DB929B datasheet(12/51 Pages) CMLMICRO

12 / 51 page

4-Level FSK Modem Data Pump

Page 12 of 50

MX929B PRELIMINARY INFORMATION

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-5

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Frequency / Bit Rate

Figure 5: RRC Filter Frequency Response vs. Bit Rate (including the external RC filter R4/C5)

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Frequency / Symbol Rate

Figure 6: RRC Filter Frequency Response vs. Symbol Rate (including the external RC filter R4/C5)

4.1.12 Tx Output Buffer

This is a unity gain amplifier used in the transmit mode to buffer the output of the Tx low pass filter. In receive

mode, the input of this buffer is connected to VBIAS, unless the RXEYE bit of the Control Register is '1', in

which case it is connected to the received signal. When changing from Rx to Tx mode, the input to this buffer

will be connected to VBIAS for 8 symbol times while the RRC filter settles.

Note: The RC low pass filter formed by the external components R4 and C5 between the TXOUT pin and the

input to the radio's frequency modulator forms an important part of the transmit signal filtering. These

components may form part of any DC level-shifting and gain adjustment circuitry. The value used for

C5 should take into account stray circuit capacitance, and its ground connection should be positioned to

give maximum attenuation of high frequency noise into the modulator.

The signal at the TXOUT pin is centered around VBIAS. It is approximately 0.2 x VDD voltsP-P for a

continuous ’+3 +3 -3 -3...' pattern with TXIMP = 0. For typical Tx Eye Diagrams refer to Section

4.7, Figure 17 and Figure 18. For typical Rx Eye Diagrams refer to Section 4.5.4.4, Figure 14.

A capacitor may be placed in series with the input to the frequency modulator if AC coupling is desired.

See Section 5.4.

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