DB929B datasheet(11/51 Pages) CMLMICRO

11 / 51 page

4-Level FSK Modem Data Pump

Page 11 of 50

MX929B PRELIMINARY INFORMATION

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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 VDD

voltsP-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 VBIAS (VDD/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

0.2,

Where

for

)

(

for

)

(

sin

)

(

for

)

(

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.

Data

Encoding

binary -

symbol

Transmit

filter

Frequency

modulator

Bit

pairs

MX929B

Symbols

-1

-3

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

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