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15th LSI Design ContestsEin Okinawa Design Specification - 4-2-1
4-2-1. Design of FFT circuit for serial input
(1) Circuit design in the case of N=4
In chapter A-1, we designed 4 point FFT circuit for parallel input.
The FFT circuit for parallel input can be designed easily, and it has an advantage that Input to Output latency is 0. However the circuit scale becomes very large.
If we designed FFT circuit with less butterfly circuits by reusing them with delay blocks and multiplexer (MUX) blocks, we could make the circuit scale smaller.
In this chapter, we design 4 point FFT circuit for serial input.
The conceptual block diagram is as follows.
The details of MUX_control and W_control blocks in Figure 19 is described as follows.
MUX1 and MUX2 output the values of d0 ~ d2 when sel=0~2. MUX_control decides the output signal of MUX1 and MUX2 multiplexers from d0 ~ d2
The output signal of MUX_control is variable at each step. Therefore, MUX_control can select the signal which is input to next butterfly circuit.
W_control decides the twiddle factor W.
This architecture computes butterfly operation changing the value of twiddle factor W (real part, imaginary part) at each step.
Each block parameters are shown in Table 5.
Table 5:  W_control block paramters
From this table, MULT1 block operates multiply by at first step and at next step. MULT2 block always operates multiply by
The timing chart of 4-points FFT circuit for serial input signal is shown in Fig. 22.
simulation result of this architecture using x(0)=0+0j, x (1)=1+1j, x(2)=2+2j, x(3)=3+3j as input signal is shown in Fig. 23.
From this figure, the output signal of this architecture is X(0)=6+6i, X(1)=-4+0i, X(2)=-2-2i, X(3)=0-4i which is same as the MATLAB simulation result (Table 6).
Table 6:  The comparison between circuit and MATLAB
And latency of this architecture is 3 clocks.