DB929B Datasheet, PDF (11/51 Pages) CML Microcircuits – Full Data Packet Framing

4-Level FSK Modem Data Pump
Page 11 of 50
MX929B PRELIMINARY INFORMATION
4.1.10 Rx Input Amp
This amplifier allows the received signal input to the modem to be set to the optimum level by suitable
selection of the external components R1 and R2. The value of R1 should be calculated to give 0.2 x V_DD
volts_P-P at the RXAMPOUT pin for a received '...+3 +3 -3 -3 ...' sequence.
A capacitor may be placed in series with R1 if ac coupling of the received signal is desired (see Section 5.4),
otherwise the DC level of the received signal should be adjusted so that the signal at the modem's
RXAMPOUT pin is centered around V_BIAS (V_DD/2).
4.1.11 RRC Low Pass Filter
This filter, which is used in both transmit and receive modes, is a linear-phase lowpass filter with a 'Root
Raised Cosine' frequency response defined by:
H(f ) =1
for
0
<
f
<
1- b
2T
H(f ) =
1
2
-
sin
(5Tf -
b
2
5
2
)
for
1- b
2T
<
f
<
1+ b
2T
H(f ) = 0
for
f
>
1+ b
2T
Where b = 0.2,
T
=
1
symbol
rate
This frequency response is illustrated in Figure 5 and Figure 6.
In transmit mode, the 4-level symbols are passed through this filter to eliminate the high frequency
components which would otherwise cause interference into adjacent radio channels. The input applied to the
RRC Tx filter may be impulses or full-width symbols depending on the setting of the Command Register
TXIMP bit. See Section 4.7.
MX929B
Data
Encoding
Bit
pairs
+3
+1
-1
-3
binary -
symbol
Symbols
Modem
Transmit
filter
Frequency
modulator
Figure 4: Translation of Binary Data to Filtered 4-Level Symbols in Tx Mode
In receive mode, the filter is used to reject HF noise and to equalize the received signal to a form suitable for
extracting the 4-level symbols. The equalization characteristics are determined by the setting of the Command
Register TXIMP bit.
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