Data Sheet

4-Level FSK Modem Data Pump Page 11 of 47 MX919B PRELIMINARY INFORMATION
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0
-5
-10
-15
-20
-25
-30
0 0.1 0.2 0.3 0.4 0.5
Frequency / Bit Rate
dB
Figure 5: RRC Filter Frequency Response vs. Bit Rate (including the external RC filter R4/C5)
0
-5
-10
-15
-20
-25
-30
0 0.2 0.4 0.6 0.8 1.0
Frequency / Symbol Rate
dB
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 V
BIAS
, 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 V
BIAS
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 V
BIAS
. It is approximately 0.2 x V
DD
volts
P-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.