User's Manual

4-Level FSK Modem Data Pump Page 10 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: [phone] Doc. # 20480170.003
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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:
rate symbol
1
= T 0.2, = b Where
2T
b+1
> f for 0 = )f(H
2T
b+1
< f <
2T
b-1
for
2
b
)
2
-Tf(
sin
-
2
1
=)f(H
2T
b-1
< f < 0 for 1 = )f(H
π
π
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
Input Data
binary to
symbol
Transmit
filter
Frequency
modulator
MX919B
Modem
Level Coded
Symbols
+3
-1
+1
-3
Bit
pairs
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.