ETTC'2003 - SEE

The processing flow for the down converter is given below :
Read_Data () ; Read data from ADC
Convert_RNS_Index () ; Prepare for index multiplication
Index_Mixer ( ) ; NCO output and sampled RF data multiplication
Decimation_Filter ( ) ; Down sampling
Conversion_RNS2Binary ( ) ; RNS to binary conversion
NCO block
The NCO is made of two parts: a phase accumulator and a waveform table in the
ROM. The phase accumulator adder needs to be very fast in order to achieve its
highest performance but for wide accumulators the propagation delay will limit the
performance. An RNS based NCO was proposed by Chren [7]. In this RNS_NCO the
phase accumulator is based on a moduli and the scalar performs the truncation
between the phase accumulator output and the ROM. The conceptual view is given
below.
Moduli
RNS
ACCU
RNS
ACCU
Phase
accumulator
Scalar
Address
Control
RNS_NCO
ROM
RNS
Processor
In the above implementation, the phase accumulator is based on n channels with a
very complex scalar hardware module. The scalar hardware is needed for the
truncation of the phase accumulator output before being addressed to the ROM. The
study currently focuses on how to reduce the complexity of the scalar circuit as well as
look at alternative hardware structures to reduce the complexity for the FPGA
implementation.
Mixer block
In a DDS, the incoming IF (Intermediate Frequency) data samples need to be mixed
(multiplier) with the NCO output signals which are performed using an index calculus
method. This index multiply technique uses the Galois field theory [8,9,10]; given a
Galois field GF(p), where p is a prime number , it is possible to generate all non zero
elements by using a primitive root of p. This approach requires following steps
Index_Calculation ( )
Addition_Modulo_p-1 ( )
Reverse Index _Calculation ( )