FX919AP4 datasheet(9/44 Pages) CMLMICRO

9 / 44 page

4-Level FSK Modem Data Pump

FX919A

D/919A/4

In receive mode, the filter is used to reject HF noise and to equalise the received signal to a form suitable for

extracting the 4-level symbols.

Tx Output Buffer

This is a unity gain amplifier used in transmit mode to buffer the output of the Tx low pass filter. In receive

mode, the input of this buffer is connected to VBIAS unless the RXEYE bit of the Control Register is '1', when it

is connected to the received signal. When changing from Rx to Tx mode the input to this buffer will be

connected to VBIAS 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 TXOP 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 capacitances, and its ground connection should be positioned to give maximum

attenuation of high frequency noise into the modulator.

The signal at the TXOP pin is centred around VBIAS and is approx 0.2 x VDD pk-pk for a continuous '+3 +3 -3

-3 ...' pattern.

A capacitor may be fitted in series with the input to the frequency modulator if ac coupling is desired, see

Section 1.6.4.

Frequency / Symbol rate

-30

-25

-20

-15

-10

-5

0.2

0.4

0.6

0.8

Figure 5 RRC Filter Frequency Response (including the external RC filter R4/C5)

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