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CMX980AL7

Digital Radio Baseband Processor

MX-COM Inc  

Digital Radio Baseband Processor

9

CMX980A Advance Information

4.2 Programmable FIR Filter Architecture

Within both, the transmit and receive data paths a common FIR filter architecture for the implementation of

the filtering requirements is employed. The filters use a small local static RAM for efficient data and

coefficient storage during filter operations, together with a dedicated hardware multiplier and accumulator for

each filter.

On reset, the coefficient RAMs are loaded with default values that provide the required response to meet the

needs of a TETRA baseband system. In the default modes the dynamic range of arithmetic units are sufficient

for all normal input data levels without causing overflows. Each filter has an odd number of default

coefficients, which are symmetrical, giving a linear phase response. These coefficients may be overwritten to

adapt to other systems or compensate for deficiencies outside the device. However the user is then

responsible for ensuring that user supplied values do not cause arithmetic overflows to occur within an

accumulation cycle. Overflow logic within each filter can detect such events and cause interrupts to be

generated under user control.

The data RAMs store the filter input data samples and operate upon these values to provide the general FIR

transfer function:

n=FL

y(k)

=

∑

n=1

An. D(n−k)

where: FL

is the filter tap length

An

is the nth filter coefficient

D(n - k)

is the data sample supplied to the

filter n-k samples previously

When a filter is de-activated, coefficient RAMs retain their state, while the data RAMs are reset to zero. This

ensures that the filters start from a quiescent state and prevents filter “memory” from a previous data frame.

Asserting the N_RESET pin will cause all programmable filter coefficients to return to default values.

Alternatively, the Tx and Rx path filter coefficients may be reset independently from each other by use of a

control bit. The data RAMs, unlike the coefficient RAMs, are not directly accessible to the user.

Read or write operations to the coefficient RAMs can be performed by accessing the base address, which

points to the MSB register of the first coefficient A1. This should be followed by a LSB register access that will

auto-index the internal RAM address pointer to A2. Successive operations will continue to auto-index the

RAM address pointer until A(FL) is reached. A further access after this point leads to a reserved location A0

that should not be altered. Continuing operations beyond this point returns the pointer to A1 again.

All filters, except the 79-tap Tx, allow access to the complete coefficient set, although the default values are

symmetrical about (FL+1)/2. This will enable users to realize non-symmetrical filter functions should this be

required.

All filters can be effectively by-passed by setting any single coefficient to normalized unity ( 211-1 in the Tx and

215-1 in Rx) and all others to zero. The chosen position of the “unity” coefficient will vary the internal group

delay, thus this feature should be used with care. For example, the Tx ramping feature has a built in delay,

which defaults to the expected group delay for the Tx filter path. Ramp delay may be bypassed, if required, by

setting the appropriate bit in the BISTControl Register. The default group delay can be retained by choosing

the central coefficient as “unity”.

The 79-tap filter has only one half of the coefficient RAM available, so can only implement symmetrical (linear

phase) filter responses. Thus, when accessing this filter only locations A1-A40 are valid. In addition, to bypass

this filter, the central coefficient (A40) should be chosen as “unity”, since this is the only unique coefficient.

4.3 Tx Data Path

The features described below give a high degree of flexibility for the user to compensate in the baseband

processing for non-ideal performance in the IF, RF and RF linear amplifier sections.

2000 MX-COM, Inc.

www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054

Doc. 20480201.001

4800 Bethania Station Road, Winston-Salem, NC 27105-1201, USA

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