Advanced Digital Radio Baseband Processor
CMX981
4.
External Components
Rx Inputs
When using the internal anti-alias filter, the following is recommended:
Figure 2a Recommended External Components - Rx Inputs
Example values (MCLK = 9.216MHz):
R1 = 220Ω
R2 = 1.2kΩ
C1 = 1.5nF (R1, C1 precise values are not critical) (-3dB at 240kHz)
C2 = 3.9nF (R2 x C2 time constant should give -3dB at 32kHz
±10%
The RC stage formed by R2 and C2 combined with the internal anti alias filter and Rx FIRs gives
a good approximation to the desired filter characteristics and near-constant group delay over the
passband. When not using the internal anti alias filter, it is suggested that the user should follow
the guidelines in Section 5.3.1. In both cases, there should be at least one filter pole close to the
chip inputs.
Tx Outputs
Figure 2b Recommended External Components - Tx Outputs
Example values (MCLK = 9.216MHz):
R3 = 6.2kΩ
C3 = 1nF (R3 x C3 time constant should give -3dB at 32kHz
±10%.
The RC stage formed by R3 and C3 combined with the internal reconstruction filter and the Tx
FIRs gives a good approximation to the desired filter characteristics and near-constant group
delay over the passband. When the default master clock frequency is not used the R2 x C2 and
R3 x C3 products may be scaled with MCLK, but care should be taken to ensure that the FIR
filter coefficients are designed to compensate for any amplitude and phase distortion due to both
on and off-chip filter components. This compensation is included in the default filter coefficients.
See section 6.3 for further details.
©
2003 CML Microsystems Plc
8
D/981/3
CMLMICRO [ CML MICROCIRCUITS ]
