CMX981 Consumer Microcircuits Limited, CMX981 Datasheet - Page 48

• Manufacturer Part Number: CMX981
• Description: Advanceddigital Radio Baseband Processor (tetra Etc.)
• Manufacturer: Consumer Microcircuits Limited

Advanced Digital Radio Baseband Processor
6.4
 2002 CML Microsystems Plc
Rx Path Details
The I and Q Rx data paths are nominally identical and consist of the following elements:
MCLK/Freq
Attenuation
(dB)
The passband attenuation caused by the Rx AAF and decimation filters is compensated in the
default filter coefficients by convolving the required FIR response with a 15-tap pre-emphasis FIR
filter. It approximately cancels the filter attenuation up to a frequency of 10kHz. This pre-emphasis
FIR filter will track with the MCLK frequency, unlike the Rx AAF, but performance should be
adequate for MCLK frequencies between 7MHz and 12MHz. The user is free to alter the external
RX filter and/or to bypass the two on-chip poles of the AAF. In wither case, the compensation FIR
filter may require adjustment. In the event of a user designing their own Rx AAF, it is suggested
that close attention is paid to the effect of component tolerances.
Two example sets of coefficients for this filter, for MCLK frequencies of 9.216MHz and 8.192MHz,
are given below. These assume that external components R2 and C2 and NOT changed from the
default values.
1. A continuous time anti-alias filter, which can be bypassed (two poles at 100kHz, Q = 0.5
2. A sigma delta ADC and decimation filter.
3. A gain/offset adjustment block.
4. A pair of programmable FIR filters (both 63-tap).
on-chip and one pole at 32kHz off-chip), with the following characteristics:
Note: the 32kHz pole is responsible for around 60% of the passband attenuation. The two
on-chip poles attenuate by approximately 0.1dB at 10kHz (equivalent to MCLK/922 for
MCLK = 9.216MHz), while the decimation filter supplies 0.2dB at this frequency. Only the
decimation filter attenuation will track with MCLK, while the other poles will remain fixed
(subject to component tolerances). There is an 800ns variation in nominal group delay of
this decimation filter up to 10kHz. This could be compensated in the FIR coefficients if
considered significant.
MCLK = 9.216MHz
0
0
-0.000113692
-0.00113692
-0.00227383
-0.00568459
-0.00909534
-0.00909534
-0.00568459
-0.00227383
-0.0147799
-0.0193276
-0.0227383
-0.0227383
-0.0193276
-0.0147799
1.16307
4608
0.03
2304
0.13
48
1536
0.29
MCLK = 8.912MHz
-0.00112409
-0.00449636
-0.00899271
-0.00899271
-0.00449636
-0.00112409
-0.0134891
-0.0191095
-0.0224818
-0.0224818
-0.0191095
-0.0134891
1.14994
1152
0.52
0
0
920
0.77
CMX981
D/981/1