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
at the RXAMPOUT pin for a received '...+3 +3 -3 -3 ...' sequence.
P-P
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
(V /2).
DD
BIAS
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:
1- b
H(f)=1 for 0 < f <
2T
5
(5Tf -
)
2
sin
1
2
1- b
2T
1+b
2T
b
2
H(f) =
-
for
< f <
1+b
2T
H(f)=0 for f >
1
Where b =0.2, T =
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.
+3
MX929B
+1
-1
-3
Bit
pairs
Symbols
binary -
symbol
Data
Encoding
Transmit
filter
Frequency
modulator
Modem
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.
©2001 MX-COM, INC.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480171.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
All trademarks and service marks are held by their respective companies.
CMLMICRO [ CML MICROCIRCUITS ]
