAS
Mode Register B5: TXRXN - Tx/Rx Mode
Setting this bit to '1' puts the modem into Transmit mode, clearing it to '0' puts the modem into Receive mode.
Note that changing between receive and transmit modes will cancel any current task.
Mode Register B4: RXEYE - Show Rx Eye
This bit should normally be set to '0'.
Setting it to '1' when the modem is in receive mode configures the modem into a special test mode, in which
the input of the Tx o/p buffer is connected to the Rx Symbol/Clock extraction circuit at a point which carries the
equalised receive signal. This may be monitored with an oscilloscope (at the TXOP pin itself), to assess the
quality of the complete radio channel including the Tx and Rx modem filters, the Tx modulator and the Rx IF
filters and FM demodulator.
The resulting eye diagram (for reasonably random data) should ideally be as shown in Figure 13, with 4 'crisp'
and equally spaced crossing points.
© 1997 Consumer Microcircuits Limited
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D/929B/1