CMX983 Datasheet, PDF (15/93 Pages) CML Microcircuits – Programmable Channel Filter

Analogue Front End (AFE) for Digital Radio
CMX983
Differential I/Q Inputs and Outputs:
Rx Inputs
R4
C4
R4
Filter 1 ^C4
R4
C4
R4
C4
IRXP
IRXN
QRXP
QRXN
Figure 5 Recommended External Components – Rx Inputs
Example values: R4 = 1.2k, C4 = 3.9nF (R4 x C4 time constant gives -3dB at 34kHz 10%)
The anti-alias filter stage formed by R4 and C4 should have a low enough cutoff frequency to give adequate
attenuation of unwanted signals near the main ADC clock frequency f_CR1, but also have a high enough cutoff
frequency to minimise the group delay variation within the passband in order to prevent intersymbol
interference (ISI). If these conflicting requirements cannot be achieved simultaneously, then the filter should
be designed to give adequate attenuation of the unwanted signals near f_CR1, and compensation for the group
delay variation should be applied in the Rx channel FIR filter. Alternatively, a higher order anti-alias filter with
a flat group delay response could be used to replace components R4 and C4. In either case, positioning the
anti-alias filter components close to the chip inputs may help to remove any pick-up from other noise sources.
Tx Outputs
ITXP
ITXN
R3
C3
R3
C3
QTXP
QTXN
R3
C3
R3
C3
Figure 6 Recommended External Components – Tx Outputs
Example values: R3 = 22k, C3 = 68pF (R3 x C3 time constant should give -3dB at 106kHz 10%)
For each transmit channel, the RC stage formed by R3 and C3 combine with the internal 2^nd-order continuous
time filter to create a 3^rd-order Bessel filter with a -3dB cutoff frequency of approximately 80kHz.
 2015 CML Microsystems Plc
15
D/983/6