CMX981 Consumer Microcircuits Limited, CMX981 Datasheet - Page 49

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

Advanced Digital Radio Baseband Processor
6.5
6.6
6.6.1
 2002 CML Microsystems Plc
Procedure For Reconfiguring FIR Filters
1. Obtain or design the required filter characteristic(s), either in s- or z-transform format. Note that
all programmable FIR filters are sampled at a frequency of MCLK/64. This is equivalent to 144kHz
for MCLK = 9.216MHz and 128kHz for MCLK = 8.192MHz.
The task may be made slightly easier if the desired filter specification can be split into two filters of
roughly similar complexity, but if this is not possible, or if it is preferable to treat the desired filter as
a single entity, then a single impulse response can be considered as a polynomial in Z and
factorised into two polynomials of appropriate length.
The Tx 79-tap filter MUST have symmetrical (hence linear phase) coefficients, but all of the 63-tap
filters allow asymmetrical coefficients. Hence any prototype filter function aimed at the Tx 79-tap
filter must be linear phase.
2. Obtain the impulse response of the desired filter shape, either by simulation of the filter
response to an impulse, or by using an inverse Fourier transform. Convolve the impulse response
with the impulse response o the appropriate compensation filter described above. This may be
accomplished either by concatenating the filters and simulating the impulse response of the
combined system, or by multiplying the Z transfer function polynomials together.
3. Use a standard windowing function (e.g. Blackman, Hamming) to limit the impulse response to
141 samples for the Tx, 125 samples for the Rx, or 79 and 63 samples (Tx), 63 and 63 samples
(Rx), if designing the filters separately. Alternatively, it may be possible to approximate small outer
coefficients to zero.
4. Scale the FIR coefficients to appropriate values. To maximise the use of dynamic range, scale
the dc gain to give values similar to those given by the default coefficients (approximately 4.5x for
Tx and 1.6x for Rx). Note that 16-bit coefficients are used in Rx filters, but only 12-bit for Tx filters,
while the use of appropriate dc gains within the system may result in one or two of the most
significant bits being redundant.
5. Load in your new coefficients, run the chip with maximum sized signals and check for internal
overflows (see Status2 and Status3 registers). Optimise individual filter gains to a level at least
5% below those which cause overflows.
Guidelines for use of Powersave Modes
The CMX981 contains a number of powersave modes. In order to maximise flexibility for
architectures and modes of operation, several register bits are available which control different
parts of the device. Operation of the various control bits is described in the appropriate sections.
These guidelines provide an overview of the powersaving features.
Auxiliary Section
When one or more Auxiliary DACs are not required, they can be individually powered down using
bits 3 to 0 of the PowerDownCtrl register.
When the auxiliary ADC channels are not required, the ADC will automatically power down if no
ADC channel is selected. If ADC conversions are only required occasionally, these can be
performed in single shot mode - the ADC will automatically power down between conversions.
When neither auxiliary channels nor the RAM DAC are required, the auxiliary section digital logic
can be powered down using bit 2 of the ClkStopCtrl register.
Note that the auxiliary ADC will power up within four master clock cycles, while the DAC circuits
will power up in less than 5 s.
49
CMX981
D/981/1