CMX981 Consumer Microcircuits Limited, CMX981 Datasheet - Page 47

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

Advanced Digital Radio Baseband Processor
6.3
 2002 CML Microsystems Plc
Tx Path Details
Data can be input via the DQPSK modulator or via the direct write port.
I and Q data is passed through the following elements:
The reconstruction filter has significant attenuation in the passband, with the following
characteristic (including the external RC):
MCLK/Freq
This attenuation is compensated in the default filter coefficients by convolving the required FIR
response with a 15-tap pre-emphasis FIR filter. The reconstruction filter and the FIR filters will
track with the MCLK frequency, provided that the external RC is scaled in proportion. The FIR filter
approximately cancels the reconstruction filter attenuation up to a frequency of MCLK/900.
Coefficients for the pre-emphasis FIR filter used in the default filter coefficients are shown below.
If there is no source of attenuation or phase distortion external to the IC for which the user wishes
to compensate, this filter can be combined with the main shaping filter as described in section 6.2.
If there are additional sources of attenuation or phase distortion, these can be catered for either by
designing a new pre-emphasis filter that incorporates gain to compensate for the attenuation in the
above table, or by designing another compensating filter and cascading with the filter described
here. Both approaches are essentially equivalent.
Attenuation
(dB)
1. A pair of programmable FIR filters (79-tap and 63-tap).
2. A gain/phase/offset adjustment block.
3. A matched pair of sigma delta DACs
4. A switched capacitor data reconstruction filter (which requires one external RC pole).
5. A two pole continuous time active filter which suppresses clock noise. This has a fixed
pole frequency of 140kHz (subject to 40% tolerance for process variations). It has no
significant effect on the passband for all allowable MCLK frequencies, but the level of
clock noise suppression supplied by this filter (48dB for MCLK = 9.216MHz) is reduced at
lower MCLK frequencies.
-0.00737876
-0.00987614
-0.0150585
-0.0206503
-0.0260154
-0.0304823
-0.0334478
1.22444
-0.0334478
-0.0304823
-0.0260154
-0.0206503
-0.0150585
-0.00987614
-0.00737876
0
0
4608
0.1
2304
0.4
47
1536
0.9
1152
1.6
1024
2.1
CMX981
D/981/1
922
2.6