FX919B Datasheet, PDF (9/47 Pages) CML Microcircuits – Excellent Radio Fading Performance

4-Level FSK Modem Data Pump
FX919B
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, the equalisation characteristics depending on the setting of the Command
Register TXIMP bit.
0
-5
-10
dB -15
-20
-25
-30
0
0.2
0.4
0.6
0.8
1
Frequency / Symbol rate
Figure 5 RRC Filter Frequency Response (including the external RC filter R4/C5)
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 V_BIAS 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 V_BIAS 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 V_BIAS and is approx 0.2 x V_DD pk-pk for a continuous '+3 +3 -3
-3 ...' pattern with TXIMP = 0.
A capacitor may be fitted in series with the input to the frequency modulator if ac coupling is desired, see
Section 1.6.4.
Figure 6 Transmitted Signal Eye Diagram (TXIMP = 0, see section 1.5.7)
© 1997 Consumer Microcircuits Limited
9
D/919B/1